A Multi-wavelength View of Star Formation in Galaxy Clusters

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Title:
A Multi-wavelength View of Star Formation in Galaxy Clusters
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1 online resource (153 p.)
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english
Creator:
Chung,Sun Mi
Publisher:
University of Florida
Place of Publication:
Gainesville, Fla.
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Thesis/Dissertation Information

Degree:
Doctorate ( Ph.D.)
Degree Grantor:
University of Florida
Degree Disciplines:
Astronomy
Committee Chair:
Gonzalez, Anthony
Committee Members:
Tan, Jonathan
Sarajedini, Vicki L
Hamann, Fredrick
Saab, Tarek

Subjects

Subjects / Keywords:
astronomy
Astronomy -- Dissertations, Academic -- UF
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Astronomy thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract:
The primary goal of this dissertation is to study the impact of environment on the evolution of star formation in galaxies. Galaxy clusters are ideal laboratories in which to examine the influence of external processes on galaxy evolution, due to the wide range of environments that are present, from the field-like outskirts to the dense cores. I take two separate approaches in order to investigate the impacts of environment on star formation in galaxies. The first approach is to conduct an in-depth study of obscured and unobscured star formation in the Bullet Cluster, which is a massive cluster merger at intermediate redshift. The second approach is to take a broader view of star formation in the cluster environment by examining the impact of global cluster properties on star formation in 69 local clusters. The Bullet Cluster is a unique system in which the main cluster and subcluster (or ``bullet''), have collided head-on in the plane of the sky, forming a well-defined supersonic shock front. Using data from Spitzer/IRAC, 24um MIPS, BVR imaging, and optical spectroscopy, I examine obscured and unobscured star formation in the Bullet Cluster out to nearly the virial radius (R200=2.2 Mpc). I compare the specific star formation rates (SFR) of post-shock versus pre-shock galaxies, and conclude that strong ram pressure induced by the supersonic gas behind the shock front, does not have a dramatic impact on recent star formation activity of Bullet Cluster galaxies (Chung et al. 2009). I also examine the global star formation properties of the Bullet Cluster by using a 90% spectroscopically complete sample of MIPS sources out to R<1.7 Mpc. I find that the mass-normalized global SFR of the Bullet Cluster is elevated relative to eight other clusters from the literature. This is explained by an excess of (U)LIRGs that is revealed in the infrared luminosity function (IR LF) of the Bullet Cluster relative to IR LFs of other massive clusters (Chung et al. 2010). I conclude that the excess of IR-bright galaxies in the Bullet Cluster likely originates from the subcluster population of galaxies, which implies that star formation is quenched on timescales longer than 250 Myr (time since merger core passage), and that slow mechanisms such as ``strangulation'' are likely to be important drivers of galaxy evolution. Finally, I compare the obscured and unobscured star forming population of galaxies in the Bullet Cluster, by utilizing MIPS 24um and H-alpha spectroscopic data, respectively. Based on the spatial distribution of the two populations, and the different timescales of SFR that 24um and H-alpha data are sensitive to, I conclude that the recent merger has quenched approximately 5% of the total SFR in the Bullet Cluster. In an effort to understand the impact of the global cluster environment on star formation in galaxies, I conduct a study of obscured star formation in 69 local galaxy clusters using mid-infrared data from the Wide-field Infrared Survey Explorer (WISE) and optical data from the Sloan Digital Sky Survey (SDSS). I find that there is no significant correlation between cluster specific SFR and total cluster mass, which indicates that physical mechanisms such as ram pressure, are unlikely to play a dominant role in the transformation of galaxies from the active star-forming galaxies in the field to the quiescent galaxies that dominate cluster cores. I also examine the radial distribution of star-forming galaxies and find that even at 3 times the cluster virial radius, cluster galaxies are quenched relative to the field population. This advocates for a scenario in which a bulk of the galaxy evolution has already occurred prior to cluster mass assembly, most likely in small galaxy groups and filamentary structures.
General Note:
In the series University of Florida Digital Collections.
General Note:
Includes vita.
Bibliography:
Includes bibliographical references.
Source of Description:
Description based on online resource; title from PDF title page.
Source of Description:
This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility:
by Sun Mi Chung.
Thesis:
Thesis (Ph.D.)--University of Florida, 2011.
Local:
Adviser: Gonzalez, Anthony.

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UFRGP
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Applicable rights reserved.
Classification:
lcc - LD1780 2011
System ID:
UFE0042823:00001


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Iamsincerelygratefultothemanypeoplewhohavemadethecompletionofthisdissertationpossible.Iwouldliketothankmyadvisor,ProfessorAnthonyGonzalez,whohasalwaysmadehimselfreadilyavailableforguidanceandinstruction,whilestillgrantingmetheindependencetoexploremyownideasandsolutions.Iamtrulythankfulforhisencouragementandpatiencethroughoutthislongjourney.IwouldalsoliketothankDr.PeterEisenhardtforofferingmethewonderfulopportunitytoworkwithWISEdata,whichgreatlyenhancedthescopeofmydissertation.ManythanksgoouttoProfessorJohnSalzer,whowastherstpersontoencouragemetopursuemyinterestsinastronomyduringmyundergraduateyearsatWesleyanUniversity.IamalsogratefultohaveworkedwithDr.JeremyDrakeattheHarvard-SmithsonianCenterforAstrophysics,whowasawonderfulsupervisorandacademicmentor.IwillalwayslookbackonLETGtea-timewithgreatfondness.Inadditiontomyacademicmentors,noneofthiswouldhavebeenpossiblewithoutthesupportofmygoodfriends.IthankHolgerSiebrechtforhisenduringfriendship,andallthetimeshehasmademelaugh,evenwhenIwantedtocry.IamalsogratefulforhavingoverlappedingraduateschoolwithsomeofthenicestandfunniestpeopleIhaveevermet.IthankMargaretMoerchen,DaveVollbach,JustinCrepp,JulianVanEyken,andDimitriVerasforallthelunch-outings,trivia,goodtimes,andgeneralabsurdity.IwouldalsoliketothankmyfriendsIzaskunSanRoman,NestorLasso,andCurtisDewitt,forallthewonderfullystrangeexcursionsinvolvingmonstertrucks,whales,andhorses.IthankDanGettingsandAudraHernandezfortheirsolidarityandlaughterduringthemanylatenightsintheofce.AndtoJesusMartinez,Iamsoverygratefulforallyourloveandsupport,andthehappinessthatyouhavebroughtintomylife.FinallyIwouldliketothankmyfamily,whohavealwaysbeensupportiveofallmypursuitsinlife.Iamgratefulformysister,YoungMiChung,andherpartnerJamesLatona,foralltherobot-hugsandmakingmytripsbackhomesomuchfunthatIusuallyneverwanttoleavetheirapartment.Andof 4

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page ACKNOWLEDGMENTS .................................. 4 LISTOFTABLES ...................................... 9 LISTOFFIGURES ..................................... 10 ABSTRACT ......................................... 12 CHAPTER 1INTRODUCTION ................................... 14 1.1TransformationMechanisms .......................... 14 1.1.1RamPressure .............................. 15 1.1.2Strangulation .............................. 16 1.1.3Harassment ............................... 17 1.1.4Galaxy-galaxyMergers ......................... 17 1.2StarFormationRateTracers ......................... 18 1.3StarFormationinGalaxyClusters ...................... 19 1.3.1ImpactofClusterDynamicalState .................. 19 1.3.2ImpactofClusterMass ......................... 20 1.4OutlineofDissertation ............................. 22 2IMPACTSOFASUPERSONICSHOCKFRONTONSTARFORMATIONINTHEBULLETCLUSTER .............................. 25 2.1GeometryandClusterProperties ....................... 26 2.2PhysicalAssumptions ............................. 27 2.2.1StarFormationRate .......................... 27 2.2.2GalaxyDistributionandProjection .................. 28 2.3Observations .................................. 30 2.3.1IRAC ................................... 30 2.3.2IMACS .................................. 31 2.4DataAnalysis .................................. 31 2.4.1IRACPhotometry ............................ 31 2.4.2IRACSampleSelection ........................ 31 2.4.3IMACSSpectroscopy .......................... 33 2.5ResultsandDiscussion ............................ 33 2.5.1ImpactoftheShock .......................... 33 2.5.2PhysicalInterpretation ......................... 39 2.6Summary:ImpactsofaSupersonicShockonStarFormationintheBulletCluster ...................................... 42 6

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............................ 43 3.1ObservationsandDataReduction ...................... 45 3.1.1SpitzerMIPS .............................. 45 3.1.2SpitzerIRAC .............................. 46 3.1.3WFIImaging .............................. 46 3.1.4IMACSSpectroscopy .......................... 47 3.2Analysis ..................................... 48 3.2.1MIPScandidatemembers ....................... 48 3.2.2SpectroscopicCompleteness ..................... 53 3.2.3TotalInfraredLuminosity ........................ 55 3.2.4StarFormationRate .......................... 56 3.3ResultsandDiscussion ............................ 57 3.3.1LIRGsandULIRG ........................... 57 3.3.2GlobalStarFormationRate ...................... 62 3.3.3SpecicSFRintheBulletCluster ................... 65 3.3.4InfraredLuminosityFunction ...................... 67 3.3.5LessonsLearned ............................ 73 3.3.6TransformationMechanisms ...................... 74 3.3.6.1TimescaleconstraintfromIRLF .............. 74 3.3.6.2SpatialDistributionofLIRGs ................ 76 3.4Summary:GlobalStarFormationRateandtheIRLFoftheBulletCluster 79 4THESPATIALDISTRIBUTIONOFOBSCUREDANDUNOBSCUREDSTARFORMATIONINTHEBULLETCLUSTER ..................... 82 4.1Observations .................................. 83 4.1.1IMACS .................................. 83 4.1.2WFIImaging .............................. 84 4.1.3SpitzerMIPS .............................. 84 4.1.4SpitzerIRAC .............................. 85 4.2Analysis ..................................... 85 4.2.1Post-starburstGalaxies ........................ 85 4.2.2H-alphaSFR .............................. 86 4.2.3MIPSSFR ................................ 86 4.2.4AGNIdentication ........................... 86 4.3ResultsandDiscussion ............................ 87 4.3.124mversusHSFR ......................... 87 4.3.2CMD ................................... 93 4.3.3SpatialDistribution ........................... 96 4.4Summary:TheSpatialDistributionofStarFormationintheBulletCluster 102 7

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.... 105 5.1Data ....................................... 106 5.1.1WISE .................................. 106 5.1.2ClusterSample ............................. 107 5.1.3SDSSDR7 ............................... 107 5.2Analysis ..................................... 109 5.2.1ClusterMembership .......................... 109 5.2.2StarFormationRatesandInfraredLuminosities ........... 109 5.2.3ExclusionofAGN ............................ 111 5.3ResultsandDiscussion ............................ 113 5.3.1RadialDependenceofStarFormation ................ 114 5.3.2ClusterMassDependenceofStarFormation ............ 117 5.4Summary:ObscuredStarFormationinLocalGalaxyClusters ....... 123 6SUMMARYANDCONCLUSIONS ......................... 128 6.1Summary .................................... 128 6.1.1Chapter2 ................................ 128 6.1.2Chapter3 ................................ 129 6.1.3Chapter4 ................................ 129 6.1.4Chapter5 ................................ 130 6.2Conclusions ................................... 131 APPENDIX:BULLETCLUSTERMEMBERS ...................... 132 REFERENCES ....................................... 145 BIOGRAPHICALSKETCH ................................ 153 8

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Table page 3-1SummaryofBulletClustermembershipforMIPSsourcesandAGN ...... 54 3-2(U)LIRGsandAGNintheBulletCluster ...................... 81 3-3SchechterparametersofclusterIRLFs ...................... 81 5-1WISEdemi-LIRGs .................................. 124 A-1CoordinatesandSFRsof357BulletClustermembers .............. 133 9

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Figure page 2-1Distancefromclustercandidatestoshockfront .................. 29 2-2IRACcolorasfunctionofdistancefromshockfront ................ 34 2-3BinnedIRACcolorasfunctionofdistancefromshockfront ........... 35 2-4Color-magnitudediagramforIRACselectedclustercandidates ......... 38 3-1Spectroscopicredshiftdistribution ......................... 47 3-2R-4.5mversusB-Rcolor .............................. 49 3-3IRACcolor-colordiagram .............................. 50 3-4BPTdiagram ..................................... 51 3-5ComparisonofSFRandLIRderivedfrom Riekeetal. ( 2009 )versus Dale&Helou ( 2002 )templates ............................... 54 3-6SpecicSFRversusstellarmass .......................... 58 3-7HSTimageofULIRG ................................ 59 3-8Color-magnitudediagramincluding90%spectroscopicallycompleteMIPSsample ........................................ 61 3-9Clustermass-normalizedglobalSFRversusredshift ............... 63 3-10IRLFoftheBulletCluster,withandwithoutAGNexclusion ........... 68 3-11IRLFoftheBulletClustercomparedwithcluster+groupIRLF ......... 71 3-12Spatialdistributionof(U)LIRGs ........................... 77 4-124mversusHSFR ................................ 88 4-224mversusextinctioncorrectedHSFR ..................... 90 4-3Ratioof24mtoHSFRversustotalSFR .................... 91 4-4Ratioof24mtoHSFRversusStellarMass ................... 92 4-5Color-MagnitudeDiagram .............................. 94 4-6ImagesofMIPSsourceswithoutHemission ................... 96 4-7SpatialDistributionPart1 .............................. 97 4-8SpatialDistributionPart2 .............................. 98 10

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.......................... 110 5-2BPTdiagramofWISEW4sources ......................... 112 5-3SpecicSFRversusprojectedradius ........................ 115 5-4Demi-LIRGfractionversusprojectedradius .................... 116 5-5Clustermass-normalizedglobalSFRversusclustermassforfullsample .... 118 5-6Clustermass-normalizedglobalSFRversusclustermassforlow-zsample .. 119 11

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Chungetal. 2009 ).IalsoexaminetheglobalstarformationpropertiesoftheBulletClusterbyusing 12

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Chungetal. 2010 ).IconcludethattheexcessofIR-brightgalaxiesintheBulletClusterlikelyoriginatesfromthesubclusterpopulationofgalaxies,whichimpliesthatstarformationisquenchedontimescaleslongerthan250Myr(timesincemergercorepassage),andthatslowmechanismssuchasstrangulationarelikelytobeimportantdriversofgalaxyevolution.Finally,IcomparetheobscuredandunobscuredstarformingpopulationofgalaxiesintheBulletCluster,byutilizingMIPS24mandHspectroscopicdata,respectively.Basedonthespatialdistributionofthetwopopulations,andthedifferenttimescalesofSFRthat24mandHdataaresensitiveto,Iconcludethattherecentmergerhasquenched5%ofthetotalSFRintheBulletCluster.Inanefforttounderstandtheimpactoftheglobalclusterenvironmentonstarformationingalaxies,Iconductastudyofobscuredstarformationin69localgalaxyclustersusingmid-infrareddatafromtheWide-eldInfraredSurveyExplorer(WISE)andopticaldatafromtheSloanDigitalSkySurvey(SDSS).IndthatthereisnosignicantcorrelationbetweenclusterspecicSFRandtotalclustermass,whichindicatesthatphysicalmechanismssuchasrampressure,areunlikelytoplayadominantroleinthetransformationofgalaxiesfromtheactivestar-forminggalaxiesintheeldtothequiescentgalaxiesthatdominateclustercores.Ialsoexaminetheradialdistributionofstar-forminggalaxiesandndthatevenat3timestheclustervirialradius,clustergalaxiesarequenchedrelativetotheeldpopulation.Thisadvocatesforascenarioinwhichabulkofthegalaxyevolutionhasalreadyoccurredpriortoclustermassassembly,mostlikelyinsmallgalaxygroupsandlamentarystructures. 13

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Dressler ( 1980 )wasthersttomeasurethedensity-morphologyrelation,observingastrongincreaseinthefractionofearly-typegalaxiesinregionsofincreasinglocaldensity.Thisresulthassubsequentlybeenconrmedbymanystudies(e.g. Dressleretal. 1997 ; Gotoetal. 2003 ; Postmanetal. 2005 ; Capaketal. 2007 ).Thestarformation-densityrelation,whichdescribesthedecreasingfractionofstar-forminggalaxieswithincreasinglocalgalaxydensity( Lewisetal. 2002 ; Gomezetal. 2003 ; Baloghetal. 2004 ),iscloselyrelatedandbothrelationsarefoundtobewell-establishedbyz1(e.g. Postmanetal. 2005 ; Poggiantietal. 2008 ; Pateletal. 2009 ).Physically,theoriginoftheserelationsremainsunclear.Anumberofdifferentphysicalprocesseshavebeenidentiedthatmaycontributetothesegalaxytransformations,whichinprincipalmaybediscriminatedbetweenbytheirvaryingefcienciesasafunctionoflocalenvironment.Becausegalaxyclustersencapsulatealargerangeofenvironmentsfromtheeld-likeoutskirtstothedense,harshenvironmentsintheclustercoretheyareinmanywaysidealstructureswithwhichtoconstraintheenvironmentalprocessesthataremostimportantforinuencingtheevolutionofstarformationingalaxies. 14

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Gunn&Gott 1972 ).Rampressureisproportionaltov2,whereistheICMdensityandvistherelativevelocityofthegalaxymovingthroughtheICM.Thereforetheeffectsoframpressurearemosteffectiveinmassiveclustersandinclustercores,wherevelocitydispersionandgasdensityareattheirpeaks.Rampressureisconsideredaviolentandfastmechanism,assimulationshaveshownthatrampressurestrippingcanremovesomeoralloftheinterstellarmedium(ISM)fromlate-typegalaxiesastheyentertheclusterpotentialwithinafewhundredMyr( Roediger&Hensler 2005 ; Bruggen&DeLucia 2008 ).ThereisalsodirectobservationalevidenceforrampressurestrippinginindividualgalaxiesfallingintotheVirgocluster. Kenneyetal. ( 2004 ), Crowletal. ( 2005 ),and Chungetal. ( 2007 )haveobservedtruncatedHIdisksorlongHItailsinspiralgalaxiesintheVirgocluster,whoseasymmetricHImorphologiesimplythatthesegalaxiesareexperiencingrampressurestripping. Maetal. ( 2008 )foundthat16outof17ofthepost-starburstgalaxiesinaz0.5galaxyclusteraredistributedwithintherampressurestrippingradiusofthecluster,andsuggestthatrampressureistheprimarymechanismforquenchedstarformationinthepost-starburstgalaxies. 15

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Gavazzietal. ( 2001 )observedatrailofionizedemissionintwoirregulargalaxiesinAbell1367,andbrightHIIregionsdistributedalongtheoppositesideofthetrails,suggestingthatrampressurecantriggerstarformationinclustergalaxies,atleastforalimitedtime.Thisisconsistentwithsimulationsby Kronbergeretal. ( 2008 )whopredictenhancedstarformationratesingalaxiesundergoingarampressureevent.TheeffectsoframpressureingalaxyclustershavebeenobservedwithHIimagingintheVirgoclusterorindirectlyviathespatialdistributionofstar-formingandpost-starburstgalaxiesinintermediateredshiftclusters.However,therehavebeennodetailedstudiesontheimpactoframpressureinmajorclustermergers,partiallybecausethereareonlyasmallnumberofknownclustermergerswithawell-constraineddynamicalhistory.TheBulletClusterhoweverisanidealsysteminwhichtostudytheimpactsoframpressureinaclustermerger,andthiswillbeexploredinChapter 2 Baloghetal. 2000 ).Becausestrangulationkillsfuturecoldgasreservoirsratherthancurrenton-goingstarformationorcoldgas,itworksonlongertimescalesthanrampressure,expectedtobe1Gyr( Bekkietal. 2002 ; Kawata&Mulchaey 2008 ).Thereareseveralstudiesthatsupportstrangulationasadominantmechanismofquenchingstarformationinclustersorgroups. vandenBoschetal. ( 2008 )foundthatquenchingmechanismsforsatellitegalaxiesareindependentofhalomass,and 16

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Moranetal. ( 2006 2007 )alsosupportstrangulationasanimportantclustermechanism,basedonthespatialdistributionofquenchedspiralgalaxiesandthelongtimescalesincequenchedstarformationinthesepassivespirals. Mooreetal. 1996 ).Theseinteractionsaremosteffectiveinthecoresofgalaxyclusters,wheregalaxydensityandvelocitydispersionareattheirhighest. Zabludoff&Mulchaey ( 1998 )rstsuggestedthatgalaxiesarepre-processedingalaxygroups,priortobeingaccretedintotheclusterenvironment.Inthispre-processingscenario,late-typegasrichgalaxiesingroupsmergetoeventuallyformearly-typequiescentgalaxies,thusexplainingthedearthofstar-forminggalaxiesinclusters.Therehavebeenmanystudiestosupporttheimportanceofgalaxypre-processing,includingby Corteseetal. ( 2006 )whoobservedaburstofstarformationinaninfallinggalaxygroupontheoutskirtsofacluster.Whilethevelocitydispersionsofmassiveclustersaregenerallytoohighforgalaxymergerstooccur,theycanstillplayaroleinclustergalaxyevolution. Martig&Bournaud ( 2008 )showedthatwhentheclustertidal 17

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Berrieretal. ( 2009 )arguethatmostgalaxiesaccretedintoclustersareaccreteddirectlyfromtheeld,ratherthanfrominfallinggalaxygroups.Thisimpliesthatgalaxypre-processingingroupscanonlyplayaminimalroleintheevolutionofstarformationhistory,andthatclustermechanismsarethedrivingforcebehindthedifferentgalaxypopulationsobservedintheeldversusclusters. Kennicutt 1998 ; Calzettietal. 2007 ).MuchoftheearlyworkonstarformationinclusterswasdoneusingHsurveystoprobestar-forminggalaxies.Hemissionisadirecttraceroftheionizingphotonsofyoung,massiveO/BstarsinHIIregions.BecauseHemissionoriginatesfromtheionizingphotonsofmassivestarswithM>20M,itissensitivetoSFRontimescalesof20Myr,whichistheapproximatelifetimeofmassiveO/Bstars.Hemissionisthereforeconsideredaninstantaneoustracerofstarformationrate. 18

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Kennicutt 1998 ).Inaddition,ithasbeenshownthatsingle-bandphotometryfrommid-infraredbandsiswellcorrelatedwiththetotalIRluminosity( Daleetal. 2005 ; Riekeetal. 2009 ).ThereforeobtainingobscuredSFRscanbeobservationallylessdemandingthanobtainingunobscuredSFRsfromHemission,whichrequireseitherspectroscopyonindividualgalaxiesornarrowbandltersthatcoveraspecicredshiftrange. 19

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Miller&Owen ( 2003 )and Owenetal. ( 2005 )ndanexcessofradiodetectedstar-forminggalaxiesinAbell2255andAbell2125,andinterpretthisabundanceofstarformationastriggeredbycluster-clustermergeractivity. Ferrarietal. ( 2005 )useopticalspectroscopytostudygalaxiesinAbell3921,andconcludefromthespatialdistributionoftheemissionlinegalaxiesthattheongoingcluster-clustermergerhastriggeredstarformationinatleastafractionofthecurrentlystar-forminggalaxies.Morerecently, Hainesetal. ( 2009a )foundanexcessofmid-IRdetecteddustystar-forminggalaxiesinthenorthernsubclusterofAbell1758,relativetothesouthernsubcluster,theformerofwhichisconsideredtohaveundergonealargeimpactparametermerger. Hainesetal. ( 2009a )concludethatstarformationwastriggeredbyapreviousclustermergerinA1758-N.Incontrast,thereisevidenceforquenchedstarformationinclustermergers. Shimetal. ( 2011 )examinethespatialdistributionandvelocitydistributionofmid-IRdetectedstar-forminggalaxiesinAbell2255andconcludethatstarformationisquenched,mostlikelyduetoanincreaseinrampressuresimilartothescenariopresentedinthesimulationsof Fujita&Nagashima ( 1999 ). Poggiantietal. ( 2004 )alsondevidenceinfavorofquenchedstarformationbasedonthespatialdistributionofyoungpost-starburstgalaxiesalongtheedgeoftwomergingsubstructuresintheComacluster. 20

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Homeieretal. ( 2005 ), Finnetal. ( 2005 ),and Koyamaetal. ( 2010 )suggestthatthereisacorrelationbetweenclusterspecicSFRandclustermass,suchthatmass-normalizedclusterSFRdecreasesasclustermassincreases.TheyestimatetotalclustermassbasedonclusterX-raytemperatureorclustervelocitydispersion,bothofwhicharereliableestimatesoftotalclustermassindynamicallyrelaxedsystems.Althoughananti-correlationisdetectedbetweenclusterspecicSFRandclustermass,theclustersamplesizeissmall(<10)inthesestudies,andtheclusterredshiftsspanalargerange(0.2
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2 ofthisdissertation,IinvestigatetheimpactofthestrongrampressureenvironmentneartheMach3shockfrontintheBulletCluster,usingIRACcolorsasaproxyforspecicstarformationrate.Becauseoftherecenttimescaleandface-ongeometryofthemerger,IamabletoconductafairlydirecttestontheimpactoframpressureonrecentstarformationactivityintheBulletClustergalaxies.ThisworkhasbeenpublishedintheAstrophysicalJournal( Chungetal. 2009 ). 22

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3 ofthedissertationpresentsamoreglobalviewofstarformationintheBulletCluster.Usinga90%spectroscopicallycompletesampleof24mMIPSsourceswithinaradiusofR<1.7Mpcfromtheclustercenter,IcalculatetheglobalobscuredstarformationrateoftheBulletClusterandcomparetotheSFRsofotherclustersandclustermergersfromtheliterature.Ialsopresenttheinfraredluminosityfunction(IRLF)oftheBulletClusterandcomparetotheIRLFsofsimilarlymassiveclustersandagalaxygroup,andconstrainthetimescaleoverwhichstarformationisquenchedinaclusterenvironment.ResultsfromChapter 3 havebeenpublishedintheAstrophysicalJournal( Chungetal. 2010 ).MynalworkontheBulletClusterinChapter 4 combinestheobscuredandunobscuredstarformationratesofclustermembersbasedonMIPS24memissionandHemission,respectively.IcomparethetwoindependentmeasuresofSFRasafunctionoftotalSFR(obscured+unobscured)andstellarmass.Ialsoexaminethespatialdistributionsofmid-IRdetectedstar-forminggalaxies,Hdetectedstar-forminggalaxies,andasmallpopulationofpost-starburstgalaxies.Usingthespatialdistributionandthedifferenttimescalesofstarformationthatmid-IR,Handpost-starburstgalaxiesaresensitiveto,IpresentageneralscenarioofhowstarformationmayhaveevolvedduringtherecentmergereventintheBulletCluster.InChapter 5 ,Iinvestigatethecorrelationbetweenclustermassandobscuredstarformationin69local(z<0.1)clusters.Ipresenttherelationbetweenclustermassandmass-normalizedclusterSFRforthe69clusters,using22mWISEdatatodetermineSFRs,andSDSSdatatodetermineclustermembership.Ialsoexaminethefractionofobscuredstar-forminggalaxiesasafunctionofdistancefromtheclustercenterouttothreetimesthevirialradiusandcomparetothetypicaleldfraction.Chapter 5 hasbeensubmittedforpublicationintheAstrophysicalJournalinAprilof2011. 23

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6 ,Iprovideasummaryofthemostimportantresultsthathavecomeoutofthisdissertation,andtheknowledgethatwehavegainedontheimpactofenvironmentonstarformationingalaxies. 24

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Dressler 1980 ),knownasthedensity-morphologyrelation.Althoughtherearevariousphysicalmechanismsthatcantransformstar-forminglate-typegalaxiesintoquiescentearly-typegalaxies,itisunknownwhetherthistransformationoccursmostlyinthegalaxygrouporgalaxyclusterenvironment.Preprocessingofgalaxiesinthegroupenvironmenthasbeenarguedbysome( Zabludoff&Mulchaey 1998 ; Kodamaetal. 2001 )tobeaprincipalmechanismdrivingthepredominanceofthepassivegalaxypopulationsobservedingalaxyclusters.Ingalaxygroups,thevelocitydispersionsaresufcientlylowthatprocessessuchasgalaxy-galaxyinteractionscanquenchstarformationandmorphologicallyaltergalaxies.Inadditiontogalaxy-galaxyinteractions, Kawata&Mulchaey ( 2008 )showthatstrangulationisanimportantmechanismthatleadstoquenchedstarformationinthegroupenvironment.Usingacosmologicalchemodynamicalsimulation, Kawata&Mulchaey ( 2008 )showthatmostofthehotgasinadiskgalaxyisstrippedaway,thuscuttingoffasourceofnewcoldgas(someofthehotgascoolstoformcoldgas),whichisnecessarytomaintainstarformation.Othershavearguedthatcluster-specicprocessesareresponsibleforthehighearly-typefraction.Thesimulationsof Berrieretal. ( 2009 )arguethatthetransformationoflate-typetoearly-typegalaxiesismostlyattributedtoprocessesinternaltothegalaxyclusterenvironment,ratherthanthegroupenvironment. Berrieretal. ( 2009 )ndthatmostofthegalaxiesinagalaxyclusterareaccreteddirectlyfromtheeld,ratherthanfrominfallinggalaxygroups.Inthiscase,mechanismssuchasgalaxyharassment( Mooreetal. 1996 )andrampressurestripping( Gunn&Gott 1972 )thataremostefcientinmassiveclustersmustdrivethistransformation. 25

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Roediger&Hensler 2005 ; Mayeretal. 2006 ; Bruggen&DeLucia 2008 ).StudiesofgalaxiesinclusterswithobservedHIdecienciesortruncateddiskssupporttheideathatrampressurestrippingcantransformagas-richspiralgalaxyintoananemicspiralgalaxy(e.g. Vollmeretal. 2001 2008 ),whichmayeventuallyevolveintoalenticular(e.g. Bekkietal. 2002 ,butsee Boselli&Gavazzi ( 2006 )).Rampressurehasalsobeenshowntotriggerburstsofstarformationinclustergalaxies( Gavazzietal. 2001 2003 ),andrecentsimulations( Kronbergeretal. 2008 )predictenhancedstarformationratesingalaxiesundergoingarampressureevent.Whilepreviousstudieshaveexaminedtheimpactoframpressureonindividualgalaxiesfallingintotheclusterpotential( Corteseetal. 2007 ; Chungetal. 2007 )theBulletClusterprovidesauniqueopportunitytoexaminetheeffectoframpressureinducedbyclustermergers.AlthoughgalaxiesinanyclusterenvironmentundergorampressureastheytravelthroughtheICM,galaxiesinamajormergereventsuchasthoseweobserveintheBulletClusterwillexperienceadramaticenhancementinthepressuresinceP/V2.Itisthuspossiblethatthisbrieftransientphasemayhaveasignicantimpactonthepropertiesoftheclustergalaxypopulation.TheBulletClusterisanidealsiteinwhichtoquantifytheimportanceofsuchmerger-inducedrampressure.Inthispaperweconductaninitialexplorationoftheimpactoftheshockfrontuponpolycyclicaromatichydrocarbon(PAH)emission,andhencestarformation,inclustergalaxies.WeanalyzeSpitzerdatatakenwiththeInfraredArrayCamera(IRAC; Fazioetal. 2004 ),inconjunctionwithopticalspectraobservedwiththeInamoriMagellanArealCameraandSpectrograph(IMACS; Bigelowetal. 1998 ). Markevitch 26

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( 2002 )[M02]observedasharplydenedbowshockfront,drivenbythesubclusterandpropagatinginthegasofthemainclusterwithavelocityof4740+710550kms1(MachnumberM=3.0+0.450.35; Markevitch 2006 ).Hydrodynamicsimulationsshowedthatthesubclusteritselfhasalowervelocityrelativetothemaincluster,2700kms1( Springel&Farrar 2007 ).Whilethesubclusterandtheshockfrontmightbeexpectedtomovetogether,thegravityofthesubclustercausesthemaincluster'sgasinfrontoftheshocktoowontothesubcluster.Asaresult,theshockfronthasahighervelocityinthereferenceframeofthatgasinow,whichiswhatismeasuredinX-rays. Markevitchetal. ( 2004 )constraintheinclinationangleofthecollision,withrespecttotheplaneofthesky,toi<8o,basedontheMachnumberoftheshockandsubclusterrelativeline-of-sightvelocity(600kms1)( Barrenaetal. 2002 ).Fromthevelocityandgeometryofthesystem,itiscomputedthatthesubclusterexitedthemainclustercore0.15Gyrago.ThesimplegeometryoftheBulletClustermakesitanidealsysteminwhichtostudyhowgalaxypropertiessuchasstarformationareaffectedbyrampressure.Becausethemergerisnearlyintheplaneofthesky,thevelocityofgalaxieswithrespecttotheintraclustermedium(ICM)iswellconstrained,providingagoodquantitativemeasureoftherampressureexertedbythegasonthegalaxies.Inaddition,therecentnessofthecollision(0.15Gyr)allowsustousesensitiveindicatorsofrecentstarformation,directandindirectmeasuresoftheionizinguxfromO/Bstarswithlifetimesof0.1Gyr. 27

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Leger&Puget 1984 ; Allamandolaetal. 1985 ).Morerecentstudieshaveshownthatthe6.2mluminosityiscorrelatedwithtotalinfraredluminosity( Peetersetal. 2004 ; Brandletal. 2006 ),whichisatracerofstarformationrate( Kennicutt 1998 ).However,PAHemissionisnotauniquesignatureofstarformation,whichcanalsobeexcitedbyvisiblephotons( Uchidaetal. 1998 ; Li&Draine 2002 ),aswellasbyactivegalacticnuclei(AGN)( Freudlingetal. 2003 ).IthasalsobeenshownthatPAHemissionisdependentonmetallicity( Engelbrachtetal. 2005 ; Smithetal. 2007 )andthusnotarobuststarformationrateindicatorforgalaxysamplesthatspanalargerangeinmetallicity( Bosellietal. 2004 ).FortheBulletCluster,botheffectsareexpectedtobesecondorder.OurdataarenegligiblyaffectedbyAGNcontamination(seeSection 2.5.1 ),andoursampleisdominatedbymassivegalaxies,forwhichthemetallicityvariationsarenotdramatic.Moreover,themetallicitydistributionshouldbethesameforgalaxiesonbothsidesoftheshock.ThePAHuxshouldneverthelessbeconsideredaroughproxyforstarformationrateratherthanaprecisiontracer.Tonormalizethe8mux,weusethe4.5muxasaproxyformass.The4.5muxoriginatesmostlyfromtheoldredgiantstars,andprobesthestellarmassofagalaxyattheclusterredshift.Byusingtheratioof8mto4.5muxorm4.5-m8color,wecanroughlytracestarformationrateperstellarmass.Specically,8memissioninexcessofthatexpectedfromtheRayleigh-Jeanstailofthecoldstellarcomponentisassumedtoarisefromstar-formationinducedPAHemission.WeestimatethelevelofimpactthattheM3shockhashadonstarformationactivityinindividualgalaxiesbylookingatthem4.5-m8colorasafunctionofdistancefromtheshockfront. 2-1 ).Thepost-shock 28

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BulletClustercandidates(lledcircles)shownwiththeshockfront(acorrespondingcontourofconstantX-raysurfacebrightness).ArrowsindicatethedistancebetweentheobjectandtheshockfrontforsourceswithIRACcolorm4.5-m8>0.5. galaxieshavealreadyexperiencedtherampressureexertedbythegasbehindtheshock,whereasthepre-shockgalaxieshavenotyetbeenaffected.Thetruephysicalsituationisclearlymorecomplex,withcontaminationfromthesubclustergalaxiesaswellasbackground/foregroundsourcesnotassociatedwiththeBulletCluster.Althoughthereissomecontaminationfromthesubclustergalaxies,weassumethatthegalaxypopulationsonbothsidesoftheshockfrontaredominatedbymainclustergalaxiesbasedonestimatesofthetotalstellarmass.Thestellarmassofthemainclusterisapproximatelyanorderofmagnitudelargerthanthestellarmassofthesubcluster.Wealsonotethatanyeffectofcontaminationfromthesubclustergalaxieswillbestrongestattheshockfront,whichbisectsthesubclusterbrightestgroupgalaxy.Subclustergalaxiesjustaheadoftheshockfrontexperienceastrongrampressureas 29

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2-1 )haveexperiencedtheeffectsoftheshock.However,becauseoftheprojectionofgalaxiesandcurvatureoftheshockfrontsurfacealongthelineofsight,thereisagradualtransitionfrompost-shocktopre-shockgalaxies.Thisprojectioneffectsoftensanystep-likeincrease/decreaseincolor(orstarformationrate)acrosstheshockboundary.BygeneratinggalaxiesinarandomdistributionaccordingtotheNavarro,Frenk,andWhite(NFW)( Navarroetal. 1997 )prole,andassumingtheshockfrontisanaxisymmetricconeprojectedintheplaneoftheskyandspanning550kpcfromtheaxisofsymmetry,weestimatethatthetransitionregionduringwhichtheobservedgalaxiesgofrombeingfullypost-shocktofullypre-shockgalaxiesis0.5Mpc. 2.3.1IRACObservationsof1E0657-56weretakenonDecember17,2004,withSpitzer/IRAC.DatawerecollectedfromallfourIRACchannels-3.6m,4.5m,5.8m,and8m,infullarrayreadoutmode.Amediumscale,cyclingditherpatternwasused,with20pointings.At2framesperpointingand100sexposuretimeperframe,thetotalintegrationtimeperIRACbandwas4000s.ThenativeIRACpixelscaleis100.22,butthenalreducedimagesweresettohaveapixelscaleof0.86arcsecpixel1,withaneffectiveeldofview(FOV)of3.73.7arcmincoveredbythefourchannels.TheframeswereprocessedusingtheSpitzerScienceCenter(SSC)IRACPipeline,andmosaicscreatedfromthebasiccalibrateddata(BCD)framesusingMOPEX. 30

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2.4.1IRACPhotometrySourcedetectionandphotometryoftheIRACimagesareperformedwithSourceExtractor(Bertin&Arnouts1996).Sourcesareidentiedinthe4.5mimagewitha3.5detectionthreshold.Magnitudesaremeasuredwithina500aperture.Wechoosetoapplynoaperturecorrectionstothephotometry.Inthisanalysisweareprimarilyinterestedincolorsratherthantotalmagnitudes,whichform8-m5.8andm4.5-m3.6wouldchangebyonly0.05magnitudeswiththecorrectionsapplied.Sincethecorrectionsthemselvesareuncertainbyupto10%(http://ssc.spitzer.caltech.edu/irac/calib/extcal),excludingsystematics,weuserawaperturemagnitudesforallanalyses.Inaddition,aperturecorrectionsareexpectedtobeminorwhenexaminingcolorsratherthanabsolutemagnitudes. 2-1 ),withamaximumandminimumdeclinationof-55.908and-55.985degrees.Objectslocatedaboveorbelow(northorsouth)theshockfrontwerenotincludedin 31

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Cloweetal. 2006 ).IntheinnerregionoftheR-bandimagewherethepointspreadfunction(PSF)issmall(000.45FWHM),objectswithahalf-lightradiusof2.6to3.2pixels(0.111arcsecperpixel)andanR-bandaperturemagnitudeof18to24.1(witha7.8pixelaperture),areaggedasstars.DuetoagradientofthePSFintheimage,half-lightradiusandaperturemagnitudecutsarescaledaccordinglytoprovideacatalogofunsaturatedstars.SaturatedstarsareidentiedmanuallybyinspectingtheMagellanR-bandimagetolookforsaturatedobjects.Tominimizecontaminationfrombrightforegroundgalaxies,weimposeamagnitudelimitofm4.5>14.20themagnitudeofthebrightestclustergalaxy(BCG)inthemaincluster.Wealsoapplyamagnitudecutofm4.5<17,whichissufcientlyfainttodetectmostofthe8msources.Whilegoingdeepertom4.5<18increasesthenumberofclustercandidates,weoptedforamoreconservativem4.5<17magnitudelimitsoastominimizethebackgroundcontamination.Mostimportantly,theanalysesdescribedinthispaperproducethesameresultwithinerrorbars,whetherweapplyamagnitudelimitofm4.5<17orm4.5<18.Ofthe758sources,18arebrighterthanthemainclusterBCG,176arestars,leavinguswith564sources.Eliminatingsourcesfainterthanm4.5=17.0,weareleftwith169sourcesinthedirectpathoftheshockfrontand31thatareabove/belowtheshockfront.Thedistancefromlikelystar-forminggalaxiestotheshockfrontisillustratedbyarrowsinFigure 2-1 32

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Tranetal. ( 2005 ),eachspectrumrepresentingadifferenttypeofgalaxygiantelliptical,E+A,Sbspiral,andanemissionlinegalaxy(ELG).Ofthe458spectra,weobtainreliableredshiftsfor326sources.Weaugmentourspectroscopicdatasetwith133redshifts( Barrenaetal. 2002 ,priv.comm),whichmoredenselysampletheclustercore,yieldingatotalof459sourceswithknownredshifts.WenextassociatethespectroscopicredshiftswithIRACsourcesforthe114spectrathatliewithintheIRACeld-of-view.Ofthese114sources,67areclustermemberswithredshiftsthatliewithin2000kms1ofthemeanclusterredshift(z=0.296),and47areinterlopers.Sixty-threeoutofthe67clustermembers,and42outofthe47interlopers,meetthem4.5<17criteria.Thisleavesuswith134clustercandidates,whichincludeallIRACsourcesthatmeetourmagnitudecriteria,arelocatedwithinthedirectpathoftheshockfront,andexcludeallknowninterlopers. 2.5.1ImpactoftheShockWeusethem4.5-m8colortoproberelativestarformationratesperunitstellarmass.Attheredshiftofthecluster(z=0.296),theprominentPAHemissionbandatrestwavelength6.2mfallsintothe8.0mIRACband.PAHemission,whichisexcitedprimarilybyUVphotons,mostoftenoriginatesfromphotodissociationregionsofstarformationsites(e.g. Howelletal. 2007 ).AlthoughPAHemissionissometimesalsoassociatedwithAGN( Freudlingetal. 2003 ),only2outof134clustercandidatesinthedirectpathoftheshockfronthaveIRACcolorswithintheAGNwedge( Lacyetal. 2004 ; Sternetal. 2005 ).BothAGNcandidateshavem4.5-m8<0.6,andthusdonotcontributespuriousstarformationsignatures. 33

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Them4.5-m8colorasafunctionofdistancefromtheshockfrontfor134clustercandidates.ThelowerandupperaxesindicateprojecteddistanceindegreesandMpc,respectively.Spectroscopicallyconrmedclustermembersarelledinboxes.Theverticaldottedlineshowsthelocationoftheshockfront.Sourcestotheleftofthislineareconsideredpost-shockgalaxies,andthosetotherightareconsideredpre-shockgalaxies.Thehorizontaldotted-dashedlinesatm4.5-m8=-0.13andm4.5-m8=2.43representtheexpectedcolorofanEllipticalandSbcgalaxyatz=0.3,respectively( Assefetal. 2008 ).Theerrorbarintheupperleftcornershowsthemeancolorerrorofthe134candidates. Figure 2-2 showscolorasafunctionofdistancefromtheshockfrontfor134clustercandidates.ThedottedhorizontallinesshowtheexpectedcolorofanellipticalandSbctypegalaxyatz=0.3( Assefetal. 2008 ).Thesepredictedcolorscomefromasetoflow-resolutionspectraltemplates,derivedby Assefetal. ( 2008 ),usingopticalandnear-infraredphotometryofover16,000galaxiesintheNOAODeepWide-FieldSurveyBootesregion,andredshiftsfromtheAGNandGalaxyEvolutionSurvey(AGES).For 34

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Them4.5-m8colorasafunctionofdistancefromshockfrontforclustercandidatesinbinsof275kpc,withthenumberofsourcesperbindenoted.Individualerrorbarsarederivedfrombootstrappingmethod.Greyshadedregionsshowtheintegratedcolorforallobjectsbehindandaheadoftheshockfront,1derivedfrombootstrapping. ellipticalgalaxiesthe8memissioncomesfromtheRayleigh-Jeanstailofthestellarcomponent,forwhichthem4.5-m80.1atz=0.3( Assefetal. 2008 ).ColorsredderthantheellipticallocusareindicativeofPAHemissioninthe8mband.AmajorityoftheIRACsourceshavecolorsconsistentwithredsequencegalaxiesintheclusteronly17%oftheclustercandidateshaveacolorm4.5-m8>0.5,and6%ofthesourceshavem4.5-m8>1.Althoughthereisaselectioneffectinfavorofredsequencegalaxieswiththespectroscopicdata,Figure 2-2 usestheentireIRACsample,forwhichthereisnobiastowardsquiescentellipticals. 35

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2-2 andFigure 2-3 )isoflimitedvalueinthecurrentanalysisbecausethespectroscopicprogrampreferentiallytargetedredsequencegalaxies.Indeed,only2ofthe23clustercandidateswithm4.5-m8>0.5,presentlyhavespectraconrmingtheirmembership.Instead,wefocusinthecurrentanalysisonthefullsampleof134candidatemembersandassesswhetherthereisevidenceforacorrelationbetweentheluminosity-weightedcolorofclustermembersandtheirdistancefromtheshockfront.Foreachgalaxywecomputethedistancefromtheshock(Figure 2-1 ),andthencomputetheintegratedcolorofallgalaxiesin275kpcwidebins(Figure 2-3 ).Foreachbinweusethebootstraptechniquewith10,000realizationstocalculatetheuncertainties.Thedottedlinescorrespondtothetotalintegratedcolorofallobjectsoneachsideoftheshockfront,withthewidthoftheshadedgreyregioncorrespondingtothe1condenceintervalcalculatedviathebootstrapmethod.FortheindividualdatapointsonFigure 2-3 ,thesizeofthebootstraperrorbarsindicatesthatthemeancolorisstronglysensitivetothesmallsubsetofstronglystar-forminggalaxies.Forexample,inthebindirectlyaheadoftheshockfront,thereare2(outof22)galaxiesthatareresponsiblefortherelativelyhighintegratedcolor.Oneofthesetwogalaxiesisaspectroscopicallyconrmed,face-onspiral;theotherisalargediskgalaxywhoseopticalcolorisbluerthantheredsequence.Thehorizontaldotted-dashedlineinFigure 2-3 showsthepredictedcolorofanellipticalgalaxyatz=0.3( Assefetal. 2008 ).ThedatashowninFigure 2-3 aremeanttoshowanexcessofcolorinthepost-shockandpre-shockgalaxies,incomparisontowhatonewouldexpectfromapurelyquiescentellipticalgalaxy.Overall,Figure 2-3 showsthatthereisnodrasticchangeincoloracrosstheshockboundary.Theintegratedcolorofthepost-shockversuspre-shockgalaxiesisconsistenttowithin2. 36

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2-4 presentsthecolor-magnituderelation(CMR)forthe88objectsoutofthe134IRACclustercandidateswithR<20.5,usingourMagellandataandagainexcludingspectroscopicallyconrmedinterlopers.AsindicatedbytheIRACcolors,Figure 2-4 conrmsthatmostoftheIRACsourcesareredsequencegalaxies.However, 37

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Theopticalcolor-magnitudediagramshownfor88IRACselectedclustercandidates.Thesolidlineisthecolor-magnituderelationwhoseslopeandzero-pointwereadoptedfrom Lopez-Cruzetal. ( 2004 ).Thedottedlinessignify2fromthet,wherethedispersionisobtainedbyttingaGaussiantotheresiduals.ObjectsoverplottedwithacrosssymbolhaveIRACcolorm4.5-m8>0.5andarethuslikelytobestar-forminggalaxies. todetermineexactlywhichobjectscanbeclassiedaspartoftheredsequence,weneedthreeparametersoftheCMRtheslope,zero-point,anddispersion.WeadoptaCMRslopeof-0.076,derivedfrom57X-rayclustersexaminedby Lopez-Cruzetal. ( 2004 ),withredshiftsrangingfrom0.02z0.18.Toobtainthezero-pointoftheCMRrelation,wetthedatawiththisxedslope.Thistyieldsazero-pointof3.927,whichis0.1magbrighterthantheexpectedzero-pointcalculatedfromthezero-point-redshiftrelationof Lopez-Cruzetal. ( 2004 ).Finally,wecalculatethedispersionbyttingaGaussianfunctiontotheCMRresiduals,excludingoutliers.We 38

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2-4 .Outofthe7objectswithm4.5-m8>0.5andR<20.5,5ofthemliemorethan2belowtheCMR(withoneobjectjustbarelywithin2).TheseredIRACsourcesconsitute5/12or42%oftheobjectsbelowthe2line.Theopticaldatathusconrmthatobjectswithredm4.5-m8colorscorrespondtoopticallyblueandlikelystar-forminggalaxies.Thereareafewcaveatsininterpretingtheresultsdiscussedabove.Foremost,thereremainscontaminationbyforegroundandbackgroundsourcesforthegalaxiesthatlackspectroscopicconrmation.Foregroundinterlopersamongthestar-forminggalaxiesmaydepresstheobservedsignicanceofanycolorchangeacrosstheshockfront.Itisunlikelyhoweverthatremovingtheseforegroundswillcreateasignicantdifferenceinthemeanm4.5-m8coloracrosstheshockboundary.Atotalof55%ofIRACsourceswithcolorm4.5-m8>0.5areknowninterlopers,whilethefractionofknowninterlopersisonly22%forsourceswithm4.5-m8<0.5.Becausethefractionofinterlopersishigherforlate-typegalaxiesthanearly-typegalaxies,eliminatingtheseobjectswilllikelycausethemeancoloronbothsidesoftheshockfront(Figure 2-3 )toapproachthequiescentvalue. 2-3 ),indicatingthatthegasbehindtheshockfrontdoesnothavealargeimpactonthecurrentorrecentstarformationactivityintheindividualgalaxies.Onepossibleexplanationisthatrampressurestrippingmaydepleteexistinggasreservoirsintheoutskirtsofgalaxieswithoutdisturbingcurrent/recentstarformationsitesthatmaybemorecentrallylocatedinthedisk.Following Gunn&Gott ( 1972 ),agalaxymovingface-onthroughanICMexperiencesarampressureofP=V2.Iframpressure 39

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Pizagnoetal. 2005 ).Therefore,wemightnotexpecttoseeevidenceofdisturbedstarformationduetorampressure,sincemuchofthecurrentstarformationoccurswithinthediskscalelength.However,thegasdensityandgalaxyvelocity(withrespecttotheICM)arehighlyuncertainatanysignicantdistancefromtheshockfront,sincebothquanitieschangewithtimeandpositioninthecluster.Iframpressurestrippingdestroysgasreservoirsingalaxiesthathostsignicantstarformation,wemightexpecttoseeanexcessofpost-starburstgalaxiesin10Myr(afteragalaxycrossestheshockfront),oncetheOstarsnolongersignicantlycontributetothestellarpopulation.Atimescaleof10Myrcorrespondsroughlyto75kpcinFigure 2-3 .Onsuchshorttimescales,itisdifculttodetectanexcessofanyparticulargalaxytypebecauseofsmallnumberstatistics(i.e.therearenotenoughgalaxieswithinasingle75kpcbin).Also,post-starburstgalaxiesaredifculttodistinguishfromquiescentellipticalgalaxieswithphotometry,sincebothpopulationslieneartheopticalCMR( Tranetal. 2007 ).However,thelifetimeofapost-starburstgalaxyis1Gyr(e.g. Quinteroetal. 2004 ),providinguswithalongertimescale(ordistancefromtheshockfront)inwhichwecansearchforanexcessofthesegalaxytypes.Infuturework,wewillspectroscopicallyidentifytheE+Agalaxiesinoursample.AhigherfractionofE+Agalaxiesbehindtheshockfrontversusaheadoftheshockfront,coupledwithnochange 40

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2-3 showsthatthereisnosignicantchangeincolorinthepre-shockversuspost-shockgalaxies,thegeometriceffectsandcontaminationfromnon-clustersourcesmaydilutethesignal.FutureworkincludesanalysisofIRACandMIPS24mdataobservedoverawiderarea,whichcoupledwithspectroscopicredshiftswillbelesssensitivetogeometriceffectsandcontamination.However,evenwithourcurrentlevelofcontamination,itisclearthattherampressureexertedfromthesupersonicgasdoesnothaveadramaticimpactoncurrentstarformationactivityinthemainclustergalaxies.Anothersourceofsignaldilutionisthelargefractionofearly-typegalaxiesinoursample,forwhichtherearetwopossibleexplanations.First,galaxypreprocessinginthegroupenvironmentmayhavealreadydoneitsworkonboththeclusterandsubclustergalaxies,explainingtheobservedlackoflate-typegalaxies( Zabludoff&Mulchaey 1998 ; Corteseetal. 2006 ).Second,thegalaxiesweobservemayhavebeenaffectedbytheclusterenvironmentinsuchamannerthattheyhavealreadyconvertedtheirgasreservoirintostars. Marcillacetal. ( 2007 )and Baietal. ( 2007 )haveinvestigatedstarformationofgalaxiesinmergingclustersatz0.83.Theyndevidenceoftriggeredstarformationininfallinggalaxiesandgalaxygroups,whichcoupledwithmechanismssuchasrampressurestrippingorgalaxyharassment,canacttoeventuallyquenchstarformation.Inthisscenario,manyoftheinitiallygas-rich,starforminggalaxiesinthemainclusterwouldhaveconsumedmuchoftheiravailablegasinanearlierepochoftriggeredstarformation,explainingthelackoflate-typegalaxiesinourcurrentsample.However,therelativelackoflate-typegalaxiescannotexplainwhywedonotseeasignalacrosstheshockfront,especiallyconsideringthatevenamongthelate-typepopulation,wedonotdetectasignicantdifferenceincolorinthepost-shockversuspre-shockgalaxies. 41

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42

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Dressler 1980 ; Gomezetal. 2003 )suggeststhatstarformationmustbequenchedatsomepointduringhierarchicalmassassembly.Galaxytransformationsarethoughttooccurpredominantlyinoneoftwobroadcategoriespriortoclusterassemblywithinthegalaxygroupenvironment,alsoknownasgalaxypre-processing( Zabludoff&Mulchaey 1998 ; Kodamaetal. 2001 ),orwithintheclusterenvironmentitself.Whilethereisevidenceinfavorofbothscenarios(e.g. Fujita 2004 ; Corteseetal. 2006 ; Koyamaetal. 2008 ; Berrieretal. 2009 ),intheendtheroleoftheclusterenvironmentonthestarformationhistoryofitsgalaxiesisunclear.Evenlessclearistheimpactofamajorclustermergerongalaxyevolution. Owenetal. ( 2005 ); Miller&Owen ( 2003 ); Ferrarietal. ( 2005 )foundevidencefortriggeredstarformationinclustermergersbasedonanenhancedfractionofstar-formingradiogalaxiesandapreferentialdistributionofemissionlinegalaxiesbetweenmergingsubclumps.Incontrast, Poggiantietal. ( 2004 )foundthatthepost-starburstpopulationofdwarfgalaxiesintheComaclusterlieneartheedgesoftwomergingsubstructures,suggestingthatthemergerquenchesstarformation,thoughthesecouldberelicsofstarburstsinducedduringanearlierphaseofthemerger( Mahajanetal. 2010 ).Therehasalsobeenmucheffortplacedinunderstandingthespecicphysicalmechanismsresponsiblefortransforminggalaxypropertiessuchasmorphologyandstarformation.Althoughthereissomeobservationaland/ortheoreticalsupportforvariousphysicalprocessesconsideredtooccurintheclusterenvironment,suchasrampressure( Gunn&Gott 1972 ),strangulation( Kawata&Mulchaey 2008 ),andgalaxy 43

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Mooreetal. 1996 ),itisstillunknownwhichoftheseplayadominantroleintransformingastar-forminggalaxyintoaquiescentone.Inthischapter,weexpandupontheworkfromChapter 2 ( Chungetal. 2009 )tostudystarformationintheBulletCluster.TheBulletClusterisagalaxyclusteratz=0.297undergoingamajormergerevent,withacollisionbetweenthemainclusterandsubclusteroccurringclosetotheplaneoftheskywithi<8( Markevitchetal. 2004 ).Awell-denedbowshockfronthasbeenconrmedby Markevitchetal. ( 2002 ),andispropagatingthroughtheX-raygasinthesubclusterregionatavelocityof4740630kms1(MachnumberM=3.00.4)( Markevitch 2006 ).Thesubclusteritselflagsbehindtheshockfrontduetoawindfromthemainclustergasandtravelsat2700kms1relativetothemaincluster( Springel&Farrar 2007 ).Atthisvelocity,thetimesincecorepassageis0.25Gyrago.Withourextensivemulti-wavelengthdatasetandopticalspectroscopy,westudystarformationouttonearlythevirialradiusofthecluster(R200=2.2Mpc).Inthisdissertation,wewillexaminevariousaspectsofstarformationintheBulletClustertounderstandhowarecentmajormergereventaffectsthestarformationofclustergalaxies.WealsoconsidertheimportanceofdifferentphysicalmechanismsthatmaydominategalaxyevolutioninthediverseenvironmentspresentintheBulletCluster.StarformationinthecoreoftheBulletClusterhasbeenstudiedinChapter 2 ,inwhichweconcludethatthestrongrampressureexertedbysupersonicgasfromtheclustermergerdoesnothaveasignicantimpactonrecentstarformationintheclustergalaxies.InthischapterweusetheSpitzerMultibandImagingPhotometer(MIPS)inthe24mband( Riekeetal. 2004 ),tocomparetheglobalstarformationrateoftheBulletCluster,astracedbythedustystar-forminggalaxies,tothoseofothermergingandnon-mergingclustersfromtheliterature.Wealsopresenttheinfraredluminosityfunction 44

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Riekeetal. 2004 )inphotometrymode.Weobservedintwocycles,with14framesat30secondsperframeforatotalintegrationtimeof940sperpixelinthemainexposedregion.Usingarastermapinwhichframesareditheredalongthescanandcross-scandirections,withhalf-arrayoffsets,weachievedatotalspatialcoverageof12.0612.06,whichcorrespondstoaradiusof1.7MpcfromthecenteroftheBulletCluster.IndividualframeswerecombinedintoanalmosaicwiththeSpitzerScienceCenter(SSC)dataanalysistoolMOsaickerandPointsourceEXtractor(MOPEX; Makovoz&Marleau 2005 ).MOPEXalsoincludesatoolforphotometrycalledAPEX,whichperformspoint-responsefunction(PRF)ttingofpointsources.UsingMOPEXandAPEX,weobtainedacatalogof418sourcesdetectedataminimumof5abovethebackgroundnoise. 45

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Fazioetal. 2004 )inallfourbands3.6m,4.5m,5.8m,and8.0mon14November2007infullarrayreadoutmodewithasmallscale,cyclingditherpattern.Witheightoffsetpointings,thetotalcoverageis150150inthefourIRACbands.WecreatedIRACmosaicsfromindividualframesusingMOPEX,withsourcedetectionandphotometrydoneusingSourceExtractor( Bertin&Arnouts 1996 ).Wecomputegalaxycolorsusingaperturephotometrywithina6pixel(3.006)diameter,whichissufcientlylargeforrobustphotometrywhileavoidingcontaminationfrompotentialnearbyneighborsinacrowdedeld.Noaperturecorrectionswereappliedbecausetheyareminimalwhenconsideringcolors(0.05magnitudes). Clowe&Schneider ( 2001 ),withtheadditionofaphotometriccorrectionbasedonthescatteredlightpresentinatelds( Kochetal. 2004 ).Photometriczero-pointsweredetermined 46

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Spectroscopicredshiftdistributionwithshadingtoindicateclustermemberregion.Insetshowszoomed-inviewofclustermembersasdenedbythecausticanalysisoftheinfallregion. usingstandardstareldsfrom Landolt ( 1992 ).WealignedallimagestoacommoncoordinatesystemandconstructaphotometriccatalogusingSExtractorontheR-bandimageforobjectdetectionandR-bandphotometry,andthetwo-imagemodetoobtainphotometryontheBandV-bandimages. 47

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Diaferioetal. ( 2005 ).Toourcatalogof362conrmedmembers,weaddanother44membersfrom Barrenaetal. ( 2002 ).Figure 3-1 showsthedistributionofspectroscopicredshifts,withtheclustermembershighlightedinred.InadditiontotheBulletCluster,wendtwoprominentredshiftpeaksatz0.21andz0.35,containingroughly90memberseach.TheHerschelSpaceObservatoryfar-infraredandsub-mmpropertiesofthebackgroundgrouparestudiedinrelationtotheBulletClusterby Rawleetal. ( 2010 ). 3.2.1MIPScandidatemembersToquantifythetotalstarformationrateoftheBulletClusterastracedby24mluminosity,werstidentifytheMIPSsourcesthatarelikelytobeclustermembers.WeuseWFIandIRACimagestoexcludegalaxieswhosecolorsareindicativeofeitherbackgroundgalaxiesoractivegalacticnuclei(AGN).WethenrenedoursamplebyspectroscopicallytargetingMIPSsourcesthatarecandidateclustermembers.WestartwithaninitialMIPScatalogof418sourcesdowntoauxof43Jy.WematchtheWFI(BVR)andIRACcatalogsandthencross-matchwiththeMIPScatalog.Figure 3-2 showstheR-[4.5]versusB-RcolorsforallMIPSsourceswithopticalandIRACcounterparts.Spectroscopicallyconrmedmembersfromthe2005,2006,and 48

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BFigure3-2. TheR-[4.5]andB-Rcolorsofspectroscopicallyconrmedclustermembers(dots),clustermemberswithMIPSemission(solidcircle),clustercandidateswithMIPSemission(opencircle),andknowninterloperswithMIPSemission(cross).Thespectroscopicallyconrmedclustergalaxies,particularlythosewithMIPSemission,formatightsequenceinthiscolor-colorspace.TominimizecontaminationfrombackgroundMIPSsources,weincludegalaxiesonlywithinthediagonalboundariesindicated.GalaxieswithBR>3arealsoexcludedsincetheseareopticallyredderthantheredsequence.TherightpanelshowstheR-[4.5]andB-Rcolorsofconrmedclustermembersonly.FiveconrmedclusterLIRGsareshownasbluestars,andoneULIRGasagreenstarwithasquareborder. rstpartof2009IMACScampaignsdemonstratedthatthestar-formingclustermembersformatightdiagonallocusintheR-[4.5]vsB-Rspace.WedeneourclustercandidatesampleasgalaxieswithinthetwosoliddiagonallinesshowninFigure 3-2 ,andbluewardofB-R=3.WethenuseIRACcolorstoidentifyAGNviatheAGNwedge( Lacyetal. 2004 ; Sternetal. 2005 ),asshowninFigure 3-3 .WeexcludeallgalaxieswithintheAGNwedgefromfurtheranalysesbecausetheirmid-infraredluminositymaybedominatedbyAGNactivityratherthanbystarformation.WealsoexcludeX-rayAGNusingacatalogof145X-raypointsourcesextractedfromChandradatathatcoverthecentral 49

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BFigure3-3. IRACcolor-colordiagram,showingtheAGNwedge( Lacyetal. 2004 ; Sternetal. 2005 ).SymbolsarethesameasforFigure 3-2 .MIPSsourcesidentiedasIRACorX-rayAGNareshownwithabluesquareandplussignoverplottedonasolidoropenredcircle.TheSeyfertgalaxyidentiedinFigure 3-4 isshownwithabluediamondbehindtheULIRG(greenstarsymbol).ThedottedlineshowstheM82trackstartingfromz=0thenprogressingtoz=0.3,0.5,1,and2,witheachredshiftmarkedbyacross.TheleftpanelincludesallMIPSsourcesandknowninterlopers,whiletherightpanelshowsonlythosesourceswhoseIRAC/opticalcolorsarewithinthediagonallinesofFigure 3-2 .ItisclearthatmanyofthehighredshiftbackgroundsourcesareremovedasaresultofourcolorcriterionillustratedinFigure 3-2 Baldwinetal. 1981 )asillustratedinFigure 3-4 .Thedottedanddashedlinesindicatetheboundariesfrom Kewleyetal. ( 2006 )thatseparatepurelystarforminggalaxiesfromSeyferts 50

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BFigure3-4. BPTdiagramswithdottedanddashedlinesseparatingpurelystar-forminggalaxiesfromSeyfertsandLINERs( Kewleyetal. 2006 ).SmallgraycirclesareobjectsthatarebeyondtheIRACFOV,andthereforeitisunknownwhethertheywouldfallintheIRACAGNwedge.LargecirclesrepresentobjectswithintheIRACFOV,withred(gray)andblackcirclesindicatinggalaxieswithandwithout24memission,respectively.ObjectsoutlinedwithalargesquareandalargetrianglehavebeenidentiedasAGNviaIRACcolorsandX-rayemission,respectively.ThestarsymbolrepresentstheULIRG,whichfallsclosetotheSeyfert/LINERboundaryintheleftpanel,andisclassiedasaLINERintherightpanel. andLINERs.NotethatmanyoftheMIPSsourcesaremissinginFigure 3-4 becauseoneormoreofthefourrequiredemissionlinescouldnotbemeasured,oftenduetoaprominentskyemissionlinethatappearsatthesamewavelengthasHattheBulletClusterredshift.Figure 3-4 revealsonlytwoMIPSsourcesclassiedasaSeyfertorLINERthathavenotbeenidentiedasAGNusingeitherIRACcolorsorX-rayemission.OneoftheseisaULIRG(starsymbol),whichappearsclosetotheSeyfert/LINERboundarywhenusingthe[SII]/Hratio,andisclassiedasaLINERusingthe[OI]/Hratio.ThetwoconrmedMIPSsourcesintheIRACAGNwedge(Figure 3-3 )arenotshownontheBPTdiagrambecausetheylackthenecessaryemissionlines.However, 51

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3-4 ,withoneclassiedasaLINER,andtheotherasanHIIdominatedgalaxy.OfthethreeX-raypointsourcesinFigure 3-3 ,oneisillustratedinFigure 3-4 asalargesquareandappearsclosetotheboundaryofHIIdominatedgalaxiesandSeyferts.Intotal,thereare8AGNidentiedamongtheconrmedMIPSsample,usingthethreemethodsIRACcolors,X-rayemission,andopticalemissionlineratios.Oftheseeight,twoareidentiedsolelyfromtheBPTdiagnostic,oneclassiedasaLINERandtheotheraSeyfert.TheLINERisincludedinoursample,assumingthat65%ofitsIRuxispoweredbystarformation(seex ).TheSeyfert,whichisexcludedfromoursample,wouldcontributeanegliblefractiontotheglobalSFR.InadditiontoisolatingtheAGNpopulation,Figure 3-3 alsoillustratesthatourselectionofclustercandidatesbasedonR-[4.5]andB-Rcolorsisaneffectivewaytocullinterlopers.TheleftandrightpanelsofFigure 3-3 showtheIRACcolordistributionbeforeandafterweapplytheR-[4.5]andB-Rcolorselection,respectively.WeshowthemodelcolorsofM82(alocalstarburstgalaxy)atz=0,0.3,0.5,and1( Devriendtetal. 1999 ; Sternetal. 2005 )toillustratethatourcolorselectioniseffectiveinremovinggalaxieswhoseIRACcolorsareconsistentwiththoseofahighredshiftstarburst(openredcirclesnearz1to2).TherightpanelofFigure 3-3 highlightstheAGNsourcesamongtheMIPSsample,includingfourgalaxiesintheIRACAGNwedge,threeX-raypointsourcesoutsidetheAGNwedge,andoneSeyfertidentiedfromtheBPTdiagram,whichissemi-hiddenbehindtheULIRGsymbol.Amongthenon-AGNMIPSclustermembers,therearetwooutliers,bothofwhichdonottinthediagonalstarformingsequence(roughlyoutlinedbytheLIRGsandULIRGshownasstarsymbols)northelocusofpassiveearlytypegalaxiesnear[3.6]-[4.5]0.2and[5.8]-[8.0]0.2.Oneoutlierhasacolorof[3.6]-[4.5]<0,duetoablendingofsourcesinthe3.6mand4.5mbands.Itisunclear 52

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3-1 ).Ofthese,362galaxiesareconrmedclustermembersand495areinterlopers.Combinedwiththe Barrenaetal. ( 2002 )catalog,wehaveatotalof406conrmedclustermembers.InestimatingthespectroscopiccompletenessofMIPSgalaxies,weconsideronlythe139MIPSsourcesthatliewithinthediagonalcolorboundariesinFigure 3-2 .Outofthose,nineliewithintheAGNwedgetwoconrmedmembers,veinterlopers,andtwoclustercandidates.ThespectroscopiccompletenesswithintheAGNwedgeforMIPSsourcesis80%.Outofthe130MIPSgalaxiesthatlieoutsidetheAGNwedge,wehaveredshiftsfor118galaxies,with39conrmedclustermembers,78interlopers,and12clustercandidateswithnospectroscopy.Weare90%spectroscopicallycompleteforMIPSsourcesoutsidetheAGNwedge,with35%beingclustermembers.Thereareatotalof43conrmedmemberswithMIPSemission,includingtwoclustermemberswhoseR-[4.5]colorsareslightlybeyondthecolorselectionofFigure 3-2 ,andvememberswhoseredshiftsarefrom Barrenaetal. ( 2002 ).Amongthese43,weexcludetwointheIRACAGNwedge,threethatareX-raypointsources,andonethatisaSeyfert.Oftheadditional14MIPSclustercandidateschosenbasedonIRACandopticalcolors,twoareintheAGNwedgeandhenceexcluded.Ournalnon-AGNMIPSsampleconsistsof37conrmedmembersand12candidatemembers, 53

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Thenumberofgalaxiesthatarespectroscopicalyconrmedclustermembers,interlopersandclustercandidates(basedonopticalandIRACcolors).ThecolumnsshownumberofgalaxiesfromtheMIPScatalogwithWFIandIRACcounterparts,numberofAGNwithintheMIPScatalogdeterminedbyIRACcolors,X-rayemission,andopticalspectroscopy,andthetotalnumberofgalaxiesfromtheentireIMACSspectroscopiccatalog. Conrmedmembers436406Interlopers835495Candidatemembers142... BFigure3-5. Theleftandrightpanelsshowacomparisonofthetotalinfraredluminosityandstarformationratederivedfromthe Riekeetal. ( 2009 )relation(redtriangles)comparedtothosefromthe Dale&Helou ( 2002 )templateswiththe Kennicutt ( 1998 )starformationrelation(blackcircles),asafunctionof24muxforthe49non-AGNMIPSconrmedmembersandclustercandidatesinoursample.Thebottomportionofbothpanelsshowthesystematicoffsetsbetweenthetwomethods. ofwhichweexpect4tobemembersbasedonoursuccessratefortargetingMIPSclustermembers.OurnaldatasampleissummarizedinTable 3-1 54

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Riekeetal. ( 2009 ),whoconstructmodelspectralenergydiagrams(SEDs)forasampleoflocalLIRGsandULIRGsandderiverelationsbetween24mux,starformationrate,andtotalinfraredluminosity.ThescatterassociatedwiththeL24LIRrelationis0.13dex. Rawleetal. ( 2010 )havefoundthatforasampleof23BulletClustergalaxies,30%arefoundtohaveanexcessof100muxrelativetothe Riekeetal. ( 2009 )and Dale&Helou ( 2002 )templates.AlthoughthedeviationbetweenHerschelderivedand24mderivedtotalinfraredluminositiescanbeuptoafactorof4forsomesources,thesearetypicallylow-luminositysourcesfainterthanourS/Ncriterionforthe24micronsample.Amongthegalaxiesusedinthispaper(withavailableHerscheldata),theratioofSFRFIRtoSFR24iscloseto1,withascatterof0.6,indicatingthatthe24muxyieldsaccuratetotalIRluminositiesandstarformationratesforoursample.AlthoughwehaveadecentunderstandingofthesystematicuncertaintyintheLIRderivedfromthe Riekeetal. ( 2009 )templates,muchofthepreviousworkintheliteratureusesthegalaxytemplatesof Dale&Helou ( 2002 )toderiveLIRfromobserved24mux.IntheleftpanelofFigure 3-5 ,weshowtherelationbetweenLIRandobserved24muxderivedfromthe Dale&Helou ( 2002 )templatesatz=0.3forour49non-AGNMIPSmembersandclustercandidates.Wecomparethe Dale&Helou ( 2002 )LIRvaluestothosederivedusingthe Riekeetal. ( 2009 )calibration.ThelowerportionoftheleftpanelinFigure 3-5 showsthedifferencebetweenthetwosetsofLIRasafunctionofobserved24mux.Amongthe49MIPSsources,the Riekeetal. ( 2009 )and Dale&Helou ( 2002 )templatesyieldadifferenceintotalinfraredluminosityofapproximately-0.5dex,-0.25dex,and0.1dexforthelow,median,andhighendof24muxesinoursample,respectively.Thebrightest24msourceshowninFigure 3-5 is65%oftheULIRGux. 55

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Riekeetal. ( 2009 ).TheuncertaintyintheSFRderivedfromthisrelationis0.2dex.ThiserrorisdominatedbythescatterintheL24LIRrelation,withanadditionalerrorduetotheunderlyingassumptionthatmostoftheyoungstellarlightisabsorbedandre-radiatedintheinfrared.Althoughanaveragecorrectionhasbeenappliedtoaccountforthelossofyoungstellarlightdirectlytotheultraviolet(andthereforeneverseenintheinfrared),thereisavariationfromgalaxytogalaxythataddsanadditionaluncertaintytothestarformationratecalibration.The0.2dexestimatedoesnotincludeuncertaintiesintheassumedinitialmassfunctionorthetheoreticalrelationbetweenSFRandLIR.Weassignanerrorof0.2dextoallstarformationratescalculatedinthispaper.Inadditiontotheuncertaintyinthestarformationrate,weneedtounderstandanysystematicoffsetsbetweentheSFRscalculatedfromthe Riekeetal. ( 2009 )relations,versustheSFRsquotedinmuchofthepreviousliteratureusingthe Dale&Helou ( 2002 )galaxytemplatesandtheoriginal Kennicutt ( 1998 )SFR-LIRrelation.StarformationratesobtainedfromtheSFR-LIRrelationpresentedin Riekeetal. ( 2009 )aresystematicallylowerby0.2dexrelativetothestandard Kennicutt ( 1998 )relationduetoadifferenceintheinitialmassfunction(IMF).AnunbrokenSalpeterIMFofslope-1.35from0.1to100Misusedintheoriginal Kennicutt ( 1998 )derivation,whereasmorerecentstudiessuggestthataSalpeter-likeIMFwithashallowerslopeforthelowmassendismoreapplicableforextragalacticstar-formingregions( Riekeetal. 1993 ; Alonso-Herreroetal. 2001 ).ThisbrokenSalpeterIMFhasatotalmassthatis0.66timesthemassofasinglepower-lawSalpeterIMF,andyieldsasimilarproportionofstarstothe Kroupa ( 2002 )IMFandthe Chabrier ( 2003 )IMF.TherightpanelofFigure 3-5 showsthecomparisonofSFRsderivedfromthe Riekeetal. ( 2009 )calibrationcomparedtothosefromthe Dale&Helou ( 2002 )templates 56

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Kennicutt ( 1998 )relation,asafunctionoftheobserved24muxoftheBulletClusterMIPSsources.TheSFRscalculatedfromthetwomethodsaresystematicallyoffsetfromeachothermoresignicantlythaninthecaseoftotalinfraredluminosities.ThebottomportionoftherightpanelofFigure 3-5 showsthatthe Riekeetal. ( 2009 )calibrationyieldssystematicallylowerstarformationratesbyafactorof2.6to1.2fortherangeof24muxesintheBulletClustersample,asnotedbythedottedverticallines.ThedifferenceinSFRsderivedfromthetwomethodsisinpartduetothesystematicallylowerinfraredluminositiesderivedfromthethe Riekeetal. ( 2009 )templates,aswellastheassumptionofadifferentIMFfromthe Kennicutt ( 1998 )relationwhichproducesfewerlowmassstars. 3.3.1LIRGsandULIRGAmongthe37non-AGNMIPSconrmedclustermembers,veareLIRGs(LIR>1011L)andoneisaULIRG(LIR>1012L).AlthoughtheULIRGdoesnotappearwithintheAGNwedgenorasanX-raypointsource,itsopticalspectrumexhibitsbroademissionlinesandemissionlineratiosontheBPTdiagrams(BPT; Baldwinetal. 1981 )thatclassifythisgalaxyasaborderlineSeyfert/LINERwhenusingthe[SII]/HratioandaLINERwiththe[OI]/Hratio(Figure 3-4 ).Mid-infraredandfar-infraredemissioninULIRGsarisesfromdustthatisheatedbyayoungstellarpopulationand/oracentralAGN.InthecaseofLINERs,somestudieshavefoundthattheinfraredemissionisdominatedbystarformationratherthannuclearactivity(e.g. Satyapaletal. 2005 ; Veilleuxetal. 2009 ).UsingsixdifferentmethodstodetermineAGNcontribution, Veilleuxetal. ( 2009 )foundthattheAGNcontributiontothebolometricluminosityinLINERsis15%to35%.Inaddition, Sturmetal. ( 2006 )foundthatIR-brightLINERstendtohavemid-IRSEDsconsistentwithstarbursts,whereasSEDsofIR-faintLINERsappearmoreAGNdominated.TheLINERinoursamplehasatotalIRluminosityof1012L,makingitmorelikelytobestarburstdominated.While 57

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Thestarformationrateperstellarmass(specicstarformationrate)asafunctionofstellarmass,withthe50%SFRcompletenesslimitindicated(dottedline).TheveLIRGsandoneULIRGarehighlightedasstarsymbols,withasquareborderaroundtheULIRG. wedonotknowthepreciseAGNcontributioninourULIRG,weadoptavalueof35%,inacautiouseffortnottooverestimatethetruestarformationrate.Intheproceedinganalysesanddiscussion,weincludethisULIRGinoursample,butalwayswithastarformationratethatassumesthatonly65%oftheinfrareduxispoweredbystarformation.TheskycoordinatesoftheveLIRGSandoneULIRGarelistedinTable 3-2 ,alongwiththeirtotalinfraredluminositiesandstarformationrates.Figure 3-6 showsthespecicstarformationrate(SSFR)andstellarmassfortheLIRGandULIRGpopulation,andtheremainingnon-AGNMIPSsampleof37conrmed 58

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TheULIRGimagedwithHST/ACSintheF606Wlter.Itisabarredspiralwithnosignsofrecentmajorinteractions. membersand12clustercandidates.ThereisacleartrendofdecreasingspecicSFRwithincreasingstellarmass.Althoughthereisaninherentselectionbiasagainstlowmass,lowspecicSFRgalaxies,thereisevidencethatthegeneralcorrelationseeninFigure 3-6 isreal.TheupperenvelopeofgalaxieswithhighSFRs(paralleltoandabovethe50%completenesslineinFigure 3-6 )shouldrepresentanearlycompletesampleofbright24msources.AmongthisnearlycompletesampleofhighSFRgalaxies,thetrendofdecreasingspecicSFRasafunctionofstellarmassisstillapparent.Inaddition,thecorrelationseeninFigure 3-6 hasbeenobservedinseveralother 59

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Feulneretal. 2005 ; Perez-Gonzalezetal. 2005 ; Noeskeetal. 2007 ; Vulcanietal. 2010 ).WenotethatFigure 3-6 isintendedtoillustrateageneraltrendbetweenSSFRandstellarmass(notaprecisetorrelation),aswellastherangeofSSFRandstellarmassvaluesinourMIPSsample.TheveLIRGsandULIRG,whicharehighlighted(starsymbols)inFigure 3-6 ,representanoutlierpopulationamongtheMIPSsources.Theyshowexcesslevelsofstarformationgiventheirstellarmass,withtheULIRGbeingthemostextremeoutlierfromthenominalSSFRversusstellarmassrelation.AllgalaxieslistedinTable 3-2 liebeyondR1Mpcfromtheclustercenter,withthetwomostluminousgalaxieslocatedatR1.7Mpc.Theintermediatetoclusteroutskirtsregionisanenvironmentsimilartothatofgalaxygroups,wheregalaxy-galaxyinteractionsaremorelikelytooccurthaninhighdensityclustercoresorinthelowdensityeld.However,thevisualmorphologiesoftheLIRGsdonotshowanyobvioussignsofrecentmergeractivity,basedonBVRimagingfromtheMagellan6.5mBaadetelescope.AtleasttwooutoftheveLIRGsareclearlyspiralgalaxies,whilemorphologiesoftheotherthreearenotentirelyclear,thoughtheydoshowsomefaintspiralstructure.Figure 3-7 showsanimageoftheULIRGtakenwiththeAdvancedCameraforSurveysontheHubbleSpaceTelescopeintheF606Wlter(P.I.HollandFord;ProposalID10996).Itisabarredspiralgalaxy,withnoobvioussignsofrecentinteractionormergers,thoughthereissomeassymetryinthespiralarms.AllsixLIRGs/ULIRGhaveopticalcolorsconsistentwiththoseofstar-forming,bluecloudgalaxies,asshowninthecolormagnitudediagraminFigure 4-5 .TheIRACcolor-colordiagram(Figure 3-3 )alsoshowsthattheLIRGSandULIRGhavecolorsconsistentwithstar-forminggalaxies,withtheULIRGhavingasimilarcolorasthestarburstgalaxyM82evolvedtoz=0.3.Table 3-2 alsoliststhesixclustermembersthatareclassiedasAGNfromIRAC,X-ray,andopticalspectroscopicdata.OneoutofthesixgalaxiesisanX-raypoint 60

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4-5 Figure3-8. Theopticalcolor-magnitudediagramwithspectroscopicallyconrmedclustermembers(dot),spectroscopicallyconrmedmemberswithMIPSemission(solidcircle)andclustercandidateswithMIPSemission(opencircle).MIPSsourcesidentiedasAGNwithIRAC,X-ray,oropticaldataareindicatedwithabluesquare,triangle,anddiamondwithaplussign,respectively.FivespectroscopicallyconrmedLIRGsareshownasbluestars,andoneULIRG/LINERasagreenstarwithasquareborder.Greysolidlineshowsthecolor-magnituderelationwithslopeandnormalizationadoptedfrom Lopez-Cruzetal. ( 2004 ). 61

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).The12MIPScandidatememberscontributeanadditional27Myr1.NormalizingbytheclustermassM(<1.7Mpc)=9.51014M,derivedfromthecausticanalysisoftheclusterinfallregion(Gonzalezetal.inprep),weobtain28Myr1per1014Mfortheconrmedmemberssample,withtheULIRGcontributing40%andtheveLIRGsanadditional30%(seeTable 3-2 ).TheBulletClusterintegratedstarformationratenormalizedbyclustermassispresentedinFigure 3-9 ,alongwithknownvaluesforeightotherclusterstakenfrom Geachetal. ( 2006 )andreferencestherein,and Hainesetal. ( 2009a ),whouse24mMIPSor15mISOCAMdatatocalculateobscuredSFRs.InordertofacilitateafaircomparisonofSFRintheBulletClusterandtheotherclustersshowninFigure 3-9 ,weimposeaninfraredluminositylowerlimitonourMIPSsample.Inaddition,wescalethestarformationratetoaccountforasystematicoffsetinthe Riekeetal. ( 2009 )relationsversusthe Dale&Helou ( 2002 )templateswiththeoriginal Kennicutt ( 1998 )relation(Figure 3-5 ). Geachetal. ( 2006 )applyalowerlimitofLIR=61010L(SFR10Myr1),foralltheirclusterswiththeexceptionofMS0451-03.Thislowerlimitwascalculatedfora200Jysourceatz=0.39usingthe Dale&Helou ( 2002 )templates.Usingthe Riekeetal. ( 2009 )calibration,thesamesourceatz=0.39yieldsatotalIRluminosityofLIR=3.81010L(SFR=3.6Myr1).AfterapplyingalowerlimitofLIR=3.81010Ltoourfullsampleof37conrmedmembersand12clustercandidates,20galaxiesremain19membersandonecandidate.WethenscaletheSFRofeachgalaxyusingtherelationpresentedinthebottomrightpanelofFigure 3-5 ,inordertoconvertSFRscalculatedfromthe Riekeetal. ( 2009 )relationtothe Dale&Helou ( 2002 )systemusedby Geachetal. ( 2006 ).Aftermakingtheseadjustments,weobtainanintegratedSFRof 62

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ThemassnormalizedintegratedSFRasafunctionofredshift,withtheBulletClustershownasabluesolidcircle.SFRscalculatedby Hainesetal. ( 2009a )forA1758isillustratedwithatriangle.Allothersarecalculatedin Geachetal. ( 2006 )andreferencestherein. 445Myr1forthe20galaxies,andaclustermassnormalizedSFRof47Myr1per1014M,asshowninFigure 3-9 .AllstarformationratesillustratedinFigure 3-9 arederivedfrommid-infrareddata,primarilywiththeMIPS24mbandandinsomecasessupplementedwithdatafromtheInfraredSatelliteObservatory(ISO)15mband.IntegratedSFRscalculatedby Geachetal. ( 2006 )includemid-IRsourcesouttoaclusterradiusofR<2Mpc,whiletheSFR 63

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Hainesetal. ( 2009a )).ThisiscomparabletotheR<1.7Mpcareathatwesurvey.ThemassnormalizedSFRoftheBulletClusterpresentedinthispaperisamorepreciseestimateoftheSFRthanwhatwaspossibleformanyoftheclustersshowninFigure 3-9 ,whichmostlylackedmid-infrareddatabeyondtheclustercore.Extrapolationofmid-infraredsourcesandeldsubtractionwereappliedinsuchcases,toplaceupperandlowerlimitsontheglobalspecicSFR,illustratedashorizontalbarsandarrowsonFigure 3-9 .Inaddition,thespectroscopywassparseforseveraloftheclusterspresentedinFigure 3-9 ,whereastheBulletClusterSFRisbasedona90%spectroscopicallycompleteMIPSsample.WhiletheintegratedSFRoftheBulletClusterexcludescontaminationfrombothmid-IRandX-rayAGN,manyoftheclustersconsideredby Geachetal. ( 2006 )and Hainesetal. ( 2009a )lackthenecessarydataforasimilarlycompleteAGNremoval.InA1758, Hainesetal. ( 2009a )identifytwoX-raypointsourcesthatareremovedasAGN,thoughnoconsiderationismadeformid-infraredAGNwithoutX-rayemission. Geachetal. ( 2006 )removeAGNfromCL0024+16andMS0451-03byexcludingmid-IRsourcesthathaveopticalandK-bandcolorssimilartotheexpectedcolorsofpassiveearlytypegalaxies(E/S0)from King&Ellis ( 1985 )models.AlthoughthismayhelpeliminateAGNwithearlytypehosts,therearetwomainissueswithusingthiscolorcriteriontoselectoutAGN.TherstproblemisthatidentifyingAGNsolelywiththismethodcanbesignicantlyincomplete.AsourowndatashowinFigure 3-3 ,noneoftheAGN(sixconrmedmembers,twoclustercandidates)haveIRACcolorsconsistentwiththelocusofearlytypegalaxiesnear[3.6]-[4.5]0.2and[5.8]-[8.0]0.2.InFigure 4-5 ,theopticalcolor-magnitudediagramshowsthatonlytwooutoftheeightAGNareontheredsequence. 64

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4-5 showsthateightoutofthe37non-AGNMIPSclustermemberslieontheredsequence.Although20%ofthenon-AGNMIPSclustermembersareopticallyred,thesegalaxieshaveatotalSFRof14Myr1,contributingonly5%totheintegratedSFR.WhiletheglobalSFRintheBulletClusterisdominatedbyLIRGs/ULIRG,opticallyredmid-IRgalaxiesmayhavemoreofanimpactontheglobalSFRinclustersthatlackaLIRG/ULIRGpopulation.Ingeneral,itisimportantnottoexcludegalaxiesasAGNbasedpurelyontheiropticalcolors,aswehaveseenthatupto20%ofstar-forminggalaxiescanlieontheredsequence.Hadweusedasimilarcriterionas Geachetal. ( 2006 )toexcludeAGNbasedonIRACandopticalcolorsofpassivegalaxies,wewouldnothavedetectedanyofthesixconrmedortwocandidateAGN,whosetotalinfraredluminosityconvertstoastarformationrateof42Myr1fortheconrmedmembers,and8Myr1forthetwoclustercandidates,usingthe Riekeetal. ( 2009 )calibration.AdjustingforthedifferentSFRcalibrationusedin Geachetal. ( 2006 ),theglobalspecicSFRoftheBulletClusterwouldincreasefrom47to57Myr1per1014MinFigure 3-9 3-9 ,secondonlytoCL0024+16.ThespecicSFRoftheBulletClusterisalsocomparabletothatofA1758,aclustermergeratnearlythesameredshift.Interestingly,thethreemostactiveclusters(CL0024+16,A1758,andtheBulletCluster)inFigure 3-9 areallclustermergers.However,allthreeareindifferentmergerstates,withdifferentmassesanddynamicalhistories.AlthoughFigure 3-9 issomewhatsuggestivethatclustermergers 65

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Kodamaetal. ( 2004 ); Geachetal. ( 2006 ); Baietal. ( 2007 ); Koyamaetal. ( 2010 )havesuggestedthatthereisaredshiftdependenceontheglobalstarformationrateofclusters,anywherefrom(1+z)4to(1+z)7,basedonplotssimilartoFigure 3-9 .AlthoughFigure 3-9 showssomesuggestionofevolutioninthespecicSFRs,thereisclearlymuchscatterbetweenindividualclusters.WeemphasizethattheintegratedSFRishighlysensitivetosmallnumberstatistics.InthecaseoftheBulletCluster,onegalaxyiscontributing40%oftheentireintegratedSFR.AnotherwaytocomparethestarformationactivityindifferentclustersistolookatthemassnormalizedintegratedSFRasafunctionofclustermass.Asfoundin Baietal. ( 2007 ); Koyamaetal. ( 2010 ),thereisadependenceofspecicSFRontotalclustermass,althoughitdoesnotdisplayamuchstrongercorrelationthanwithredshift.OfalltheclustersinFigure 3-9 ,CL0024+16andtheBulletClusterhavethehighestnumberofspectroscopicallyconrmed24mMIPSsources,with45and44clustermembersrespectively,priortoexcludingAGN.AlthoughCL0024+16hasaspecicSFRthatisafactorof4higherthantheBulletCluster,thiscouldinpartbedrivenbyitssmallclustermass(M(<2Mpc)=6.11014; Kneibetal. 2003 ),asseveralauthorshavenotedacorrelationbetweenglobalspecicSFRandclustermass(e.g. Finnetal. 2005 ; Baietal. 2007 ; Koyamaetal. 2010 ).AmongthethreeclusterswithhighestspecicSFR(A1758,BulletCluster,andCL0024+16),wenotethatthecorrectionforAGNcontaminationinCL0024+16andA1758islesscompletethanfortheBulletCluster,asisthespectroscopiccompletenessofA1758.Inaddition,theSFRforA1758isintegratedfromcandidatemembersouttoR<3Mpc,comparedtoR<1.7MpcfortheBulletCluster.Ithasbeenshowninseveralstudiesthatinfraredandsubmmluminousgalaxieswithhighlevelsofstarformationratearepreferentiallydistributedinclusteroutskirts(e.g. Maetal. 2010 ; 66

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, 2010 ).ThereforeacomparisonofintegratedSFR(notnormalizedbymass),withinR<3MpcversusR<1.7Mpcisnotnecessarilyafairviewontherelativestarformationactivitybetweentwoclusters.TheintegratedSFRoftheBulletClusterisstronglydrivenbyasmallnumberofinfraredluminousgalaxies.Amongoursampleof37MIPSconrmedmembersexcludingAGN,oneULIRGcontributes40%ofthetotalSFR,whileveLIRGscontributeanadditional30%.WewillseeinthesubsequentsectionsthatthesefewgalaxiesnotonlydominatetheglobalSFR,buthaveasignicantimpactontheinfraredluminosityfunctionaswell. 3-10 ,with1Poissonerrorbars.WeincludetwoversionsoftheBulletClusterIRLFonewhichexcludesallknownAGNwedgeandX-raypointsources(leftpanel),andonewhichdoesnot(rightpanel).TheaimofthelatterIRLFistoillustratetheimpactofAGNcontamination.Therstsamplecontains39conrmedclustermembersand12clustercandidates,whilethelattersamplehas43conrmedmembersand14clustercandidates.Inbothcases,weweighttheclustercandidatesby0.35,whichistheprobabilitythataMIPSsourcewithinthecolorboundariessetinFigure 3-2 isanactualclustermember.WeapplyacompletenesscorrectionderivedfromarticialstartestsfordatafainterthantheMIPS80%completenesslimit,shownasadottedverticallineinFigure 3-10 67

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BFigure3-10. TheinfraredluminosityfunctionoftheBulletCluster(solidcircle),Comacluster(solidsquare),CL1358+62(solidtriangle),andSG1120(asterisk),withtheIRLFoftheComaclusterevolvedtoz0.3.Allinfraredluminositiesshownherearebasedonthe Dale&Helou ( 2002 )galaxytemplates.TheSchechterttotheBulletClusterIRLFisshownasthedashedcurve,andtheMIPS80%completenesslimitisindicatedasadottedverticalline.TheLeftpanelshowstheBulletClusterIRLFexcludingallknownAGNcandidatesandtherightpanelincludessixAGNconrmedmembersandtwoAGNcandidatemembers(scaledby0.35toaccountforprobabilityofbeingactualclustermembers).TheinclusionofjustasmallnumberofAGNcansignicantlyelevatetheIRLF,preferentiallyinthebrightend.However,wenotethatevenwithouttheAGN,theBulletClusterIRLFexhibitsanexcessofIRluminoussourcesrelativetoComaandCL1358+62.ThebrightestBulletClustergalaxyisaULIRGandisnotincludedintheSchechtert. 68

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3-10 aretheIRLFsofCL1358+62(z=0.328)andtheComacluster(z=0.024)from Tranetal. ( 2009 )and Baietal. ( 2009 ).BothCL1358+62andComahaveapproximatelythesametotalmassastheBulletCluster(M1015M; Kuboetal. 2007 ; Hoekstraetal. 1998 ).TheComaIRLFshowninFigure 3-10 isevolvedtoz=0.3usingtherelationsLIR/(1+z)3.2andIR/(1+z)1.7from Baietal. ( 2009 ).InadditiontotheCL1358+62andComa,wealsooverplottheIRLFofSG1120(z=0.37)from Tranetal. ( 2009 ).SG1120consistsoffourgalaxygroupsintheprocessofclusterassembly,predictedtoformaComa-likeclusterbyz=0( Gonzalezetal. 2005 ).TheinfraredluminosityfunctionofComaandCL1358+62arederivedfrom24mMIPSdatathatcover33and2.52.5Mpc,respectively.ThisiscomparabletotheBulletClusterMIPSdata,whichcoversanareaof3.43.4Mpc.ThespatialcoverageofSG1120is66Mpcbecausethefourgalaxygroupsrequireamoreextensivesurveyareathantheclusters.InadditiontothesimilarspatialcoverageofCL1358+62,Coma,andtheBulletCluster,weemphasizethatthetotalmassofallsystemsshowninFigure 3-10 arecomparabletothemassoftheBulletCluster. Tranetal. ( 2009 )donottakeAGNcontaminationintoaccountintheIRLFsofCL1358+62andSG1120,while Baietal. ( 2009 )cross-matchedtheirComadatawiththeCatalogueofQuasarsandActiveGalacticNuclei( Veron-Cetty&Veron 2003 ).WetaSchechterfunctiontotheBulletClusterdata,xingthefaintendslopeto=1.4,adoptedfromtheIRLFofComa( Baietal. 2009 ).TheULIRG,whichoccupiesthebrightestbininFigure 3-10 ,isexcludedfromthetbecauseitisaclearoutlierfromthesmoothdistributionofIRstar-forminggalaxies.ResultsfromttingaSchechterfunctiontotheBulletClusterdatawithandwithoutAGNaresummarizedinTable 3-3 ,alongwiththeSchechterparametersforComa,CL1358+62andSG1120.WenotethattheSchechterparametersfortheBulletClusterremainconsistentwithin1whetherornotweallowtovaryduringthet. 69

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3-10 showsthattheBulletClusterluminosityfunctionisenhancedrelativetotheIRLFsofCL1358+62andComaatallvaluesofLIR.InadditiontoshowingahighersurfacedensityofIRstar-forminggalaxiesinallLIRbins,theIRLFoftheBulletClusterextendstologLIR>45(notincludingtheULIRG),whereastheIRLFsofCL1358+62andComatruncateatlogLIR44.4andlogLIR44.6,respectively.FortheIRLFexcludingallknownAGN,weobtainL=44.680.11,,whichis0.25and0.35dexbrighterthantheLvalueofevolvedComaandCL1358+62,respectively.Figure 3-10 alsoillustratesthatinclusionofevenasmallnumberofAGNsourcescanhaveanoticeableimpactontheIRLF,particularlyatthebrightend.IfweincludeallknownAGN,(sixconrmedmembersandtwoclustercandidatesweightedby0.35theprobabilitythattheyareclustermembers),Lincreasesto44.810.13,andis0.4and0.5dexbrighterthanLofComaandCL1358+62,respectively.AlthoughtheBulletClusterIRLFhasavalueofLthatisbrighterthanthatofevolvedComaorCL1358+62,itisstillfainterthanLofSG1120by0.30(0.18)dex,excluding(including)theAGNpopulationintheBulletClustersample.Ithasbeenshownby Tranetal. ( 2009 )thatSG1120exhibitsanexcessof24msourcescomparedtoCL1358+62andevolvedz0clusters,indicatingthattheinfraredgalaxiesofSG1120representagalaxygrouppopulationwhosestarformationhasnotbeenquenchedtotypicalclusterlevels.WeseefromFigure 3-10 thatwhiletheBulletClusterhasahighernumberdensityofIRstar-forminggalaxiescomparedtootherclusters,itisstillsuppressedrelativetothesegalaxygroupIRLFs.OneexplanationfortheexcessofIRbrightsourcesintheBulletClusterrelativetootherclusterscanbethepresenceofgalaxiesbelongingtotheinfallinggrouppopulationassociatedwiththebullet,whichmaycontributesignicantlytotheoverallelevatedIRLF.Inthiscase,theIRLFwouldberepresentativeofacombinedgalaxygroupandgalaxyclusterenvironment.Totestthisscenario,wescaledowntheSG1120 70

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Figure3-11. TheinfraredluminosityfunctionoftheBulletCluster(solidcircle)excludingknownAGN.Overplottedwithsolid(red)squareistheIRLFofComaevolvedtoz=0.3addedwiththeIRLFofSG1120,afterthelatterhasbeenscaleddownbyafactorof4tomatchtheapproximatemassoftheinfallinggrouppopulationintheBulletCluster. 71

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Springel&Farrar 2007 )andatotalclustermassofM200=1.51015(Gonzalezetal.inprep).ThisyieldsasubclustermassofMsub=1.41014M.WithalowerlimitonthemassofSG1120(5.31014M; Gonzalezetal. 2005 ),thesubclustertoSG1120massratioisapproximately1:4.Figure 3-11 showstheIRLFoftheBulletClusteroverplottedwiththeIRLFofevolvedComaaddedwiththeIRLFofSG1120afterithasbeenscaleddownbyafactoroffour.Overall,theIRLFofcombinedComaandscaledSG1120isagoodmatchtotheobservedBulletClusterIRLF,withthetwoLFshavingasimilareffectiveL.ThissupportstheideathattheBulletClusterIRLFisacombineddistribution,representinggalaxiesfrombothaclusterandgrouppopulation.Quantitatively,theBulletClusterLFisstillafactorof1.5higherthantheevolvedComa+scaledSG1120modelatmostluminositybins.Thereareseveralposssibleexplanations.First,theintrinsicvariationintheIRLFbetweengroupsremainspoorlyconstrainedonewouldonlyneedafactorof1.5highersurfacedensityinthesubclustertoreproducetheobservedBulletClusterIRLF.Second,theexcessmaybeevidenceoftriggeredstarformationintheBulletClusterduetothemerger.Inthiscasethefactorof1.5canbeconsideredanupperboundonthetotalenhancementinducedbythemerger.InChapter 2 wefoundthatrampressureassociatedwiththesupersonicshockfrontdoesnothaveasignicantimpactonrecentspecicstarformationratesofgalaxieswithinthecentralR0.5Mpcregion.However,thisdoesnotexcludethepossibilitythatgalaxiesintheclusteroutskirtsmayexperiencelowleveltriggeringofstarformationduetorampressureeffectsuponrstenteringtheclustermergerenvironment.Sincerampressureisafast-actingmechanismthatcanleadtoquenchedstarformationinasshortatimeas100Myr,galaxieswithtriggeredstarformationintheoutskirtsregioncouldalreadybetransformedintoquiescentgalaxieswithlittletonostarformation,bythetimetheyreachtheclustercore. 72

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3-11 ,whichareconsistentwithinafactorof1.5.GrouptogroupvariationinIRLFsand/orasmalleffectfromtheclustermergeritselfmayalsocontributeasmallexcessofIRstar-forminggalaxiesintheBulletCluster. ,thecontaminationfromanunknownAGNpopulationcanhaveasignicantimpactontheinferredintegratedSFRandthebrightendoftheIRLF.AlthoughthefractionofexpectedAGNinaclustermaybeaslowasafewpercent,evenasmallnumberofAGNcandramaticallyincreasethemeasuredglobalSFRofacluster,sinceAGNarepreferentiallyIRbrightcomparedtothenormalstar-formingpopulation.WealsowarnagainstexcludingredsequencegalaxiesinanattempttoremoveAGNfromamid-IRsampleofgalaxies.IntheBulletCluster,wehavefoundthat20%ofthenon-AGNMIPSmemberslieontheredsequence.Inaddition,itisimportantwheninterpretingtheintegratedspecicSFRsofclustersthatonehasagoodunderstandingoftheinfallinggalaxypopulation.InthecaseoftheBulletCluster,afewgalaxiesmostlikelybelongingtoaninfallinggroupheavilydrivetheintegratedspecicSFR,makingtheBulletClusteroneofthemoreactiveclustersinFigure 3-9 .WehavebeenabletoidentifytheimportanteffectoftheinfallingpopulationbycomparingtheIRLFoftheBulletClustertoothersystems,andknowingthemassratiooftheinfallinggrouptomaincluster.Incaseswheretheclusterdynamicsarenotwellcharacterized,particularlyinclustermergerswhereinfallinggroupsmaybeprevalent,itisimportanttoconsiderthattheintegratedspecicSFRisnotnecessarily 73

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3.3.6.1TimescaleconstraintfromIRLFTheIRLFoftheBulletClusterrevealsanexcessofIRstar-forminggalaxiesrelativetoothermassiveclusters.However,theobservedBulletClusterIRLFisconsistentwithacombinedclusterandgroupIRLF(appropriatelyscaledformass)withinafactorof1.5.Thissupportsthehypothesisthatgalaxiesfromtheinfallinggrouppopulation(orsubcluster)arepredominantlyresponsiblefortheenhancedstarformationobservedintheBulletClusterrelativetootherclusters.Inthiscase,wecanplacealowerlimitonthetimescalerequiredforaclustermechanismtoquenchstarformationinrecentlyacquiredgroupgalaxies.Sincesubclusterstar-forminggalaxiesareobserved250Myraftercorepassage,wecanruleoutfast-actingmechanismssuchasrampressure,asbeingadominantphysicalprocessindrivingtheevolutionofstarformationrateinaclustermerger.Insteadwerequireaslow-workingprocessthatcanexplaintheexcessofstar-formingLIRGsobservedintheBulletCluster250Myrafteramajormerger.TheneedforaclusterprocessthatworkstoquenchstarformationoverlongtimescalesofafewGyrhasbeensupportedbymuchobservationalandtheoreticalwork.Therearetwomainphysicalmechanismsthatworkoversuchtimescalesintheclusterenvironmentgalaxyharassmentandstrangulation(alsoknownasstarvation).Ofthesetwo,wepreferstrangulationtobethemoreplausiblemechanismtoeventuallyquenchtheremainingstarformationintheBulletClusterandtruncatethebrightendoftheIRLF.Strangulationisaslowprocessthatbeginstoworkintheclusteroutskirts.TheinteractionbetweentheclusterICMandthelooselyboundhothalogasofagalaxycancausethehalotobegentlypushedout,eventuallyleadingtotransformationsinbothmorphologyandstarformation( Larsonetal. 1980 ; Baloghetal. 2000 ).Simulationsby Bekkietal. ( 2002 )haveshownthatrampressureandglobaltidaleffectscanbean 74

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Treuetal. ( 2003 )whoclassiedmorphologiesofover2000galaxiesinCl0024+16outtoa10Mpcdiameterregion,anddiscoveredmildgradientsinthemorphologicalfractionsatlargeradii.Theyarguethatsuchatrendcouldonlybeexplainedbyaslowworkingmechanismsuchasstrangulation. Moranetal. ( 2006 2007 )alsosupportstrangulationasanimportantclusterprocess,basedonthespatialdistributionofquenchedspiralgalaxiesandusingultravioletandspectroscopicsignaturesofstarformationtoshowthatstarformationinpassivespiralsmusthavedecayedover<1Gyr.Inadditiontotheobservationalstudies,thereisalsomountingtheoreticalevidenceinfavorofstrangulation,suchasworkby Baloghetal. ( 2000 )whoareabletoreproducethesystematicobserveddifferenceincolorsofclusterversuseldgalaxies,iftheyassumeagradualdeclineofSFRoverafewGyrinthegalaxiesaccretedintothecluster.Ithasalsobeenshownbysimulationsfrom Kawata&Mulchaey ( 2008 )thatstrangulationcanoccureveninlowmassgroups,wheretheICM-galaxyhalointeractionisweakerthaninclusters.Morerecently, McGeeetal. ( 2009 )havedemonstratedthroughsemi-analyticgalaxyformationmodelsthatstrangulationisafavorablemechanismtoquenchstarformationoverlongtimescalesinordertomatchtheobservedevolutionofredgalaxyfractionsinclusters.Furthertheoreticalevidenceinfavorofstrangulationincludesworkby Weinmannetal. ( 2010 )whohavedemonstratedthatmodelsincludingslowremovalofhothalogasinagalaxycanmatchobservedfractionsofpassiveclustersatelliteandcentralgalaxies. 75

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Mooreetal. 1996 ; Haynesetal. 2007 ).Itisaprocessmostefcientinclustercores,wheregalaxydensitiesarehighandnearencountersoccurfrequently.However,allofourLIRGsliebeyondR1Mpc.AssumingthattheLIRGstake1Gyrtoreachtheclustercorewhereharassmentcanbegintohaveanimpact,itislikelythatotherICMprocesseseffectiveintheintermediate/outskirtsregion(suchasstrangulationorgalaxy-galaxyinteractions)wouldhavealreadybeguntotakeeffect.Inaddition,ithasbeensuggestedthatharassmentisonlymildlyeffectiveinthecaseofdwarfclustergalaxies(Mtot1010M),withonly10%stellarmasslossonaverage( Smithetal. 2010a ).Giventhatharassmenthassuchanegligibleeffectondwarfclustergalaxies,itisunlikelythatitcansignicantlyquenchstarformationinintermediatemassLIRGs. 3-10 ,itisparticularlyusefultoexaminethespatialdistributionoftheLIRGs/ULIRG.LookingatthespatialdistributionoftheseLIRGs/ULIRGisagoodwaytoisolatethegalaxiesthatmostlikelydonotbelongtoanormalgalaxyclusterIRLF.ThisisevidentinFigure 3-10 ,whichshowsthattheIRLFsofevolvedComaandCL1358+62truncateatamuchlowerLIRthantheBulletClusterIRLF.ThespatialdistributionoftheLIRGsandULIRG,illustratedinFigure 3-12 ,showsthatallsixIRbrightgalaxiesliebeyondthecentralMpcregion,whereaMach3shockfrontpropagatesthroughthemainclustergalaxies,creatingastrongrampressure 76

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R-bandWFIimageshownwithX-raysurfacebrightnesscontours.ThelargeboxshowstheFOVofour24mMIPSdata,andsmallboxesindicatethelocationoftheveLIRGsandoneULIRG. environment.ThereisalsonoclearcorrelationbetweenthedistributionoftheLIRGsandtheclustermergerstructureasshownbyX-raysurfacebrightnesscontours.ThisimpliesthattheLIRGswerenottriggeredbythemergereventitself,andinparticularnottriggeredbyrampressure.Thisisconsistentwithresultsfrom Chungetal. ( 2009 )whoanalyzedthespecicSFRsofpost-shockversuspre-shockgalaxiesintheBulletClusterandfoundthatrampressuredidnotsignicantlytriggerrecentstarformationinthe 77

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Hainesetal. ( 2009b )similarlyconcludedthatclustermergersdonothaveastrongimpactonstar-formingIRgalaxiesbyndingnocorrelationbetweenthedynamicalstateofacluster(whetheritisrelaxedormerging),andthefractionofIRluminousgalaxiesinasampleof30clusters.Incontrast, Moranetal. ( 2005 )foundevidenceinfavoroftriggeredstarformationtracedby[OII]emissioninfaintearlytypegalaxiesresidingnearthevirialradiusofCL0024+16,whereclustermergereffectsmayhavegeneratedshocksand/orinducedgalaxyharassment.While Moranetal. ( 2005 )favorafastmechanismsuchasmerger-inducedrampressuretotriggerstarformationingalaxiesneartheclusteroutskirts,theyalsorequireagradualfadingofstarformationontimescalesof1GyrbasedontheradialdistributionofBalmerabsorptionlinegalaxiesinCL0024+16.ThisissimilartothelongtimescalerequiredtoquenchstarformationintheBulletClusterLIRGs,iftheyareindeedfromthesubclusterpopulation.Galaxy-galaxymergersortidalinteractionsalsodonotexplaintheLIRGpopulationintheBulletCluster,despitethepreferentialdistributionofLIRGsintheoutskirts,wherethechanceofmergersisrelativelyhighwithinaclusterenvironment.WendnoobvioussignsofrecentinteractionsthatwouldhavetriggeredstarformationfromthemorphologiesoftheLIRGsandULIRG. Geachetal. ( 2009 )foundthatintheclustermergerCL0024+16,severalLIRGsbelongtosmallergroupsthatmaybefallingintothecluster,withsomeevidencethatstarformationistriggeredbyinteractionswithinthesesmallgroups.However,theyalsondnostatisticaldifferenceinthelocalenvironmentofLIRGsversusquiescentspiralgalaxies.ThecurrentdatathusprovidenoevidencethattheobservedLIRGSaretriggeredbyeithertheclustermergerorgalaxy-galaxyinteractions.ItalsoseemsimplausiblethattheseLIRGSareeldgalaxiesfallingintotheclusterforthersttime,basedonacomparisonoftheIRLFsoftheBulletClusterwiththoseofevolvedComaandCL1358+62.TheIRLFsofbothevolvedComaandCL1358+62truncateatlowervalues 78

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3-12 showsthattheLIRGsarealldistributedatR>1Mpc,consistentwithrecentstudieswhichndthatmid-IRandsub-mmdustystar-forminggalaxiesavoidtheclustercoreandarepreferentiallydistributedintheintermediateandoutskirtsregions(e.g. Bragliaetal. 2010 ; Koyamaetal. 2010 ).However,thespatialdistributionoftheLIRGsshowninFigure 3-12 alsocorrespondstotheoutskirtsregionofthesubcluster,whereitismorelikelytonddustystar-forminggalaxies.AlltheLIRGsarewithin1to2timestheR200radiusofthesubcluster,fromthecenterofthebullet.Overall,thespatialdistributionoftheBulletClusterLIRGsshowsnoevidenceofbeingstronglyinuencedbytheclustermerger.CombinedwithresultsfromcomparingtheIRLFoftheBulletClustertotheIRLFsofothersystems,theLIRGsareunlikelytobeeldgalaxies.However,thespatialdistributionoftheLIRGsisconsistentwiththegalaxiesbelongingtotheoutskirtsregionofthesubcluster. Riekeetal. ( 2009 ).Normalizingbyclustermassweobtainaspecicstarformationrateof28Myr1per1014M.Usingthe Dale&Helou ( 2002 )modelswiththe Kennicutt ( 1998 )SFRcalibration,andapplyingalowerlimiton24muxcomparabletopreviousworks,weobtainamassnormalizedintegratedSFRof47Myr1per1014M.WendthatthespecicSFRoftheBulletClusterisoneofthehighestamongothermergingandnon-mergingclustersfromtheliterature,withtheexceptionofCL0024+16.IntheBulletCluster,asmallpopulationofLIRGsandULIRGcontribute30%and40%ofthetotalobscuredstarformationrate.Thesegalaxies 79

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80

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SpectroscopicallyconrmedmemberswithMIPSemissionthatareeitherLIRGsand/orAGN.Column1noteswhetheragalaxyisaLIRG/ULIRG,andifitisdetectedaseitheranX-raypointsourceoranIRACAGNwedgesource.Columns2-5showtherightascension,declination,totalinfraredluminosity,andstarformationrateforeachgalaxy.SFRoftheULIRGassumesthat35%ofthetotalIRluminosityispoweredbyAGN,andtherestbystarformation.FortheLIRGsandAGN,the1errorofthemeasured24muxisbetween1-10%,andthusthenalerrorofthetotalinfraredluminosityisdominatedbytheuncertaintyintheL24toLIRcalibration,whichis<0.15dex. ULIRG/LINER6:58:30.87-56:03:36.311.31012104LIRG6:59:13.45-55:56:31.192.5101128LIRG6:58:55.33-55:55:43.041.5101116LIRG6:58:29.30-55:53:15.921.4101114LIRG6:57:47.86-55:55:10.831.1101111LIRG6:58:37.06-56:00:45.661.0101110LIRG,X-rayAGN6:58:03.52-56:01:14.231.01011...X-ray&IRACAGN6:58:35.22-56:01:4.788.71010...X-rayAGN6:58:17.48-56:02:49.358.11010...OpticalAGN(Seyfert)6:58:42.50-56:00:28.227.61010...IRACAGN6:58:08.48-55:53:36.451.71010...X-rayAGN6:58:26.67-56:00:0.189.2109... SchechterparametersfortheclustersandSG1120showninFigure 3-10 .SchechterparametersforComaareadoptedfrom Baietal. ( 2009 )andevolvedtoz=0.3usingL/(1+z)3.2evolution,whilevaluesforCL1358+62andSG1120arefrom Tranetal. ( 2009 ).WetaSchechterfunctiontotheobservedBulletClusterIRLF,withoutandwiththeAGN,xingthefaintendslopetothematchthatofComa. Coma(evolvedz=0.3)-1.444.44+0.270.24...CL1358+62-1.444.33+0.320.251.92+1.41.3SG1120-1.444.99+0.190.191.25+0.50.3BulletCluster-1.444.68+0.110.111.77+0.50.5BulletCluster(withAGN)-1.444.81+0.130.131.36+0.40.4

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Kennicutt 1998 ).However,themaindrawbackofusingHasaSFRtraceristhatitispronetoextinctionandsensitiveonlytotheionizingphotonsthatescapeunobscuredbydust.Incontrast,infrareddatatracetheobscuredcomponentofstarformation.Theinfraredemissionofdustystar-forminggalaxiesoriginatesfromtheultra-violetemissionofyoungstars,whichisabsorbedbydustthenre-emittedininfraredwavelengths.Monochromaticmid-infraredluminosityfromMIPS24mdatahasbeenshowntocorrelatewellwithtotalinfraredluminosity,fromwhichanobscuredstarformationratecanbecalculated( Calzettietal. 2007 ; Riekeetal. 2009 ).InfraredemissionfromheateddustbyyoungstarsissensitivetoSFRontimescalesofafew100Myr,roughlyafactorof10longerthanHderivedSFR.InadditiontousingHand24mdatatostudythestarformationhistoryoftheBulletCluster,weuseourspectraldatatoidentifypost-starburstgalaxies.Post-starburstore+agalaxieswererstdiscoveredingalaxyclustersasobjectswhosespectrahad 82

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Bigelowetal. 1998 ),andimagingdatafromSpitzerMIPS( Riekeetal. 2004 )andIRAC( Fazioetal. 2004 ).TheIMACSdataareusedtoconrmclustermembershipforMIPSsources,deriveunobscuredHSFRs,andidentifypost-starburstgalaxies.TheMIPS24mdataareusedtocalculateobscuredSFRs,andtheIRACdataareusedtoisolateMIPSsourceswhosemid-IRemissionisdominatedbyAGNratherthanstarformation. 83

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Diaferioetal. ( 2005 ). 3 fordetailsonthedatareduction.) 84

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Makovoz&Marleau 2005 ).UsingAPEX,whichisthestandardtoolforMIPSphotometry,weproducedanalcatalogof418sourcesthataredetectedatleast5abovethebackgroundnoise. 4.2.1Post-starburstGalaxiesPost-starburstgalaxies,whicharealsoknownase+aork+agalaxies,areclassiedfromtheiropticalspectraasgalaxiesthathavestrongBalmerabsorptionlinesandlackemissionlines.Thespectraofpost-starburstgalaxiesshowastrongpresenceofA-typestarsandnosignicantpopulationofO/Bstars,indicatingthatstarformationwasabruptlyquenchedbetween0.1-1Gyrpriortoobservation.Whilethereissomevariationintheliteratureonhowexactlytodeneapost-starburstgalaxy,weadoptacommonwaytoclassifye+aspectrabyusingthefollowingthresholdofequivalentwidth(EW):AverageEWBalmer>4AandnoHor[OII]emissiondetectedinthespectra.WemeasuretheEWsofH(4861),H(4341),andH(4102)absorptionlinesbyusingtheIRAFtasksplottotgaussianstotheabsorptionproles.Amongthe301clustermembers(excludingtheDecember2009masks),wendonlyninegalaxieswithspectrathatfallintothepost-starburstclassication. 85

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Kennicuttetal. ( 2009 )relationSFR[Myr1]=5.51042LH(ergss1),whichassumesa Kroupa ( 2002 )stellarinitialmassfunction(IMF). Riekeetal. ( 2009 ).Theserelationsarederivedfromasetofspectralenergydistribution(SED)templatesoflocalLIRGsandULIRGs,aswellasmodiedversionsofthelowerluminositytemplatespresentedin Dale&Helou ( 2002 ).TheoverallerroronobscuredSFRis0.2dex,whichisdominatedbythescatterintheL24LIRrelation.ThestellarIMFassumedinthe Riekeetal. ( 2009 )SEDsissimilartothe Kroupa ( 2002 )or Chabrier ( 2003 )IMFs. 3-10 ofChapter 3 .OnecommonwaytoidentifyAGNisviatheAGNwedge( Lacyetal. 2004 ; Sternetal. 2005 ),whichisaneffectivewaytoisolateAGNbasedonlyontheirIRACcolors.AllgalaxieswithintheAGNwedgeareexcludedfromfurtheranalysessincetheir24memissionisnotatruereectionofthestarformationrate.WealsoexcludeX-rayAGNbyusingacatalogofX-raypointsourcesextractedfromChandradata.Atotalof8AGN 86

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Relationbetween24mSFRversusHSFRnotcorrectedforextinction,withaone-to-onerelationoverplottedindottedline.TheredstarsareLIRGs,andgreenstarwithsquareoutlineisaULIRG. ThedifferenttimescalesprobedbythesetwoindependentSFRtracerswillallowustoexaminethegeneralstarformationhistoryoftheBulletClusterandhowitmayhavebeendrivenbytherecentmergerthatoccurred250Myrpriortoobservation.Thisisthetimeelapsedsincecorepassage,orwhenthesubclusterexitedthecoreofthemaincluster,basedonasubclustervelocityof2700km/s( Springel&Farrar 2007 )andadistanceof720kpcbetweenthemainclusterandsubclustermasspeaksderivedfromlensinganalyses( Cloweetal. 2006 ).Figure 4-1 illustratesthecomparisonbetweentheobscuredSFRderivedfrom24memissionandtheunobscuredSFRderivedfromHluminosity.TheHluminosity 88

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4-1 indicatesthatroughly50%ormoreofthetotalSFRinthesegalaxiesisabsorbedandre-emittedintheinfraredfrominterstellardust.Thisisconsistentwithresultsfrom Perez-Gonzalezetal. ( 2006 )whoexaminedtheHand24mSFRsofindividualHIIregionsinthemoderatelystar-formingM81galaxyandfoundthatintegratedovertheentiregalaxy,50%ofthetotalSFRisobscuredbydust.AlthoughthereisaclearcorrelationbetweenobscuredandunobscuredSFR,Figure 4-1 showsasignicantamountofscatter,withinafactorof2to3.Thisisdominatedbytheintrinsicscatterintheinternalextinctionoftheclustermembers.Whileitiscommontoattributeacanonicalvalueof1magnitudeofHextinctioninlocalgalaxies,thereisknowntobealargescatterininternalextinctionthatcanrangebyasmuchas4magnitudes( Bell&Kennicutt 2001 ).ThelargescattershowninFigure 4-1 illustratetheimportanceofcorrectingforextinctionifusingsolelyHdatatoestimateaSFR.SeveralstudiesrelyontheknownintensityratiooftheBalmerdecrement(H=H)inordertoderiveanextinctioncorrection( Kewleyetal. 2002 ).AnotherwaytoquantifyHextinctionwithouttheuseofhighqualityspectraistoquantifytheextinctioncorrectedHluminosityasalinearcombinationofobservedHluminosityandtotalinfraredluminosity.Figure 4-2 showsthe24mSFRversusHSFR,wherethelatterhasbeencorrectedforextinctionusingthe Kennicuttetal. ( 2009 )relation.Inprinciple,theextinctioncorrectedHSFRisameasureofthetotalSFRinagalaxy,sinceittakesintoaccountbothunobscuredandobscuredstarformation.ThedatainFigure 4-2 showconsiderablylessscatterthanFigure 4-1 ,nowthatextinctioncorrectionisappliedtoH.Therelationbetween24mandcorrectedHSFRmatchesreasonablywellwithtsadoptedfrom Zhuetal. ( 2008 )and Kewleyetal. ( 2002 ).Thetightrelationbetween24mandextinction-correctedH

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RelationbetweenSFRderivedfrom24mSFRversusHSFRwithcorrectionappliedforextinction.Thetwolinesoverplottedaretsadoptedfrom Zhuetal. ( 2008 )and Kewleyetal. ( 2002 ).TheredstarsareLIRGs,andgreenstarwithsquareoutlineisaULIRG. SFRillustratesthattheobscuredSFRderivedfromsingle-bandMIPSphotometryisareliabletraceroftotalSFRformoderatelystar-forminggalaxiestoLIRGs.Usingthe24mtoobserved(notcorrectedforextinction)HSFRratio,wecanexploretheamountofobscurationinstar-forminggalaxiesasafunctionofgalaxyproperties,suchastotalSFR.Ingeneral,therearetwomethodstoderiveatotalSFRfromopticalandinfrareddata.OneistoapplyanextinctioncorrectiontoHluminositythencalculateSFRfromoneoftheknownrelations(e.g. Kennicutt 1998 ).TheotheristoaddtheobservedorrawHSFRwithobscuredSFRderivedfrominfrareddata. 90

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RatioofobscuredtounobscuredSFRasfunctionoftotalSFR.ThedottedlineshowsthemeanSFRratioof1.9,excludingtheULIRG. Figure 4-3 illustratestheobscuredtounobscuredratioofSFRsasafunctionofthecombinedSFR,wheretheSFRratioprovidesanestimateofthelevelofobscurationineachgalaxy.TheSFRratiosappearuniformlydistributedaroundameanof1.9,andshownosignicanttrendwithtotalSFR,excludingtheULIRG.TheULIRGhasanobscuredSFRthatis50timeslargerthantheSFRinferredfromHalone.OtherstudieshavesimilarlyshownthatSFRsderivedfromHcanseverelyunderestimatethetotalSFRofLIRGsandULIRGs,duetoahighlevelofobscurationfrominternaldust( Geachetal. 2006 ; Koyamaetal. 2010 ). 91

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RatioofobscuredtounobscuredSFRasfunctionofstellarmass.Thedottedlineshowsalinearttothedata,excludingtheULIRG. ThescatterintheSFR24toSFRHratioishigh,rangingfrom0.6to4.Resultsfrom Geachetal. ( 2006 ); Garnetal. ( 2010 )and Koyamaetal. ( 2010 )alsondlargescatterintheobscurationlevelsoftheirgalaxies.However,thesestudiesalsoobserveatrendofincreasingobscurationwithincreasingSFRatrendthatisnotapparentinourdata.Wealsoinvestigatethedependenceofthe24mtoHSFRratioonstellarmassinFigure 4-4 .Stellarmassisobtainedfromthecorrelationsbetweenopticalcolorandstellarmass-to-lightratiosthatarepresentedin Belletal. ( 2003 ).TheserelationsarederivedbyttingarangeofgalaxytemplatestoalargesampleofopticallybrightSDSSgalaxiesandcomparingthebest-ttemplatestoevolvedzeroredshifttemplatesinorder 92

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4-4 showsalinearttothedata(excludingtheULIRG),whichsuggeststhattheremaybeatrendofincreasingobscurationwithincreasingstellarmass.WhilesomestudieshavesuggestedthatHextinctionincreaseswithstellarmass Brinchmannetal. ( 2004 ),othershavefoundthatobscurationofSFRisnotstronglydependentonstellarmass Garnetal. ( 2010 ).Ifthetwoquantitiesareindeedpositivelycorrelated,itcouldbeduetothemass-metallicityrelation,wheremoremassivegalaxiestendtoretainmoredustandarethereforemorehighlyobscured.However,thedatapresentedinFigure 4-4 areonlysuggestiveofamildtrendbetweenobscurationandstellarmass,andisnotdenitive.WenotealsothatthereisaknowncorrelationbetweenstellarmassandtotalSFR,whichmeansthatanytrendbetweenextinctionandstellarmassmayjustbeareectionofatrendbetweenextinctionandtotalSFR. 4-5 hasaclearlydenedredsequencepopulationofgalaxiesthatprimarilyconsistofearly-typequiescentgalaxieswithopticallyredcolors.Theredsequenceisdenotedbyagraysolidline,whoseslopeandzero-pointareadoptedfrom Lopez-Cruzetal. ( 2004 ).Theregionbetweentheredsequenceandthegraydashedline,whichis0.3magnitudesbelowtheredsequence,islooselyreferredtoasthegreenvalley(e.g. Brammeretal. 2009 ; Baloghetal. 2009 ).Thegreenvalleyrepresentsatransitionregionbetweenthedenselypopulatedredsequencethatconsistsofmostlyofpassivegalaxiesandthebluecloudthatconsistprimarilyoflate-typestar-forminggalaxies.InFigure 4-5 ,thelargeredcirclesrepresentclustermemberswith24memission,andbluesquaresrepresentmemberswithHemission.Asexpected,nearlyallofthe 93

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Opticalcolor-magnitudediagramof220clustermemberswithintheMIPSFOV(R<1.7Mpc).Blackcirclesareclustermembers,mostofwhichpopulatetheredsequence,whichisdenotedbyagraysolidline.Theregionbetweentheredsequenceanddashedgraylineisknownasthegreenvalleyandrepresentsatransitionregioninwhichopticallybluestar-forminggalaxiestransformintoopticallyredquiescentgalaxies.Belowthedashedlineisthebluecloud,comprisedmostlyofstar-formingspirals.Redcirclesaregalaxieswith24memission,andbluesquaresaregalaxieswithHemission.ThegreenplussymbolsareAGN,andtheorangediamondsarepost-starburstgalaxies. bluecloudgalaxies(galaxiesbluewardofthedashedline)areMIPSand/orHsources.However,whilealmostallopticallybluegalaxiesareHemitters,mostofthebluecloudgalaxiesfainterthanR20.2arenotdetectedin24mMIPS.Themostlikelyreasonforthisisthatopticallyfaintgalaxiestendtobelessmassivethanbrightgalaxies,andthereforegenerallycontainlessdustandarethusfaintinthe24mband. 94

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4-6 showsthumbnailimagesofalleightgalaxies,withlabelsdenotingB-Rcolor,Rmagnitude,andAGNpresencebasedonX-rayorIRACdata.ThetwoAGNhostgalaxiesandonenon-AGNgalaxyappeartohaveearly-type,ellipticalmorphologies.Oftheremainingvegalaxies,atleastthreehaveobviousdisk-likestructureand/orsomeasymmetry.Itisunclearwhetherthesegalaxieshavespiralstructureorarelenticulargalaxies.TheMIPSsourcesthatlackHemissionmaybesimilartotheopticallypassivespiralgalaxypopulation( Couchetal. 1998 ; Dressleretal. 1999 ; Gotoetal. 2003 ).Opticallypassivespiralgalaxiesshowlittletonosignofstarformationbasedonalackofemissionlinesintheiropticalspectra.Howevermanyofthesespiralsexhibitasubstantialamountofstarformationrateasprobedbyfar-ultraviolet(FUV)orinfraredwavelengths( Moranetal. 2006 ; Wolfetal. 2009 ).Thesestudieshavesuggestedthatpassivespiralsaretransitiongalaxieswhosestarformationhasrecentlyceasedandmorphologicaltransformationhasnotyetoccurred.SinceHtracesstarformationrateonshorttimescalesof10Myr,whileFUVandIRdatatracelongertimescalesof100Myr,thesimultaneouslackofemissionlinesandpresenceof24memission(i.e.dustobscuredstarformation)isconsistentwithascenarioinwhichstarformationhasbeenquenchedwithinthelastfewhundredMyr.Thistimescalematchesthetimeelapsedsincecorepassageofthesubclusterandmaincluster,approximately250Myrpriortoobservation.Ifstarformationwasindeedquenched250Myrago,thenaviablequenchingmechanismwouldberampressurestrippingthatoccurredduringcorepassage.Anotherexplanationforthepresenceof24memissionandlackofHemissionisahighlevelofdustobscuration.OpticallyredMIPSsourcesmaybeextremelydustgalaxiesthatsufferfromheavyextinction,suchthatHemissioniscompletely 95

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R-bandimagesfromtheMagellan6.5mtelescopeofthe8sourcesthataredetectedin24mwhosespectradonothaveHemission.TheB-RcolorandRmagnitudeareindicatedforeachgalaxy.Twooutofthese8areclassiedasAGN. attenuated.Thiswouldalsoexplaintheopticallyredcolors,sincesuchdustygalaxieswillalsosufferfromreddening. 4-7 ,withtheshockfrontandacircleofradiusR=1Mpcoverplotted.Mostofthestar-forminggalaxiesaredistributedbeyondthecentralR<1Mpcregion, 96

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Spatialdistributionof220clustermemberswithintheMIPSFOV.GraycircleshowsR<1Mpcregionfromthecenterofthecluster.BlackxsymbolsshowthelocationofthetwomainclusterBCGsandthesubclusterBCG.ThesubclusterBCGroughlycoincideswiththeleadingedgeoftheshockfront,overplottedinpink.RedcirclesareMIPSsources,bluesquaresareHsources,andgreenplussymbolsareAGN. avoidingtheclustercore,whichisconsistentwithexpectations(e.g. Bragliaetal. 2010 ; Rawleetal. 2010 ).However,thereisaconspicuousoverdensityofMIPSsourcesthatlackHemissioninthecentral1Mpcregion,containing5outofthe8galaxies.ThespatialdistributionoftheMIPSsourcesthatlackHemissionshowninFigure 4-8 isroughlyalignedinthedirectionofthemajormergerandisconsistentwiththeideathatthemajormergerhasrecentlyquenchedstarformationinthesegalaxies.BecausetheHand24memissionprobesstarformationrateondifferenttimescales, 97

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Spatialdistributionof220clustermemberswithintheMIPSFOV.GraycircleshowsR<1Mpcregionfromthecenterofthecluster.BlackxsymbolsshowthelocationofthetwomainclusterBCGsandthesubclusterBCG.ThesubclusterBCGroughlycoincideswiththeleadingedgeoftheshockfront,overplottedinpink.RedcirclesareMIPSsourcesthatdonothaveHemission,orangediamondsarepost-starburstgalaxies,andgreenplussymbolsareAGN.Arrowpointsingeneraldirectionofapreviousmergerthatoccurredwithinthemaincluster. wecanplaceaconstraintonwhenstarformationmayhavebeenquenched.Hissensitivetostarformationrateoveraveryshorttimescaleof20Myr,whereas24mdatatracesstarformationoverafew100Myr.ThereforewecandeducethatthecentralMIPSsourcesshowninFigure 4-8 havehadtheirstarformationquenchedwithin20-200Myrpriortoobservation. 98

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Springel&Farrar ( 2007 ).Thesubclustermovesslowerthantheshockfrontbecausethegasjustinfrontoftheshockfrontowsontothesubcluster,thusslowingitdown.Assumingaconstantvelocityof2700kms1,thesubclusterwouldhavecrossedthefourMIPSsourcesshowninFigure 4-8 approximately250to200Myrago.Wenotethattheseareapproximatetimescales,sincethevelocityofthesubclusteritselfchangeswithtime(i.e.,thesubclusterwasmovingfasterthan2700kms1inthepast,andiscontinuallyslowingdown).ThesetimescaleestimatesareconsistentwiththeabsenceofHemissionandthepresenceofmid-IRheateddustemissiondetectedinthefourcentralMIPSsources,thataredistributedinthepathoftheshockfront.Basedonthespatialdistribution,andthetimescalesforquenchedHstarformationrateandtherecentmerger,itisplausiblethatthecentralMIPSsourceswithoutHemissionaregalaxieswhosestarformationratewasquenchedbythemergermorethan20Myrandlessthan200Myrago.Inthiscase,wecanmakearoughestimateofhowmuchstarformationisquenchedduetothemerger.Thetotal24mSFRofthefourMIPSsourcesinthecoreregionoftheclusteris10Myr1.MostofthisstarformationcomesfromonegalaxythathasameasuredSFRof7.5Myr1.However,theMIPSemissionofthisparticulargalaxyisblendedwithanearbyMIPSsource,andthereforeits24muxandSFRmaybeoverestimatedbyasmuchasafactorof2.ThetotalamountofobscuredSFRfromthefourcentralMIPSsourcesisthen6to10Myr1,withanassociateduncertaintyofafactoroftwo.Thisuncertaintyisbasedonthesystematicuncertaintyinherentinthe Riekeetal. ( 2009 )calibrationthatconverts24muxintoaSFR. 99

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Zhuetal. ( 2008 )relationbetweenobscuredSFRandtotalSFR(whichistheequivalentoftheextinction-correctedHSFR)showninFigure 4-1 ,wecanobtainthetotal(obscuredandunobscured)SFRofeachMIPSgalaxyviainterpolation.ThecombinedSFRofallfourcentrallylocatedMIPSsourcesis8to12Myr1(again,dependingonthetrue24muxoftheblendedsource).Incomparison,thetotalobscuredSFRintheBulletCluster(withinR<1.7Mpc)is270Myr1,whereveLIRGsandoneULIRGcontribute30%and40%ofthetotalobscuredSFR,respectively.Wenotethatourestimateof8to12Myr1representsalowerlimitonthetotalamountofSFRthatmayhavebeenquenchedduringcorepassage,becausethe24memissionthatwecurrentlymeasurewillhavefadedafterthemassivestarformationrateisturnedoff.SinceHSFRissensitivetomassivestarswithM>20M,andmid-IRSFRissensitivetolessstarswithM>3M,theamountof24mfadingafterthemostmassivestarsdiedependsontheslopeoftheIMF.ForaKroupaIMF,theratioofbolometricluminositytoHluminosityisafactoroftwobetween5Myrand100Myr( Kennicuttetal. 2009 ).Byapplyinganapproximatefactoroftwointhefadingof24memission,afterthemassiveO/Bstarshavedied(i.e.,thelifetimeofHSFR),weestimatethatthetotalamountofstarformationratequenchedbytherecentmergeris20Myr1.InadditiontoanalyzingtheMIPSandHstar-forminggalaxies,weexaminethestarformationhistoryoftheBulletClusterbyidentifyingpost-starburstgalaxieswhichhelptoconstraintimescalesforquenchedstarformation.Figure 4-5 showsthesixpost-starburstgalaxiesthatwehaveidentiedamongthe220clustermemberswithintheMIPSFOV.Threeofthesixe+agalaxiesareontheredsequenceandtheremainingthreeareintheintermediateregionbetweentheredsequenceandthebluecloud,knownasthegreenvalley.Thesignicantfractionofpost-starburstgalaxiesinthegreenvalleysigniesthatthesegalaxiesrepresentatransitionphasebetweenactivelystar-formingbluecloudgalaxiesandpassivelyevolvingredsequencegalaxies. 100

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4-5 ,noneareMIPSsources,whichimpliesthatthee+agalaxieshadtheirstarformationquenchedmorethan100-200Myrago.Thespatialdistributionofthepost-starburstgalaxiesshowninFigure 4-8 suggesttwopossiblescenariosforquenchedstarformation.Therstisthatthesegalaxieswerequenchedbytherecentmerger,whichisconsistentwiththefactthatmostofthepost-starburstgalaxiesarelocatedonthepost-shocksideofFigure 4-8 .Mechanismssuchastidalstripping,galaxyharassment,orrampressurestrippingareprocessesthatmayhaveabruptlyquenchedstarformationinthesepost-starburstgalaxiesastheICMofthesubclusterinitiallycollidedwiththeICMofthemaincluster.However,thelackof24memissionfromthepost-starburstgalaxiesnearthemainclustercorewouldsuggestthatthesegalaxieswerequenchedatadifferentepochthantheMIPSsourcesthatlackHemission.Whileitispossiblethatthepost-starburstgalaxieswerequenchedbytherecentmerger,itseemsunlikelythattwodifferentmechanismsworkingondifferenttimescalesareresponsibleforboththepost-starburstgalaxiesandtheMIPSsourceswithoutHemissionnearthecoreofthemaincluster.Another,perhapsmoreplausibleexplanationforthespatialdistributionofpost-starburstgalaxiesisthatstarformationwasquenchedfromapreviousmergerthatoccurredroughlyinthedirectionperpendiculartothepathoftheshockfront.ThearrowinFigure 4-8 indicatestheapproximatedirectionofthepreviousmerger,whichisparalleltothetidalbridgeofstellarmaterialbetweenthetwoBCGsassociatedwiththemaincluster.Fouroutofthesixpost-starburstgalaxiesinFigure 4-8 followthepathofthepreviousmergerremarkablywell,includingonepost-starburstgalaxythatisadjacenttothebrightestBCGinthemaincluster.Ifthepost-starburstgalaxieswerequenchedfrom 101

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Springel&Farrar 2007 ).ThereforetheoveralleffectissuchthatthespatialdistributionofMIPSorpost-starburstgalaxiesrelativetothelocationoftheshockfrontisdominatedbyimpactsoftheshockonthemainclusterpopulation.Also,Figure 4-8 showsthatallthepost-starburstandMIPSgalaxieswithoutHemissionlagbehindthebulletorsubclustercore,whichwouldbehighlyunlikelyifthesegalaxieswereassociatedwiththesubclusterinsteadofthemaincluster. 102

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2 ,thattheintegratedspecicSFRisnotsignicantlydifferentinpost-shockversuspre-shockgalaxies.Wendsixpost-starburstgalaxieswithintheMIPSFOV.Fouroutofthesixarewell-alignedinthegeneraldirectionofapreviousmergerthathasoccurredwithinthemaincluster.Ifthesepost-starburstgalaxieshadtheirstarformationabruptlyquenchedbythepreviousmerger,wecansetaconstraintthatthepreviousmergermusthaveendednomorethan1Gyrago. 104

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Lewisetal. 2002 ; Gomezetal. 2003 ; Baloghetal. 2004 ),thiscorrelationhasbeenconrmedinmanystudies,primarilyusingopticalandUVdatatotracestarformationinmassivegalaxyclustersandeldenvironments.Mid-infrareddatafromtheInfraredSatelliteObservatory(ISO)andtheMulti-bandImagingPhotometerforSpitzer(MIPS)havealsorevealedthepresenceofhighlyobscured,dustystarforminggalaxies,previouslyundetectedbyopticalorUVsurveys(e.g. Cedresetal. 2009 ; Smithetal. 2010b ).WhilethesensitivityofMIPShasenableddetailedstudiesofobscuredstarformationinindividuallocalanddistantgalaxyclusters(e.g. Geachetal. 2006 ; Marcillacetal. 2007 ; Dressleretal. 2009 ; Hainesetal. 2009a ; Chungetal. 2010 ; Finnetal. 2010 ),therearestillonlyasmallnumberoflowredshiftclustersthathavebeensystematicallysurveyedfordustystar-forminggalaxiesouttothevirialradius.Thereremainmanyuncertaintiesintherelationshipbetweenstarformationinclustersandtheirglobalclusterproperties.Inparticular,severalstudieshavetriedtounderstandthecorrelationbetweenclustermassandthemass-normalizedclusterstarformationrate(SFR).Whileresultsfrom Baietal. ( 2009 )suggestthatthereisnostrongcorrelationbetweenclusterspecicSFRandclustermass,otherssuchas Finnetal. ( 2005 ), Poggiantietal. ( 2006 ),and Koyamaetal. ( 2010 )arguethatclusterspecicSFRdecreaseswithclustermass.Thelargespatialcoveragerequiredtoobservedustystar-forminggalaxiesinlowredshiftclustersouttotheclusterinfallregionshasthusfarhinderedourabilitytounderstandhowstarformationisaffectedbyglobalclusterpropertiessuchasclustermass.InthispaperweexploitdatafromtheWide-eldInfraredSurveyExplorer(WISE; 105

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, 2010 )toovercomethisobservationalchallengeandpresentresultsonobscuredstarformationandhowitrelatestoclustermassandradiusoutto3R200inasampleof69clustersatz<0.1.R200andM200arecommonlyusedinterchangeablywithvirialradiusandtotalclustermass,respectively.R200istheradiuswithinwhichtheaveragedensityis200timesthecriticaldensityoftheuniverseandM200isthemassenclosedwithinthatradius. 5.1.1WISEWISEisamedium-classexplorermissionfundedbyNASAandhascompletedobservationsoftheentireskyinfourinfraredbands:3.4,4.6,12,and22m(W1toW4,respectively).WISEscannedtheskywith8.8secondexposures,eachwitha47arcmineldofview,providingatleasteightexposuresperpositionontheeclipticandincreasingdepthtowardstheeclipticpoles.Theindividualframeswerecombinedintocoaddedimageswithapixelscaleof1.375arcsecperpixel.Cosmic-raysandothertransientfeatureswereremovedviaoutlierpixelrejection.Thephotometryusedforouranalysesispointspreadfunction(PSF)ttedmagnitudesfromtherst-passoperationscoaddsourceworkingdatabasecreatedbytheWISEdatareductionpipeline.Galaxiesinourclustersamplehaveadiffractionlimitedresolutionof1200(fullwidthhalfmaximum)inthe22mband.WehaveconrmedfromW4coaddedimagesthatallstar-forminggalaxiesconsideredinouranalysesappearunresolvedinthe22mband,andhavePSFphotometryreduced2valueslessthan1.5.ThereforeweusethePSFmagnitudesfromtherst-passphotometriccatalogtoobtainestimatesoftotalux.Fortheminimumcoverageof8overlappingframes,thesensitivityforS=N=5intheW4bandis6mJy,includinguncertaintyduetosourceconfusion( Wrightetal. 2010 ).ToensureanunbiasedcomparisonofglobalSFRsandtotalIRluminositiesofclustersatdifferentredshifts,weimposealowerlimitofSFR=5Myr1onourentire 106

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Rines&Diaferio 2006 )samplebecauseitprovideshigh-delitymassestimates,isatsufcientlylowredshifttoenabledetectionwithWISEofstronglystar-forminggalaxies,andhasextensivespectroscopyformembershipdetermination.TheCIRSsampleconsistsof72low-redshiftX-raygalaxyclustersidentiedfromtheROSATAll-SkySurveythatarewithinthespectroscopicfootprintofSDSSDataRelease4.TheredshiftrangeoftheCIRSclustersis0.003
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Belletal. ( 2003 ).Withasampleofmorethan10,000opticallybrightgalaxies(13
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5.2.1ClusterMembershipSpectroscopicredshiftsfromSDSSDR7areusedtodeterminemembershipforeachofourclusters.Figure 5-1 illustratesanexampleofusingthecausticsinfallmethodtodetermineclustermembershipforAbell119,themostmassiveclusterinoursample.Itshowsthedifferenceinradialvelocity(cz)ofgalaxiesintheclustereldwithrespecttotheclustersystemicvelocity,asafunctionofprojecteddistancefromtheclustercenter.Galaxiesthataredynamicallyboundtotheclusterformawell-denedregionthatdecreasesindelta-velocityasafunctionofprojectedradius.Wedenethegalaxiesthatarewithintheedgeofthisenvelopeandwithinaprojectedradiusof3R200,asclustermembers.WhiletheaverageturnaroundradiusfortheCIRSsampleis5R200,werestrictourclustergalaxysampletowithin3R200,sinceclusterinfallpatternsaregenerallybetterdenedcloserintotheclustercenter.OnlygalaxieswithSDSSspectroscopicredshiftsareconsideredinthefollowinganalyses.WealsolimitallclusterandeldgalaxiestobebrighterthanMr=20.3,whichcorrespondstothe90%spectroscopiccompletenesslimitofr=17.77atz=0.1. Riekeetal. ( 2009 )thatarecalibratedforMIPS24mdata. Riekeetal. ( 2009 )constructedmodelaveragespectralenergydistribution(SED)templatesfromasampleoflocalLIRGsandULIRGsandderivecorrelationsbetween24mux,SFR,andLIR.Theux-SFRrelationgivenintheirequation14canbeusedforbothMIPS24mandWISE22mdata( Riekeetal. 2009 ).Thesimilarityofthetwobandpassesareconrmedby Gotoetal. ( 2011 )whoquantifythecorrelationbetweenLIRandtheinferredMIPS24mandWISE22mluminosities.TheMIPSandWISEluminositiesareinferredbyapplyingacolor-correction 109

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VelocityoffsetversusprojecteddistancefromtheclustercenterofAbell119,forallSDSSgalaxiesbrighterthanr<17.77withspectroscopicredshifts,withina10Mpcradiusoftheclustercenter.Filledcirclesindicatetheclustermemberswithin3R200.Crosssymbols(blue)highlighteldgalaxiesthatareatleast5000kms1awayfromtheclustersystemicvelocity.Redandgreenstarsymbolsrepresentdemi-LIRGsintheclusterandeldpopulations,respectively.OpencirclesaregalaxiesthatareincludedintheSDSSspectroscopicsamplebutarenotchosenasclustermembersnoreldgalaxies. tothe18mAKARIux,andLIRisderivedfromttingtheIRSEDtemplatesof Chary&Elbaz ( 2001 )toallsixAKARIbandpasses(9,18,65,90,140,and160m)for600galaxiesatz<0.1.TheresultingcorrelationsbetweentheLIRLMIPS24andLIRLWISE22arenearlyidentical,withonlya4%offset.Thereforeweproceedbyusingtheux-SFRandLIRLMIPS24relationsof Riekeetal. ( 2009 ),assumingthecalibrationdeterminedfromMIPS24mdata.ThetotalerrorattributedtotheSFRis0.2dex,whichisdominatedbyscatterintheLMIPS24-LIRrelation,anddoesnotincludeuncertaintiesinherentintheassumed 110

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Riekeetal. ( 2009 )issimilartothe Kroupa ( 2002 )and Chabrier ( 2003 )IMFs,whichhaverelativelyfewerlowmassstarscomparedtoaSalpeterIMF,andismoreapplicableforextragalacticstar-formingregions( Riekeetal. 1993 ; Alonso-Herreroetal. 2001 ). 3 ,evenafewIR-brightAGNcansignicantlybiastheinferredglobalstarformationrateofacluster.ThereareseveralmethodsbywhichwecanidentifyAGNinourdataset.TherstmethodreliesuponWISEcolors.SimilartotheAGNwedgeinSpitzer/IRACdata( Sternetal. 2005 ),AGN-dominatedsourceswillhaveredcolorsinW1-W2([3.4]-[4.6]).Specically,weusethecriteriaW1-W2>0.5(Vega)toidentifycandidateAGNintheWISEdataset.Thiscolorselectionissimilarto,thoughslightlybluerthan,theAGNselectiondeterminedinJarrettetal.2011(submitted)andSternetal.2011(inprep).WhilethissinglecolorcutisingenerallessrobustthanthefullAGNwedge,atthelowredshiftsthatarethefocusofthiswork,star-forminggalaxiesshouldhavecolorsuniformlybluewardofthisthreshold.WeillustrateinFigure 5-2 thatourW1-W2>0.5criterionworkswellinselectingoutAGNfromstar-forminggalaxies.AsecondapproachtoAGNidenticationisuseofopticalspectroscopytoidentifysourcesthatlieintheAGNregionoftheBaldwin-Phillips-Terlevich(BPT)diagram(BPT; Baldwinetal. 1981 ).HereweusetheBPTdiagramasacross-checkontheWISEcolorselectionforthesubsetofsources.Figure 5-2 showstheemissionlineratiosof[OIII]/Hand[SII]/H,forasampleof105clustermemberswithLIR>51010L,ofwhich22haveW1-W2>0.5.Boundariesfrom Kewleyetal. ( 2006 )areshownasdottedlinesandseparateregionswherenarrowemissionlinesarisefromthepresenceofSeyferts,LINERs(lowionizationnarrowemissionlineregions),andHIIregions(star-forming 111

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TheBPTdiagramfor105clustergalaxieswithLIR>51010L.Dottedcurvesindicateboundariesfrom Kewleyetal. ( 2006 )thatseparategalaxieswhoseemissionlinesoriginatefromSeyferts,LINERs,andHIIregions.Solidcircles(red)highlightgalaxiesthatareidentiedasAGNbasedonhavingaredWISEcolor(W1-W2>0.5),andtriangles(purple)representsourcesaggedasquasarsintheSDSScatalog. galaxies).GalaxiesthatareselectedasAGNcandidatesbasedpurelyonhavingaredWISEcolor(W1-W2>0.5)areindicatedaslled(red)circles.Figure 5-2 showsthatoutofseventeenopticallyidentiedSeyferts,eleven(65%)arealsoidentiedasAGNbasedontheWISEW1-W2>0.5selection.Sourcesaggedasquasars(orQSOs)intheSDSScatalogarehighlightedwith(purple)trianglesymbols,withallsevenquasarsindependentlyidentiedasAGNusingtheWISEcolorselection.SincetheBPTdiagnosticismeanttoclassifygalaxiesbasedonlyonnarrowemissionlineratios,itisnotsurprisingthatnearlyalloftheSDSSquasarsarenotproperlydiagnosedintheBPTdiagram.TheHandHemissionlinesinsixout 112

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5-2 thattheWISEcolorselectionisaneffectivemethodofexcludingAGNfromoursampleofbrightW4sources,identifying70%oftheopticallydetectedAGNdowntotheSeyfertlevel. 5-1 113

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Lewisetal. 2002 ; Gomezetal. 2003 ; Baloghetal. 2004 ).Themainstrengthsofthecurrentanalysisaretheuniform22mWISEdata,spectroscopiccompleteness,andexistenceofR200measurementsforthefullsample,whichtogetherprovideuswithaninfrared-basedviewofstarformationforahomogeneoussampleextendingwellbeyondthevirialradius.Acommonapproachintheliteraturehasbeentolookattheradialdependenceofthefractionofstar-forminggalaxies.Hereweinvestigateboththedependenceofthestar-formingfraction,andalsothemSSFRoutto3R200.Forcomparison,theidenticalquantitiesforaeldpopulationarecalculated,withtheeldsamplechosenfromacatalogofgalaxieslocatedwithinaprojectedradiusof5Mpcfromtheclustercenter,attheclusterredshift.Amongthesegalaxies,wechoosethosewitharadialvelocitygreaterthan5000kms1awayfromtheclustersystemicvelocity,redshiftz<0.1,andabsolutermagnitudeMr<20.3.Thesearethesameredshiftandmagnitudelimitsoftheclustergalaxysample.Thehighestvelocitydispersionofourclustersampleis960kms1,whichmeansthatthechoseneldgalaxiesaremorethan5awayfromtheclusterredshift.Figure 5-1 showsdelta-velocity(cz)versusprojecteddistancefromtheclustercenterforgalaxieswithina5MpcprojectedradiusofAbell119.Theeldgalaxiesindicatedwith(blue)crosssymbolsareclearlynotassociatedwiththeclustergalaxies,whichappeardistinctlyconnedtoatrumpet-shapedregion.Bygatheringeldgalaxieswithinaprojected5Mpcradiusfrom69differentregionsofthesky,wehavecompiledalargeenoughsampletoobtainarepresentativeeldvalueofmSSFRandthefractionofstar-forminggalaxieswithLIR>51010L.Figure 5-3 showsthemSSFRasafunctionofr/R200forourensembleof69clusters,includingstarformationonlyforgalaxieswithLIR>51010L(SFR>5Myr1).We 114

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MeanspecicSFR(mSSFR)ofspectroscopicallyconrmedclustermembersasafunctionofprojectedradius,withnumbersindicatinghowmanystarforminggalaxieswithLIR>51010Lareineachbin. emphasizethatbyconstructionourSFRlimitthereforemeansthattheobservedmSSFRisalowerboundbutaconsistentlowerboundacrossthesample.Asexpected,themeanspecicSFRincreaseswithprojectedradius,withthemostcentralbincontainingonlythreegalaxies(outof83star-formingdemi-LIRGsinthe69clusters)withSFR>5Myr1.ThemSSFRincreasesbynearlyanorderofmagnitudebetweenthecenterandR200beforeatteningatlargerradii.SucharadialtrendcanbedrivenbytwofactorsanincreaseintheSSFRofthesub-populationofstar-forminggalaxies,oranincreaseinthefractionofstar-forminggalaxieswithradius.WendthatthereisnostatisticallysignicantchangeintheSSFRofindividualstar-forminggalaxieswithradius,implyingthatthetrendisdrivenprimarilybyaradialgradientinthestarformingfraction. 115

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Ratioofstar-forminggalaxiestoallspectroscopicclustermembersasafunctionofprojectedclusterradius,withthetypicaleldvalueshownasadashedline. ThiscanbeseeninFigure 5-4 ,wherewedirectlyplotthefractionofstar-forminggalaxiesasafunctionofradius.Approximatelyonethirdofthestar-forminggalaxiesresideat2R200
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Lewisetal. ( 2002 )and vonderLindenetal. ( 2010 )similarlyconcludethatclusterstarformationissuppressedatseveraltimesthevirialradiusrelativetotheeld. Gavazzietal. 2001 ; Bekkietal. 2002 ; Owenetal. 2005 ; Haynesetal. 2007 ).Furthermore,ithasbeenobservedthatwhilethetotalbaryonfraction(stellarandgascontent)remainsroughlyconstantwithclustermass,thestellarmassdecreasesandgascontentincreasesingalaxyclustersasafunctionofclustermass( Gonzalezetal. 2007 ; Giodinietal. 2009 ; Andreon 2010 ).Thisimpliesthattheintegratedstarformationefciencyofthehistoryofaclusterisdirectlytiedtoclustermass.Inthissectionweexaminetherelationbetweenclustermassandtotalcurrentclusterstarformationtobetterunderstandhowstronglythepresent-daystarformationratedependsuponclustermass.OurapproachistocomputetheclusterSFRnormalizedbyclustermass(cSSFR;equation1)asafunctionofclustermass.Foreachcluster,theclusterSFRisthesumofSFRsforallmembergalaxiesthathaveSFR>5Myr1andliewithinaprojectedradiusof3R200.Figure 5-5 showsthecSSFRforthefullsampleof69clusters.ThedottedcurveshowsthelimitingdetectablecSSFRasafunctionofM200correspondingtoasingledemi-LIRGwithaSFRof5Myr1inacluster.ClustersthathavenomembersabovetheSFR>5Myr1limitareassignedacSSFRofzero.Thebinneddata(largesquares)indicatetheaveragecSSFRineachM200bin,includingclusterswithacSSFRofzero. 117

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ClusterspecicSFRversusclustermassforfullsampleof69clusters,withtheaveragevaluepermassbinoverplottedwithlargegreensquaresymbols.TheaverageclusterspecicSFRcalculatedusingonlyclustersathigheclipticlatitude(andthereforegreatercoveragewithWISE)areshownaspurplediamonds.Thesubsampleofclustersathigheclipticlatitudeshowsasimilartrendrelativetothefullsampleofclusters,indicatingthatthegapspresentintheWISEcoadddataatloweclipticlatitudesdonothaveasignicantimpactonourresult.ThedottedlineshowstheclusterspecicSFRwhenSFR=5Myr1correspondingtothe6mJyW4-banddetectionthresholdatz=0.1. ThusthecSSFRofthelowestM200binisstronglybiasedbyincompleteness,sincenearlyalloftheclustersatlowmassfallbelowtheSFR>5Myr1limitandhaveaclusterSFRsettozero.Toensurethatourresultsarenotsignicantlybiasedbythisincompletenessatlowclustermass,wealsoshowtheclusterspecicSFRversusM200forasub-sampleofclustersatz<0.06intherightpanelofFigure 5-5 .Forthese30low-zclusters,wearesensitivetostar-forminggalaxiesdowntoSFR>1.5Myr1, 118

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Figure5-6. ClusterspecicSFRversusclustermassforalowredshiftsubsampleof30clustersatz<0.06.ThedottedlineshowsadetectionlimitofSFR=1.5Myr1,correspondingtothe6mJyW4-banddetectionthresholdatz=0.06.ThereisnoapparenttrendbetweenclusterspecicSFRandM200evenwithaloweredSFRthreshold. Usingthefullsampleof69clusters,Figure 5-5 showsnosignicantcorrelationbetweenclusterspecicSFRandclustermass,overmorethanoneorderofmagnitudeinM200.TherstpasscoaddprocessingusedherehasthesamecalibrationasintheWISEPreliminaryDatarelease,butwasruneveryotherdayusingonlyonedayofdata,creatinggapsincoverageatloweclipticlatitude.Thesegapsshouldnotsignicantlyimpactthisresult,sincelocationofthegapsarerandomwithrespecttotheclustercenters.WeconrmthisexpectationbyexaminingtheclusterspecicSFRs 119

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5-5 insmallpurplediamonds.ThesehigheclipticlatitudeclustersshownosignicantcorrelationbetweenclusterspecicSFRandclustermass,similartothelackofcorrelationseenwiththefullclustersample.ThelackofcorrelationbetweencSSFRandM200isalsoconrmedwiththez<0.06sub-sample,forwhichwehaveappliedasignicantlylowerdetectionlimitofSFR>1.5Myr1(Figure 5-6 ).ThebinneddatainFigure 5-5 andFigure 5-6 showclusterspecicSFRsthatareconsistentwithin1ofeachother,acrossthefullrangeofM200.TheerrorbarsassociatedwiththeaveragecSSFRarethePoissonuncertaintyonthenumberofstar-forminggalaxiesdetectedabovetheSFRlimitineachM200bin.ThelackofasignicanttrendbetweencSSFRandM200indicatesthattransformationmechanismswhichscalestronglywithclustermassmaynotplayadominantroleintheevolutionofstarformationinclusters.Twoclusterprocessesthatstronglyscalewithclustermassaregalaxyharassmentandrampressure.Rampressureisamechanismthatactstocompressand/orstripcoldgasreservoirsingalaxies.Theimpactoframpressureisproportionaltov2,whereisthedensityoftheintraclustermedium(ICM)andvisthevelocityofagalaxywithrespecttotheICM( Gunn&Gott 1972 ).Galaxyharassmentreferstothecumulativeeffectofhighvelocitycloseencountersbetweengalaxiesandhasthepotentialtodisturbmorphologiesandquenchstarformation( Mooreetal. 1996 ).Theeffectsofgalaxyharassment,likerampressure,areexpectedtoscaleupwithclustermass,sincebothclustervelocitydispersionandICMgasdensityincreasewithclustermass.Whiletherehasbeenevidenceinsupportofbothrampressureandharassmenthavinganimpactonstarformationinindividualclustergalaxies(e.g. Roediger&Hensler 2005 ; Vollmeretal. 2008 ; Haynesetal. 2007 ),thelackofacorrelation 120

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5-5 indicatesthatneitherofthesemechanismssignicantlytrigger/quenchstarformationinlocalgalaxyclusters,withinthemassrangeof1014to71014M.Ourresultsareconsistentwiththoseof Goto ( 2005 )and Popessoetal. ( 2007 )whondthattheH-derivedclusterSFRnormalizedbyclustermassisnothighlycorrelatedwithclustermassfor100localgalaxyclusters.Similarly, Balogh&McGee ( 2010 )demonstratethatthepassivegalaxyfractionisroughlyconstantasafunctionofsystemmass,fromM1013Mto1015M.Assumingthatthenon-passivegalaxyfractionisdominatedbystar-forminggalaxiesratherthanAGN,theresultsof Balogh&McGee ( 2010 )supportourndingthatclusterspecicSFRisnotstronglydependentonclustermass.Incontrast,severalotherstudieshavefoundevidenceforananti-correlationbetweenclusterspecicSFRandclustermass. Finnetal. ( 2005 )and Koyamaetal. ( 2010 )obtainclusterSFRsbyintegratingHderivedSFRsformembersoutto0.5R200,andclustermassesfromobservedvelocitydispersionsforasampleofeightandnineclusters,respectively.Thetrendobservedintheirdataissignicantlyweakenedthough,whenclustersofthesameepochareconsidered,indicatingthatevolutionaryeffectsmaybethedominatingfactor.Inadditiontointeractionsbetweengalaxiesandtheclusterenvironment,studieshavedemonstratedthattidalinteractionsbetweengalaxiesorgalaxy-galaxymergerscantriggeraburstofstarformation(e.g. Owenetal. 2005 ; Martig&Bournaud 2008 ).Galaxy-galaxyinteractionsaregenerallyoptimizedinintermediatedensityregionswithlowvelocitydispersions,suchasingalaxygroups,whereasthemoremassiveclustersarelesslikelytohostgalaxymergers.Figure 5-5 includesasubstantialnumberoflowmasssystems(57clusterswithM200<31014M),yetshowsnosignofsignicantlyelevatedclusterspecicSFRinlowmassclustersrelativetointermediatemassclusters.Anysignatureofenhancedstar 121

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5-5 ),combinedwiththesuppresseddemi-LIRGfractionat3R200relativetotheeld(Figure 5-4 ,wouldsuggestascenarioinwhichstarformationisquenchedinasignicantpopulationofgalaxieswithinsmallgroupsorlaments.Thenoncetheremainingstar-forminggalaxiesareaccretedintotheclusterenvironment,amechanismthatisnotstronglydependentonclustermassmustoperate.Wesuggeststrangulationasaplausiblecandidateforquenchingstarformationintheclusterstar-forminggalaxies.AsagalaxyenterstheclusterICM,itshothalogasisremoved,therebycuttingoffitsresourceforfuturecoldgassupplies,sincethehalogaswouldotherwiseeventuallycoolandsettleontothedisk( Baloghetal. 2000 ; Bekkietal. 2002 ).Theeffectivenessofstrangulationislessdependentonclustermassthanrampressureorharassment.Simulationsfrom Kawata&Mulchaey ( 2008 )haveshownthatstrangulationcanoccurinlowmassgroups,wheretheICM-galaxyhalointeractionisweakrelativetoclusters.Strangulationhasbeensuggestedbyseveralclusterstudiesasanimportantclustermechanism(e.g. Moranetal. 2006 2007 ; Chungetal. 2010 ),includingby Treuetal. ( 2003 )whodiscoveredmildgradientsinthemorphologicalfractionsofgalaxiesinCl0024+16outtolargeclusterradii,whichwerebestexplainedbyaslow-workinggentlemechanismsuchasstrangulation.Whilestrangulationmayquenchstarformationinmassiveclusters,ourresultssuggestthatthedominantprocessforregulatingstarformationindenseenvironments 122

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Riekeetal. ( 2009 ).Outof69clusters,wendatotalof83star-formingdemi-LIRGswithSFR>5Myr1within3R200.Boththefractionofdemi-LIRGsandthemeanspecicSFRofclustergalaxiesincreaseswithprojecteddistancefromtheclustercenter.However,thefractionofdemi-LIRGsremainsbelowtheeldvalueevenat3timestheclustervirialradius.Oneplausibleexplanationforthesuppresseddemi-LIRGfractionat3R200isthatevengalaxiesthatresidesignicantlybeyondthevirialradiushavealreadybeenquenchedintheirpreviousenvironments,suchassmallgroupsorlaments.WealsoinvestigatetheimpactofclustermassonstarformationbypresentingthetotalclusterSFRnormalizedbyM200asafunctionofM200.WendnoevidenceofacorrelationbetweentheclusterspecicSFRandclustermassinthisrstuniformdatasettodetectobscuredstarformationouttoseveraltimesthevirialradiusinalargesampleoflowredshiftclusters.Ourresultindicatesthatclustermechanismswhichscalewithclustermass,suchasrampressureorharassment,arenotlikelytoplayadominantroleintheevolutionofstarformationinlocalclusters. 123

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6.1.1Chapter2MyrststepininvestigatingtheeffectsofamajorclustermergeronstarformationintheBulletClusterwastofocusonthecentralregionoftheclusterneartheMach3shockfrontanextremelyhighrampressureenvironment.UsingdeepSpitzer/IRACdata,Icomparedtherelativestarformationrates(SFR)perstellarmassofthepre-shockversuspost-shockgalaxies,andfoundthatthespecicSFRsofthetwogalaxypopulationsareconsistentwithin2.ThemainresultfromthisworkisthatthestrongrampressureinducedbythesupersonicshockfrontdoesnothaveadramaticimpactontherecentstarformationratesoftheBulletClustergalaxies.Thisarguesagainstrampressurebeingadominantdriverofgalaxyevolutionshortlyafteramajorclustermerger( Chungetal. 2009 ),thoughwedonotprecludethepossibilitythatitmayhavesomeimpact(whichwereturntoinChapter 4 ). 128

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Chungetal. 2010 ). 129

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5 ,Iexploretheimpactofclustermassonstarformationforasampleof69localgalaxyclusters,using22mWISEdatatodetermineobscuredstarformationrates.ClustermembershipisdenedwithspectroscopicredshiftsfromtheSDSS.Indthatthereisnostrongdependenceofthemass-normalizedclusterSFRonthetotalclustermass.Thisindicatesthatclustermechanismswhoseefciencyisstronglydependentonclustermass,suchasrampressure,arenotlikelytobestrongdriversofgalaxyevolutioninclusters.Ialsoexaminetheradialdependenceofstarformationinthe69localclusters,outto3timesthevirialradius(R200).Thefractionofstar-forminggalaxiesincreaseswithprojecteddistancefromtheclustercenter,thoughremainsbelowtheeldvalueevenat3R200.Thissuggeststhatstarformationhasalreadybeenquenchedevenintheveryoutskirtsofclusters,perhapsviainteractionswithinsmallinfallinggroupsorlaments.ThisworkhasbeensubmittedforpublicationtotheAstrophysicalJournalinAprilof2011. 130

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SunMiChungwasborninSeoul,SouthKorea,butgrewupformostofherlifeinQueens,NewYork.SheattendedtheBronxHighSchoolofSciencewherehermainextra-curricularinterestsincludedplayingultimatefrisbeeandeditingthestudentyearbook.SherstbecameinterestedinastronomyafterwatchingthemovieContact,starringJodieFoster.ThisinterestgrewmuchstrongerduringherfreshmanyearatWesleyanUniversityinMiddletown,CT,whereshetookherrstastronomycoursewithProfessorJohnSalzer,whowouldlaterbecomeherundergraduatethesisadvisor.Duringherthirdsummerasanundergraduate,herenthusiasmforbothastronomyandthemovieContactculminatedinaninternshipattheAreciboObservatory,whereshewasintroducedtotherealworldofradioastronomy.Aftergraduatingwithhonorswithadouble-majorinastronomyandphysics,shewasemployedattheHarvard-SmithsonianCenterforAstrophysicsforthreeyears,wheresheworkedonthecalibrationofinstrumentsonboardtheChandraX-rayObservatoryandtheX-raycoronaeofyoungstarswithDr.JeremyDrake.ShethenmovedtoGainesville,FLtobegingraduateschoolattheUniversityofFloridaandsoonafterstartedherresearchonstarformationingalaxyclusterswithProfessorAnthonyGonzalez.AfterreceivingherPh.D.fromtheUniversityofFloridaintheSummerof2011,shewillmoveontoOhioStateUniversitytostudytheevolutionofAGNwithProfessorChrisKochanekandcontinueherworkongalaxyevolutioninclusters. 153