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Internal Kinematics and Stellar Populations of Dwarf Early-Type Galaxies in the Coma Cluster

Permanent Link: http://ufdc.ufl.edu/UFE0019762/00001

Material Information

Title: Internal Kinematics and Stellar Populations of Dwarf Early-Type Galaxies in the Coma Cluster
Physical Description: 1 online resource (125 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: age, alpha, coma, dispersion, dwarf, elliptical, galaxies, index, kinematics, line, metallicity, velocity
Astronomy -- Dissertations, Academic -- UF
Genre: Astronomy thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: In this dissertation, we characterize the dwarf early-type galaxies (dE/dS0s) in the Coma cluster based on their internal kinematics (velocity dispersion or sigma) and properties of their underlying stellar populations. We derive scaling relations of dEs including the luminosity--velocity dispersion (the Faber-Jackson relation), color--sigma, line strength index--sigma, age-metallicity, and relations between age, metallicity and alpha/Fe with velocity dispersion. We also assess how these relations are affected by cluster environment. Our galaxy sample consists of 74 early-type galaxies in the 30 arcmin diameter region in the center of the Coma cluster and 48 early-type galaxies in a 1 degree region centered around NGC 4839, a region believed to be infalling into the cluster. We measure velocity dispersions and line strength indices for these galaxies. To distinguish between luminous ellipticals and their less luminous counterparts, we refer to them as high- and low-sigma galaxies respectively, where sigma > = 100 km/s represents the luminous galaxies and galaxies with 30 < sigma < 100 km/s the dE/dS0s. We determine that the L--sigma relation for the dE/dS0 galaxies is different from the canonical Faber-Jackson relation, L proportional to sigma^4, followed by high-sigma galaxies. We find that low-sigma galaxies, including dE/dS0s, in the Coma cluster follow a relation where L is proportional to sigma^{2.01 +- 0.36}. We discuss possible causes for this change of slope between luminous ellipticals and dE/dS0s and show that this change cannot be explained by rotation. Galaxies in the infalling region of the cluster follow the same L--sigma relation. The line strength index--sigma relations, I--sigma, for dE/dS0 galaxies in the Coma cluster exhibit three different relations depending on the index. We find that one set of metallic indices show tight linear relations with sigma (C4668, Mg1, Mg2, Mgb, and MgFe'). Another group can be described by linear fits which have a large scatter and shallower slopes than the first group of indices (Ca4227, Fe5015, Fe5335 and < Fe > ). We find no linear relations in the third group of indices (G4300, Fe4383, Ca4455, Fe4531 and Fe5270). We were unable to determine the cause of these different trends, although we note that the micro-turbulent velocity of stellar atmospheres may in fact play a key role here. Through the use of stellar population models and line strengths we derive ages, meatllicities and alpha/Fe ratios of the Coma cluster dEs/dS0s. We find trends of younger ages, lower metallicities and solar of subsolar alpha/Fe ratios for low-sigma galaxies located in the core of the cluster. On the other hand, dE/dS0s in the low-density region of Coma form a less homogeneous population. We find an unusually high fraction of dE/dS0s with high alpha/Fe ratios. This suggests short time scales for the star formation histories for these galaxies. Further, the dE/dS0s with high alpha-ratios have a range of ages and metallicities implying multiple formation scenarios where some galaxies have experienced their short star formation bursts at more recent epochs. Finally, we investigate the age-metallicity relation of dE/dS0 galaxies in the Coma cluster. We find that high-sigma galaxies in the center of the cluster show an anti-correlation. However, this relation is purely a consequence of correlated errors. Dwarf early-type galaxies show a similar anti-correlation which is due to correlated errors, but is offset towards younger ages and lower metallicities. We argue that the effect of younger ages and metallicities for low-sigma galaxies in comparison to more massive ellipticals is real since it is in the direction opposite of the correlated errors in age and metallicity.
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.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Guzman, Rafael L.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2009-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0019762:00001

Permanent Link: http://ufdc.ufl.edu/UFE0019762/00001

Material Information

Title: Internal Kinematics and Stellar Populations of Dwarf Early-Type Galaxies in the Coma Cluster
Physical Description: 1 online resource (125 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: age, alpha, coma, dispersion, dwarf, elliptical, galaxies, index, kinematics, line, metallicity, velocity
Astronomy -- Dissertations, Academic -- UF
Genre: Astronomy thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: In this dissertation, we characterize the dwarf early-type galaxies (dE/dS0s) in the Coma cluster based on their internal kinematics (velocity dispersion or sigma) and properties of their underlying stellar populations. We derive scaling relations of dEs including the luminosity--velocity dispersion (the Faber-Jackson relation), color--sigma, line strength index--sigma, age-metallicity, and relations between age, metallicity and alpha/Fe with velocity dispersion. We also assess how these relations are affected by cluster environment. Our galaxy sample consists of 74 early-type galaxies in the 30 arcmin diameter region in the center of the Coma cluster and 48 early-type galaxies in a 1 degree region centered around NGC 4839, a region believed to be infalling into the cluster. We measure velocity dispersions and line strength indices for these galaxies. To distinguish between luminous ellipticals and their less luminous counterparts, we refer to them as high- and low-sigma galaxies respectively, where sigma > = 100 km/s represents the luminous galaxies and galaxies with 30 < sigma < 100 km/s the dE/dS0s. We determine that the L--sigma relation for the dE/dS0 galaxies is different from the canonical Faber-Jackson relation, L proportional to sigma^4, followed by high-sigma galaxies. We find that low-sigma galaxies, including dE/dS0s, in the Coma cluster follow a relation where L is proportional to sigma^{2.01 +- 0.36}. We discuss possible causes for this change of slope between luminous ellipticals and dE/dS0s and show that this change cannot be explained by rotation. Galaxies in the infalling region of the cluster follow the same L--sigma relation. The line strength index--sigma relations, I--sigma, for dE/dS0 galaxies in the Coma cluster exhibit three different relations depending on the index. We find that one set of metallic indices show tight linear relations with sigma (C4668, Mg1, Mg2, Mgb, and MgFe'). Another group can be described by linear fits which have a large scatter and shallower slopes than the first group of indices (Ca4227, Fe5015, Fe5335 and < Fe > ). We find no linear relations in the third group of indices (G4300, Fe4383, Ca4455, Fe4531 and Fe5270). We were unable to determine the cause of these different trends, although we note that the micro-turbulent velocity of stellar atmospheres may in fact play a key role here. Through the use of stellar population models and line strengths we derive ages, meatllicities and alpha/Fe ratios of the Coma cluster dEs/dS0s. We find trends of younger ages, lower metallicities and solar of subsolar alpha/Fe ratios for low-sigma galaxies located in the core of the cluster. On the other hand, dE/dS0s in the low-density region of Coma form a less homogeneous population. We find an unusually high fraction of dE/dS0s with high alpha/Fe ratios. This suggests short time scales for the star formation histories for these galaxies. Further, the dE/dS0s with high alpha-ratios have a range of ages and metallicities implying multiple formation scenarios where some galaxies have experienced their short star formation bursts at more recent epochs. Finally, we investigate the age-metallicity relation of dE/dS0 galaxies in the Coma cluster. We find that high-sigma galaxies in the center of the cluster show an anti-correlation. However, this relation is purely a consequence of correlated errors. Dwarf early-type galaxies show a similar anti-correlation which is due to correlated errors, but is offset towards younger ages and lower metallicities. We argue that the effect of younger ages and metallicities for low-sigma galaxies in comparison to more massive ellipticals is real since it is in the direction opposite of the correlated errors in age and metallicity.
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.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Guzman, Rafael L.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2009-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0019762:00001


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IwouldliketothankmyadviserRafaelGuzmanforhissupportbothpersonallyandscientically.ItrulyappreciatethatRafaelhasgivenmeagreatprojecttoworkon,encouragedmetoattendconferencesandpresentmywork,tocollaboratewithothers,encouragedmetowriteandnishmypapers,andalwaysansweredmyquestionsaboutscience.RafaelwasalwayswillingtomeetwithmeanddiscussanyproblemsthatIwouldrunintoduringmyresearch.ItrulyappreciatehisenthusiasmwhichwouldalwaysmotivatemetokeepworkingevenonresearchaspectsthatIdidnotenjoymuch.IhopetotakeawaysomeofRafael'sabilityto,onthespot,immersehimselfintoascienticproblemandcomeupwithcreativewaystosolveit.IoweagreatdebtofgratitudetoDr.PatriciaSanchez-BlazquezforallthescienticandpersonalhelpIhavereceivedovertheyearsfromher.Shehasbeenmorethanavaluableresourceforscienticdiscussionsandtechniquesinvolved.Ithasbeenatruepleasurelearningmanyaspectsofastronomyfromandwithher.IalsoacknowledgethesupportIhavereceivedfrommycollaboratorsinMadrid:NicolasCardielandJavierGorgas.IthankAlisterGrahamforhisscienticsupportduringmyearlyyearsingraduateschool.Iamgratefulforhisguidancethroughthescienceinvolvedinmyrstpublishedpaper.Iamalsogratefulforlearningmanyaspectsofastronomyfromhim.Igivespecialthanksallmygraduatestudentcolleaguesandpost-docshereattheAstronomydepartment,whethertheyhavealreadyleftorarestillhereattheUniversityofFlorida.Yourcompanyhasmademygrad-schoolexperiencemuchmoreenjoyable.Ithankthiswonderfulgroupofpeopleforalltheirhelpwithwork,fortheirsupport,thefunwehad,andtheirfriendship.IalsothankCatherineCassidyformakingmydealingswiththeuniversitybuerocracymucheasier.Finishingmydoctoralstudieswouldnothavebeenpossiblewithoutthesupportofmyfamilyandfriends.Ithankmyparents,familymembersandmyfriendswhohavealwaystriedtheirbesttosupportmeineverythingthatIwoulddo.Iamdeeplygrateful 4

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page ACKNOWLEDGMENTS ................................. 4 LISTOFTABLES ..................................... 8 LISTOFFIGURES .................................... 9 ABSTRACT ........................................ 11 CHAPTER 1INTRODUCTION .................................. 13 1.1IntroductiontoDwarfEllipticalGalaxies ................... 13 1.2FormationScenariosforDwarfEllipticalGalaxies .............. 15 1.3MethodsofDeterminingtheStarFormationHistoriesofEarly-TypeGalaxies 18 1.4GoalsandMotivationforthisDissertation .................. 20 2OBSERVATIONSANDDATA ........................... 23 2.1SampleSelection ................................ 23 2.2SpectroscopicObservations ........................... 25 2.3DataReduction ................................. 28 2.4RadialVelocityandVelocityDispersionMeasurements ........... 29 2.4.1TheFourierQuotientMethod ..................... 29 2.4.2ChoiceofParametersandFinalMeasurements ............ 31 2.4.3ErrorMeasurementsforInternalKinematics ............. 33 2.4.4DeterminingTheSignal-to-NoiseRatio ................ 40 2.4.5QuantifyingTheEectsofNoiseonKinematics ........... 41 2.4.6ComparisonofMeasurementswithOtherAuthors ......... 41 2.5Absorption-LineStrengthsMeasurements ................... 44 2.5.1MethodofMeasuringtheIndices ................... 44 2.5.2ErrorMeasurementsforLineStrengthIndices ............ 53 2.5.3ComparisonofIndexMeasurementswithLiterature ......... 54 3INTERNALKINEMATICSOFCOMADWARFEARLY-TYPEGALAXIES 59 3.1TheL{Relation ................................ 59 3.1.1DerivingtheL{Relation ....................... 59 3.1.2DiscussionofL{Relation ....................... 61 3.2Color{Relation ................................ 66 3.2.1DerivingtheColor{Relation ..................... 66 3.2.2DiscussionofColor{Relation ..................... 68 3.3EnvironmentalEectsonKinematicsofdE/dS0Galaxies .......... 72 3.3.1EnvironmentalEectsontheL{Relation .............. 72 3.3.2EnvironmentalEectsontheColor{Relation ............ 73 6

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...... 77 4.1Index{VelocityDispersionRelations ...................... 77 4.1.1ResultsofI{Relations ........................ 77 4.1.2DiscussionofIndex{relations ..................... 82 4.1.2.1RelationBetweenHand 83 4.1.2.2RelationbetweenMg2and 84 4.1.2.3Index-trendsingeneral ................... 84 4.2Ages,MetallicitiesandElementAbundancesofEarly-TypeGalaxies .... 86 4.2.1O-GridGalaxies ............................ 90 4.2.2MethodofDerivingSPMParameters ................. 93 4.3ModelParametersvs. 95 4.3.1Age{Relation ............................. 97 4.3.2Metallicity{Relation ......................... 98 4.3.3Relationbetween[=Fe]and 98 4.4TrendsBetweentheAge,Metallicityand{Ratio .............. 99 4.5EnvironmentalEectsonStellarPopulations ................. 102 4.5.1Index{RelationsforOutskirtsGalaxies ............... 102 4.5.2StellarPopulationParametersvs.Relations ............ 104 4.5.3Age{MetallicityRelationforOutskirtsofComa ........... 110 5CONCLUSIONS ................................... 113 5.1InternalKinematics ............................... 113 5.2UnderlyingStellarPopulationsofdE/dS0Galaxies ............. 114 5.3FutureWork ................................... 116 5.4Credits ...................................... 117 REFERENCES ....................................... 118 BIOGRAPHICALSKETCH ................................ 125 7

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Table page 2-1Centralsamplemeasurements ........................... 37 2-2Outerregionmeasurements ............................ 39 2-3Linestrengthindexdenitions ............................ 45 2-4Cenralregionindexmeasurements,part1 ..................... 47 2-5Centralregionindexmeasurements,part2 ..................... 49 2-6Outerregionindexmeasurements,part1 ...................... 51 2-7Outerregionindexmeasurements,part2 ...................... 53 2-8Linestrengthcomparisonwithotherstudies .................... 58 3-1Luminosity{least-squarests ........................... 61 3-2Color{leastsquarests ............................... 67 3-3ComparisonofcentalandouterregionL{relations ............... 73 3-4Color{andcolor-magnituderelations ....................... 75 4-1GroupIstatisticsforI{relations ......................... 78 4-2GroupIIlineartsandstatisticsforI{relations ................. 80 4-3GroupIIandBalmerlineslineartsandstatisticsforI{relations ....... 80 4-4Binsinandmodelparametersforcentralgalaxies ................ 87 4-5Modelparametermeasurements ........................... 93 4-6Modelparametersvs.relations .......................... 95 4-7Relationsbetweenageandmetallicity ........................ 100 4-8Outersamplemodelparametersvs.relations .................. 106 4-9Outergalaxybinsinandmodelparameters ................... 106 4-10Outerregionrelationsbetweenageandmetallicity ................ 110 8

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Figure page 2-1Color{magnitudediagramoftheComacluster ................... 24 2-2Spatialdistributionofobservedgalaxies ...................... 25 2-3SamplespectraofdEgalaxies ............................ 27 2-4Clustermembership ................................. 28 2-5Fitbetweenagalaxyspectrumandoptimaltemplate ............... 30 2-6DerivingkinematicparameterswithMOVEL ................... 32 2-7SpectralregiontestsforthecentralComagalaxies ................. 33 2-8Spectralregiontestsfortheoutskirtsgalaxies ................... 34 2-9TaperingfractiontestsforcentralComagalaxies ................. 35 2-10Taperingfractiontestsforoutskirtsregiongalaxies ................ 36 2-11ModellingtheeectsofS/Nratioonmeasurements ............... 42 2-12Externalcheckofmeasurements .......................... 43 2-13Comparisonwith Smithetal. ( 2008a ) ........................ 55 2-14Comparisonwith Poggiantietal. ( 2001a ) ...................... 56 2-15ComparisonwithNFPS ............................... 57 3-1Luminosity{relation ................................ 60 3-2RotationaleectsonMR{ 65 3-3Color67 3-4ModellingthestarformationepochofdEs ..................... 70 3-5Luminosity{plotswithoutskirts .......................... 74 3-6EnvironmentaleectsonC{relation ....................... 75 3-7Histogramofg-rcolor ................................ 76 4-1Index{logplots .................................. 79 4-2Plotsof[MgFe]0vs.HandMgbvs.hFei 89 4-3PlotofHfortheo-gridgalaxiescombined .................... 91 9

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............................. 92 4-5Velocitydispersionvs.age,metallicityand[=Fe] ................. 96 4-6Relationsbetweenthemodelparameters(age,metallicityand[=Fe]) ...... 100 4-7Index{logplotsincludingoutskirtsgalaxies ................... 103 4-8Environmentaleectsonvs.age,metallicityand[=Fe] ............ 105 4-9Comparisonofmodelmeasurementswithadierentmethod ........... 107 4-10Consistencycheckswiththe2method ....................... 108 4-11Spatialdistributionofgalaxiesafteranalysis .................... 111 4-12Relationsbetweenthemodelparameters(age,metallicityand[=Fe]) ...... 112 10

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Inthisdissertation,wecharacterizethedwarfearly-typegalaxies(dE/dS0s)intheComaclusterbasedontheirinternalkinematics(velocitydispersion,)andpropertiesoftheirunderlyingstellarpopulations.WederivescalingrelationsofdEsincludingtheluminosity{velocitydispersion(L{,theFaber-Jacksonrelation),color{,linestrengthindex{,age-metallicity,andrelationsbetweenage,metallicityand[=Fe]withvelocitydispersion.Wealsoassesshowtheserelationsareaectedbyclusterenvironment. Ourgalaxysampleconsistsof74early-typegalaxiesinthe300diameterregioninthecenteroftheComaclusterand48early-typegalaxiesina1regioncenteredaroundNGC4839,aregionbelievedtobeinfallingintothecluster.Wemeasurevelocitydispersionsandlinestrengthindicesforthesegalaxies.Todistinguishbetweenluminousellipticalsandtheirlessluminouscounterparts,werefertothemashigh-andlow-galaxiesrespectively,where>100kms1representstheluminousgalaxiesandgalaxieswith30<<100kms1thedE/dS0s. WedeterminethattheL{relationforthedE/dS0galaxiesisdierentfromthecanonicalFaber-Jacksonrelation,L/4,followedbyhigh-galaxies.Wendthatlow-galaxies,includingdE/dS0s,intheComaclusterfollowarelationwhereL/2:010:36.WediscusspossiblecausesforthischangeofslopebetweenluminousellipticalsanddE/dS0sandshowthatthischangecannotbeexplainedbyrotation.GalaxiesintheinfallingregionoftheclusterfollowthesameL{relation. 11

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Throughtheuseofstellarpopulationmodelsandlinestrengthswederiveages,meatllicitiesand[=Fe]ratiosoftheComaclusterdEs/dS0s.Wendtrendsofyoungerages,lowermetallicitiesandsolarofsubsolar[=Fe]ratiosforlow-galaxieslocatedinthecoreofthecluster.Ontheotherhand,dE/dS0sinthelow-densityregionofComaformalesshomogeneouspopulation.WendanunusuallyhighfractionofdE/dS0swithhigh[=Fe]ratios.Thissuggestsshorttimescalesforthestarformationhistoriesforthesegalaxies.Further,thedE/dS0swithhigh-ratioshavearangeofagesandmetallicitiesimplyingmultipleformationscenarioswheresomegalaxieshaveexperiencedtheirshortstarformationburstsatmorerecentepochs. Finally,weinvestigatetheage-metallicityrelationofdE/dS0galaxiesintheComacluster.Wendthathigh-galaxiesinthecenteroftheclustershowananti-correlation.However,thisrelationispurelyaconsequenceofcorrelatederrors.dE/dS0galaxiesshowasimilaranti-correlationwhichisduetocorrelatederrors,butisosettowardsyoungeragesandlowermetallicities.Wearguethattheeectofyoungeragesandmetallicitiesforlow-galaxiesincomparisontomoremassiveellipticalsisrealsinceitisinthedirectionoppositeofthecorrelatederrorsinageandmetallicity. 12

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Sandage&Binggeli 1984 )thathavenoactivestarformationandlittleornocoolgasanddust.TherstdwarfellipticalgalaxiesdiscoveredwereFornaxandSculptor,companionstotheMilkyWaygalaxy.Theywerediscoveredby Shapley ( 1938a ).Soonafter,moredEswereidentiedintheLocalGroupofgalaxiesfollowedbyincreasingevidenceoftheirabundanceinthenearbyclusters( Shapley 1938b ; Reaves 1956 ; Hodge 1959 ).Today,itiswidelyacceptedthatdEsarethemostnumerousgalaxiesinclusters,withtheirnumbersexceedingthoseofthehigh-luminositygalaxiesbyafactorofsix( Mateo 1998 ). Dwarfellipticalgalaxiesarediculttoobserveduetotheirlowsurfacebrightness,e;V>22magarcsec2( Ferguson&Binggeli 1994 ).Despitethisdiculty,considerableinterestintheseobjectshasariseninthepasttwodecades.Inparticular,anumberofextensivephotometricstudieshaveconcentratedoncomparingdEstobrightellipticalgalaxieswithMB20:50mag( Graham&Guzman 2003 )asitisbelievedthatthetwoarestructurallydistinctclasses.Themainargumentsforthisdisparityare: ManyauthorsinthepastarguedthatthelightprolesofbrightEsarebestdescribedbythedeVaucouleur'sR1=4law( deVaucouleurs 1948 ),wherethesurfacebrightnessofagalaxyscalesasitsradiustothepowerof1/4.Meanwhile, Faber&Lin ( 1983 )and Binggelietal. ( 1984 )showedthatdwarfellipticalgalaxiesarebettertbyanexponentialprole.dEsandbrightEsfallalmostperpendiculartoeachotherintheeectivesurfacebrightness{luminosityplot(eL)andareclearlydierentintheluminosity{central 13

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Kormendy 1985 ).Furthermore,dEsandbrightEsfollowadierenteRerelation( Wirth&Gallagher 1984 ; Capaccioli,Caon,&D'Onofrio 1992 )whereReisthehalf-lightradius.Moreevidencetostrengthentheproposeddichotomyexists.DwarfellipticalsseemtolieotheFundamentalPlane,therelationbetweenthesurfacebrightnessattheeectiveradius,e,eectivehalf-lightradius,Re,andthevelocitydispersion,( Bender,Burstein,&Faber 1992 ; deCarvalho&Djorgovski 1992 ; Peterson&Caldwell 1993 ).ThisdisparityhasbeeninterpretedasadierenceintheformationmechanismfordwarfandbrightEs. Nonetheless,therearemanystudiesarguingforcontinuityinthedwarf-brightfamily.Justtomentionafew: Caldwell ( 1983b 1987 ); Caldwell&Bothun ( 1987 ); Ferguson&Sandage ( 1988 ); Hudsonetal. ( 1997 ); Jerjen&Binggeli ( 1997 ); Jerjenetal. ( 2000 ); Karachentsevetal. ( 1995 ).SomeofthesestudiesshowthatdEsexhibitthesamecentralsurfacebrightnessandabsolutemagnituderelationasbrightEs. Caldwell ( 1983b )alsopointedoutthatacontinuoustrendexistsbetweencolorandluminosity,while Caldwell&Bothun ( 1987 )showthesamecontinuityfortheluminosity{metallicityrelation. Graham&Guzman ( 2003 )(alsosee Guzmanetal. 2003 )oeredapossibleresolutionofthedieringviews.Theypointoutthatthedichotomyintheluminosity{eectivesurfacebrightnessrelation(MBe)andtheluminosity{eectiveradiusrelation(MBRe),isadirectconsequenceofthelinearrelationsbetweentheluminosity,thecentralsurfacebrightness(0),andthelightproleshape(n).Furthermore,theyarguedthatdEsandintermediateluminosityellipticalsfollowacontinuoussequenceuptoMB20:5mag.Atthispointbrightellipticalsstartshowingevidenceofevacuatedcores,possiblycoalescingblackholes,causingtheircentralsurfacebrightnesstodecreasewithincreasingluminosity( Graham&Guzman 2003 ; Graham 2004 ,andreferencestherein).ThemostmassiveEsmaythusbetheexceptionandnottheruletotheempiricalcorrelationsdenedbyearly{typegalaxiesthatincludethelowluminositygalaxiesandrangeover8magnitudes(alsosee Graham&Guzman 2003 ). 14

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White&Rees 1978 ; White&Frenk 1991 ).Dwarfgalaxiesarealsopredictedtoformrstinthedensestenvironments,i.e.clusters.Alternatively,thedownsizingtrendof DeLuciaetal. ( 2004 )wherestarformationoccurslaterandovermoreextendedtimescalesinlessmassivegalaxies,suggeststhatthedwarfgalaxiesformedorenteredtheclustersafterthegiantgalaxies. Sinceellipticalgalaxiesaremostlyfoundinclusters,theirformationandevolutionislikelyinuencedbytheirenvironment.Thegiantellipticalsmaybemainlyaectedbygalaxy-galaxyinteractions,whileinteractionswiththeclusterpotentialortheInter-ClusterMedium(ICM)maybeofmoreimportanceinformationandevolutionofdEs( vanZee,Skillman,&Haynes 2004a ).Forexample,thegalaxyharassmentmodelof Moore,Katz,&Lake ( 1996 )and Mooreetal. ( 1998 )explainshowdwarfgalaxiesinclusterscouldactuallyberemnantsofstrippeddisksystemsordwarfirregulargalaxies.Inthisscenario,theprogenitorgalaxiesaredisruptedduetoencounterswithbrightergalaxiesandthecluster'stidaleld;theylosetheirstars,arestrippedotheirinterstellarmediumandaremorphologicallytransformedintotoday'spopulationofclusterdEs(alsosee Conselice,Gallagher,&Wyse 2001 ). Viarecentstudies,dEshavealsobeenlinkedtotheButcher-Oemlereect( Butcher&Oemler 1978 1984 ).Observationsofdistantclustersrevealtheexistenceofnumerousstar-forming,low-mass`bluedisk'galaxiesinclustersatz0:4.Thesegalaxies,asshownbyHubbleSpaceTelescope,aredistortedsmallspiralswhichhavedisappearedfromthepresentdayclusters.Thefateofthesegalaxiesremainsoneofthemostimportantunansweredquestionsinmoderncosmology.Thegalaxyharassmentmodel Mooreetal. ( 1996 1998 )couldpossiblyexplainhowthedwarfspiralgalaxiesinclustersmayevolve 15

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Babul&Rees 1992 ).However,observationsofsubsolarandsolarabundancesofelementsofdEgalaxiescontradictthisscenario,sincesolarabundancesimplythatthesegalaxieshavehadmoreextendedstarformationhistories.Although,arecentpaperintheliterature, Bosellietal. ( 2008 ),predictsthatstar-formingdwarfgalaxiescanlosemostorlikelyalltheirgaswithin150Myrand,inshorttimescales,becomequiescentdEs. AnumberofscenarioswheredEsaretheevolvedcounterpartsofdwarfirregulargalaxieshavealsobeenproposed( Dekel&Silk 1986 ; Silk,Wyse,&Shields 1987 ; Daviesetal. 1988 ).Dwarfirregulargalaxies,dIs,areasymmetricintheirappearance,theycontaingasandyoungbluestarsandhavealuminosityfainterthan108L.Dwarfellipticalsandirregularshavesimilarstellardistributionsandtheyshowsimilaritiesinstarformationhistories( Skillmanetal. 2003 ).InorderfordEstoformfromdwarfirregulars,theywouldhavetolosetheirgasatsomepoint.Thiscouldhappenviasupernova-drivenwinds( Dekel&Silk 1986 )orbyexternalfactorswhichincluderampressurestripping( Lin&Faber 1983 )andinteractionswiththeclusterenvironment.SignaturesoftheseeventsshouldbeseenthroughthestellarpopulationsofdEssincetheirages,metallicitiesandchemicalabundancesdependonwhentheirstarformationoccurred(beforeorafteraccretionintothecluster)and,incaseofenteringtheclusteraftertheirstarformationevents,ontheamountoftimepasseduntiltheyenteredtheclusterenvironment. Despitethesubstantialnumberofstudiesofdwarfellipticalgalaxies,theirformationscenariosremainsomewhatofamystery.InrecentyearsauthorshavepresentedevidencethatthereexistmorethanonetypeofdEgalaxy.Forexample,nucleateddEsseemtoclustermorearoundtheluminousEsandtowardsthecentersofclustersthanthenonnucleateddEswhichhavewideclustervelocitydispersions,similarlytolate-typegalaxies(e.g. Binggeli,Tammann,&Sandage 1987 ).Thisimpliesdierentformationscenariosforthesetwosubclasses.Further,afractionofdEshaveasignicantrotational 16

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Geha,Guhathakurta,&vanderMarel 2003 ; vanZee,Skillman,&Haynes 2004b );anditispossiblethattheserotatingdEsformedfromdIsthatenteredthecluster,whilenonrotatingdEsformedfromaccreteddEs( vanZee,Skillman,&Haynes 2004a ).SomestudiesalsonddEswithspiralremnants(e.g. Graham&Guzman 2003 ; Lisker,Grebel,&Binggeli 2006 ).Further, Aguerrietal. ( 2005 )ndtwodierenttypesofdwarfearly-typegalaxiesintheComacluster.OnetypearethedwarfellipticalswhosesurfacebrightnessprolescanbedescribedbyasimpleSersiclaw,denedasR1=n,wherentakesonvaluesbetween1and10.Theothertypearedwarflenticulargalaxies(dS0s)whichonlydierfromdEsbyhavingadisk.ThesegalaxiesarealsodevoidofstarformationandaredescribedbySersic+exponentialproles.This,again,suggestsdierentformationscenariosfordwarfearly-typegalaxies. Perhapsthemostconvincingevidenceofdierentsubclassesandformationscenariosofdwarfearly-typegalaxiescomesfromtheSloanDigitalSkySurvey(SDSS). Liskeretal. ( 2007 )analyze400early-typedwarfgalaxiesobservedbySDSSintheVirgocluster,13Mpcaway.TheydiscoverthreesubclassesofdEgalaxiesbasedontheirmorphologyandclusteringproperties: ThesethreetypesofdEshavedierentspatialdistributionswithinthecluster.WhilethedE(nN)s,dE(di)s,anddE(bc)aremostlyfoundinlow-densityregionswithsimilardistributionstotheirregularandspiralgalaxiesinthecluster,thedE(N)sarepreferentiallylocatedinhigh-densityregionsandhavespatialdistributionssimilartoEandS0galaxies.Basedonthis,theauthorsndthatallthetypesexceptfordE(N)galaxiesarenotyetrelaxed.SincedE(N)'sarerelaxed, Liskeretal. ( 2007 )concludethattheyarelikelytohaveformedbeforeallotherclasseswheretheyeitherweretherstones 17

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Evenbeforethisdetailedstudy vanZeeetal. ( 2004b )proposedthatVirgoclusterdEsformedthroughatleastthreedierentprocesses.DwarfellipticalsfoundintheproximityofgEsarelikelytohaveformedinsitu,whiletheothertwoscenariosincludeinfallingdEsordIs.TheassumptionisthatthegroupsofgalaxiesfallingintotheclusteraresimilartotheLocalGroupdwarfsandincludebothdEsanddIs,wherethedIswouldlosetheirgasviaram-pressurestrippinguponenteringthecluster.Further,theseauthorssuggestedthatratherthanasimilarformationprocess,theuniformstructuralpropertiesofthesegalaxiesmayinsteadbeduetothesimilarityoftheirdarkmatterhalos. Worthey 1994 ),abundanceratiodierencesbetweenthecalibrationstarsusedtocalculatethemodelsandthegalaxyspectra,andstrongBalmerlineswhichcanbecausedbyeitheryoungstellarpopulationsoranextendedhorizontalbranch.Themostrecentstudieshaveincorporatediterativeproceduresand/orsimultaneousttingofasmanyindicesaspossiblewhilederivingages,metallicitiesandrelationsbetweenthelinestrengthsandvelocitydispersions()ofearly-typegalaxies( Proctoretal. 2004 ; Thomasetal. 2005 ; Nelanetal. 2005 ; Denicoloetal. 2005 ; Sanchez-Blazquezetal. 2006a b ).However,despitetheseimprovedtechniques 18

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Oneofthemostusefulparametersindeterminingthelengthofstarformationingalaxiesistheratiobetweenthesocalled-elementsandFe([=Fe]).FeismostlyproducedinTypeIasupernovatogetherwithotherFe-peakelementsincludingCr,Mn,Fe,Co,Ni,CuandZn.Whilethelighter-elementsincludingO,Ne,Mg,Si,S,Ar,Ca,TiareproducedinTypeIIsupernova.Therefore,theobserved[=Fe]ratiostracethetimescaleofthestarformationactivity( Gilmore&Wyse 1991 ).Galaxieswithsupersolar-ratiosprobablyformedtheirstarsbeforetheFeproducedintheTypeIasupernovaehadachancetobeincorporatedinthestarformationprocess.Thus,theirstarformationmusthavehappenedrapidlywheretheISMwasenrichedmostlybytheelementsproducedinSNTypeII.Ontheotherhand,low[=Fe]ratiosimplyaslowerchemicalenrichment,i.e.moreextendedstarformationhistory. Massiveellipticalgalaxiesexhibithigh[/Fe]ratiosimplyingthattheirstarformationactivityhappenedrapidlyandalongtimeago.MostdEgalaxies,ontheotherhand,exhibitsolarorsubsolar-ratios(e.g. Gehaetal. 2003 ; Thomasetal. 2003 ; vanZeeetal. 2004a ).Signaturesofsupernova-drivenwinds( Dekel&Silk 1986 )orrampressurestripping( Lin&Faber 1983 )andinteractionswiththeclusterenvironmentshouldalsobeseenthroughthestellarpopulationsofdEssincetheirages,metallicitiesandchemicalabundancesdependonwhentheirstarformationoccurred(beforeaccretionoritwastriggeredafterwards)andhowlongaftertheirstarformationtheyenteredtheclusterenvironment.Forexample,iftheprogenitorsofdEshadashortstarburstwhichwasquicklyfollowedbytheirinfallintothecluster,mostoftheircoolgaswouldbelostthroughram-pressurestripping.Inthiscase,wewouldexpecttoobservehighabundancesofthelightelementsandlowmetallicities.SincetheywouldnothaveenoughtimetoincorporateFefromSNTypeIaintothenewgenerationofstars.Inconclusion, 19

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TheComaclusterisanidealtestbedforstudiesofgalaxyformation.Thecenterofthiscluster,providesoneofthedensestenvironmentsinthelocaluniverse.Thedensityofgalaxiesintheinner1diameteris20timeshigherthantheouter12:5region.Thus,objectsintheinnerregionwillhaveexperiencedmoreinteractionswithothergalaxies,comparedtothoseintheouter,lessdenseregions.Inthiswork,webasethecharacterizationofinternalkinematicsandstellarpopulationsofdEs/dS0sontheirpropertiesinthecenteroftheComacluster,whileweassesstheenvironmentaleectsbycomparingthecentralsampleofthesegalaxiestotheoneslocatedjustoutsidethevirialcoreoftheComacluster. 20

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Poggiantietal. 2001a ; Caldwelletal. 2003 ; Nelanetal. 2005 ; Sanchez-Blazquezetal. 2006a ),wepresent,forthersttime,astatisticallyrepresentativenumberofdE/dS0sinComa,reaching=30kms1.UpuntilthepastcoupleofyearsmostinformationonspectroscopicpropertiesofdEscamefromtheirlinestrengthindices( Held&Mould 1994 ; Gorgasetal. 1997 ; Mobasheretal. 2002 ; Mooreetal. 2002 )andahandfulofvelocitydispersionmeasurements.Althoughmorediculttoobtainduetolowsurfacebrightnessoftheseobjects,thenumberofpapersintheliteraturewhichincludevelocitydispersionmeasurementsofdEsindierentclustershasincreased( Bender&Nieto 1990 ; Brodie&Huchra 1991 ; Heldetal. 1992 ; Bender,Burstein,&Faber 1992 ; Peterson&Caldwell 1993 ; Bernardietal. 1998 ; Mehlertetal. 2002 ; Hudsonetal. 2001 ; DeRijckeetal. 2001 ; Simien&Prugniel 2002 ; Pedrazetal. 2002 ; Mooreetal. 2002 ; Geha,Guhathakurta,&vanderMarel 2002 2003 ; Guzmanetal. 2003 ; Bernardietal. 2003 ; Smithetal. 2004 ; vanZee,Skillman,&Haynes 2004b ; DeRijckeetal. 2004 ; Smithetal. 2008b ).Unfortunately,noneofthesestudieshaveastatisticallylargenumberofgalaxiesintheirsamples.Incontrast,theNOAOFundamentalPlanesurvey(NFPS)of Smithetal. ( 2004 )isthelargestcompilationincludingvelocitydispersionmeasurementsinlow-redshiftgalaxyclustersuptodate.However,thissampleincludesonlythemostluminousofthedwarfearly-typepopulationwithvelocitydispersions.DescribedinthisdissertationistheComaclustersampleofdwarfearly-typegalaxieswithmeasurementswhichreach=30kms1( Matkovic&Guzman 2005 ).ThiswillallowustodeterminetheFaber-Jacksonrelation,arelationbetweenvelocitydispersionandluminosity,fordE/dS0galaxies. Itiswellknownthatlower-luminosityearly-typegalaxiesshowawiderrangeinagethantheirmoreluminouscounterparts(e.g. Caldwell 1983a ; Benderetal. 1993 ; Worthey&Ottaviani 1997 ; Kuntschner&Davies 1998 ; Poggiantietal. 2001a ; Caldwelletal. 2003 ).Somestudiesndthatthelowermassgalaxiesalsodisplayyoungerages( Poggiantietal. 21

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; Caldwelletal. 2003 ; Proctoretal. 2004 ; Thomasetal. 2005 ; Nelanetal. 2005 ),whileothersdonotndthisrelationorndthatitdependsontheenvironment( Trageretal. 2000b ; Kuntschneretal. 2001 ; Sanchez-Blazquezetal. 2006a ).Similarlyanumberofstudiesshowthatlowermassgalaxieshavelowermetallicities( Brodie&Huchra 1991 ; Poggiantietal. 2001a ; Kuntschneretal. 2001 ; Mehlertetal. 2003 ; Proctoretal. 2004 ; Nelanetal. 2005 ; Thomasetal. 2005 ; Sanchez-Blazquezetal. 2006b ; Bernardietal. 2006 ).Further,thesegalaxiesdisplaylower(closertosolar)abundanceof-elementsthantheluminousellipticalgalaxies(Es)whichhaveanoverabundanceof-elementswhencomparedtothevaluesinthesolarneighborhood( Gorgasetal. 1997 ; Jorgensen 1997 ; Trageretal. 2000b ; Nelanetal. 2005 ; Thomasetal. 2005 ; Denicoloetal. 2005 ; Bernardietal. 2006 ; Sanchez-Blazquezetal. 2006b ).Thelowervaluesfortheabundanceratio,/Fe,forlow-massgalaxiessuggeststhatthesegalaxieshavehadmoreextendedstarformationhistoriesthantheirmassivecounterparts(seealso 1.3 ). InChapter 2 wedescribeoursampleselectionandourspectroscopicdata,togetherwithmeasurementsofvelocitydispersionsandthelinestrengthindicesofdE/dS0galaxiesinthecluster.Chapter 3 describestheinternalkinematicsincomparisontothemoremassivegalaxiesandeectsoftheenvironmentontheLuminosity{relation.InChapter 4 weinvestigatetheunderlyingstellarpopulationsofdE/dS0galaxiesandassessenvironmentaleectsontheseproperties.Chapter 5 containstheconcludingremarksofthiswork. 22

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The&White 1986 ).Ourgalaxiesspanawiderangeinvelocitydispersion(30-260kms1)andluminosity(17:017magatthedistanceoftheComacluster 23

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Color-magnitudediagramofgalaxiesintheComacluster.ThesmalldotsareobjectsintheGMPcatalog( Godwinetal. 1983 ),thediamondsareconrmedmembersoftheComacluster.Themiddlelinedescribestheleastsquaresttotheconrmedmembersofthecluster,whilethetwolinesaboveandbelowitare2awayfromthederivedline. donotbelongtotheComacluster.Tominimizecontaminationatthefaintendbythebackgroundelddiskgalaxiesatz<0:2,weappliedanothercutousingthe(U{B)vs.(B{R)color-colordiagramat0:2<(UB)<0:6mag,and1:3<(BR)<1:5mag. Wedidnothavephotometricobservationsoftheoutskirtssample,soweemployedacatalogoftheComaclustergalaxiesby Godwinetal. ( 1983 ,hereafterGMP).Thiscatalogcontainsapparentmagnitudesinbandrbands.Weappliedthesamplemagnitudecutoasforthecentralsampleandderivedourowncutosinbr.Weplottedthecolor-magnituderelationforallthegalaxiesintheComacluster( 2-1 ).ThecolorsequenceofgalaxiesintheComaclusterisdenedbytheconrmedmembersofthecluster.Wehadthisinformationasbythetimeweobservedtheoutskirtssample,wealreadyhadmeasurementsofthegalaxiesinthecenteroftheComacluster. 24

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Spatialdistributionofobservedgalaxies.Redopencirclesdenoteearly-typegalaxiesweobservedinthecenterofthecluster.Thelledbluecirclesarethegalaxiesinouroutskirtssample.ThelargeblackcirclemarksthevirialcoreoftheComacluster. 2-2 .Werefertothegalaxiesinthecoreoftheclusterasourcentralsample,andthegalaxiesjustoutsidetheviralcore,asthe\outskirts"orthe\outer"sample. ThethecentralComaclusterfaintearly{typegalaxieswereobservedduring1998May23{26,and1999May14{19onthe3.5mWIYNtelescopeatKittPeakNationalObservatorywiththemulti-berspectrographHYDRA.Weusedthe600lmm1gratinginthe2ndorder,andthebluebercable,whichwechoseforitstransmissionatthedesiredwavelengths.Theselectedgratingallowedustoobserveinthewavelengthrangeof=41205600A,whichisoptimalfordiscerningsomeofthemostprominentabsorptionfeaturesofthefaintearly{typegalaxiesincludingmolecularG-band,H,H, 25

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TheoutskirtssamplewasobservedinApril2003andMay2004.Weusedthesametelescopeandinstrument.However,weusedadierentgrating,630lmm1inthe2ndorder.Thisgratinggaveushigherdispersion,butlowerspectralcoverage.However,withthewavelengthrange=41205600A,wewerestillabletoobservealltheabsorptionfeaturesaswedidinthecentralsample. TheHYDRAmulti-berspectrographhas100berseachwith300diameter.Thus,itiswellsuitedforthedetectionofthefaintearly{typegalaxies,whichtypicallyhaveahalflightradiusof200atthedistanceofComa( Graham&Guzman 2003 ).Weobserved45galaxiesand45adjacentskyspectraforeachHYDRAsetup.Thegalaxysamplewasdividedinto3dierentgroupsdependingontheexposuretimestoachieveSNR>15.Weobservedthebrightestgalaxiesbj617:5magforatotalintegrationtimeof4hours,objectswith17:5018:5magfor16hours.Inaddition,weobtainedspectraoftemplatestarsrepresentativeoftheprevailingstellar-populationofdEs,primarilyGandK{typestars.Thesamplealsoincluded30brightellipticalgalaxiesobservedinpreviousstudiesinordertoassessanysystematiceects.Samplespectraof4galaxieswithdierentluminosityandSNRaregiveninFigure 2-3 .Allthecandidatesinourcentralsamplehadenoughsignalforradialvelocitymeasurementsfromwhichweconcludethat100percenthavetherangeofrecessionvelocitiesof4,000{10,000kms1,consistentwithmembershipintheComacluster( Colless&Dunn 1996 ).Onegalaxyintheoutskirtssample,GMP4427doesnotbelongtotheCluster.PleaseseeFigure 2-4 forclustermembership. Wealsoclassiedgalaxiesinoursampledependingontheirbulge-to-totalratios,B/T.Thisinformationwasonlyavailableforourcentralsample.Thereare33galaxies 26

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SamplespectraofdEgalaxies.Spectraforearlytypegalaxiesofdierentluminosity.TheGMPnumber( Godwinetal. 1983 )andthesignal-to-noiseratios(SNR)aregivenaboveeachgalaxyspectrum.[Reproducedfrom Matkovic&Guzman ( 2005 ).] withbulge-dominatedluminosityproles,15withbulge+singleexponentialcomponent,20withasingleexponentialcomponent,and6galaxiesforwhichnomorphologywasavailable.Theluminosityprolesarefrom Gutierrezetal. ( 2004 ); Graham&Guzman ( 2003 ). 27

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Clustermembership.Theplotshowsourcentralsampleasredopencirclesandtheoutskirtsgalaxiesasthebluelledcircles.ThelinesseparatingtheComaclustergalaxiesfromforegroundandbackgroundaredenedasthelocalescapevelocityateachradius.Theselineswereapproximatedfrom Kent&Gunn ( 1982 )andGuzman(1993). 28

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ThroughoutthispaperwerefertogalaxiesfainterthanMB18andwith<100kms1as`faint',orlow-galaxies.Thisgroupincludesthe36dE/dS0galaxiesand6intermediateearly-typegalaxiesinthecenterofthecluster.Theremaining32objectswith>100kms1werefertoasbright,orhigh-galaxies.Inouroutskirtssamplewehave48galaxiesoutofwhich10arehigh-galaxiesandtheremaining38arelow-galaxies.ThisisoneofthelargestsamplestodateofclusterdE/dS0galaxieswithvelocitydispersionsreachingdownto30kms1. Cardiel 1999 ).ThisprogramimplementstheFourierquotientmethod,originallyintroducedby Sargentetal. ( 1977 ),tomeasuretheradialvelocitiesandvelocitydispersions.Wealsodeterminedasetofparametersbestsuitedforourgalaxiesandwetestedthesoftwareforinconsistencies.Thisisdescribedinthefollowingsectionstogetherwiththemethodofdeterminingtheerrorsintheradialvelocityandmeasurements. Gonzalez 1993 )whichareapartofREDUCEME.TheFourierquotientmethodassumesthattheobservedgalaxyspectra,G,canbedescribedasaconvolutionbetweenthespectralcharacteristicsofthestellarpopulation,SG,andthebroadeningfunction,B: where*standsforconvolution,IistheeectiveresponsefunctionoftheinstrumentandBrepresentsthedistributionofstellarradialvelocitiesalongthelineofsight.Ontheother 29

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Fitbetweenagalaxyspectrumandit'scorrespondingoptimaltemplate.PanelA)showstheresidualsbetweenthegalaxyspectrumandtheoptimaltemplatet.Theverticaldottedlinesrepresentthepositionoftypicalemissionlines.TheredandthegreenspectrainB)correspondtothenormalizedgalaxyandtheoptimaltemplatespectra,respectively,aftertheircontinuumhasbeenremoved. hand,templatestellarspectracanberepresentedasaconvolutionbetweentheobservedstellarspectra,ST,andtheinstrumentalresponsefunctiononly: SincethespectraofellipticalgalaxiesisverysimilartothatofGandKtypestars,itispossibletomodelthegalaxylightbyusingthesestarsastemplates.Infact,oneofthemainadvantagesofthismethodisthesuccessfulmatchingbetweenthegalaxyandthetemplatespectra.ThisisintroducedthroughtheOPTEMAalgorithmwhereanoptimaltemplateforeachgalaxyisconstructedasalinearcombinationofstellarspectraofdierenttypesandluminosityclasses.Foranexampleofatbetweenthegalaxyandtheoptimaltemplate,seeFigure 2-5 30

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2{1 and 2{2 .Thisisdonebydividingtheobservedgalaxyspectra(G)bythetemplate(T)inFourierspace,whereconvolutionistransformedintoaproduct: ~B=~G=~T+Noise(2{3) where~B,~Gand~TareFouriertransformsofthebroadeningfunction(i.e.thevelocitydispersion),thegalaxyandthetemplatespectra,respectively.ThisprocessisimplementedbytheMOVELalgorithm.MOVELdeterminesthevelocitydispersionsviaaniterativeprocedureinwhichagalaxyspectrumtogetherwithagalaxymodelareprocessedinparallel.Themodelgalaxycanbeexpressedas: wherethemodelgalaxy,MG,representsaconvolutionbetweentheoptimalstellartemplateandthebroadeningfunction. Usinganinitialguessofthevelocitydispersion,theprogramcalculatesthebroadeningfunctionandmodelsagalaxyspectrumasaconvolutionofthebroadeningfunctionandanoptimalstellarspectrum.Thebestvalueofthevelocitydispersion(broadeningfunction)isthendeterminedviathe2minimizationbetweenthegalaxyandthemodelspectra. 2-6 ,panelsA,BandC).Weused7thorderpolynomialstotthecontinuumtoourgalaxiesandtemplatestars. 31

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DerivingkinematicparameterswithMOVEL.PanelA)showsthenormalizedgalaxyspectruminred,thetemplatestaringreenandthemodelgalaxyinblue.B)showsthettedcontinuumtoeachspectrumwherethecolorsarethesameasinA).PanelC)showsthethreespectraaftertheircontinuumhasbeenremoved,whileD)illustratesthecosinebellremovalprocedure. Wenoticedadierenceinthemeasurementswhenwechangedthewavelengthrangeusedtocalculatethevelocitydispersionsandwhenwealteredthetaperingfraction.Thetaperingfractionisusedtoeliminatetheeectofthehigherandlower-orderharmonicsbeingaddedduringtheFouriertransform.InMOVELthiseectiscorrectedbymultiplyingthenormalizedandcontinuum-removedspectrabyacosine-bell-likefunction(seepanelCofFigure 2-6 ).Wefoundthatthemeasuredvelocitydispersionvariedupto20%forthefaintestgalaxieswhenvaryingthetaperingfractions.Furthermore,valuesvariedby10percentdependingonthewavelengthrangeused. 32

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SpectralregiontestsfortheCentralComagalaxies.Thedierentsymbolsrepresentgalaxieswithawiderangein.Thisgureshowshowthederivedvalueofforeachgalaxychangesforagiventaperingfractionoverthedierentspectralregions.Thex-axisrepresentstheregioninpixelswhichwasusedinderivationofvelocitydispersionsandradialvelocities. Weconductedaseriesoftestsalteringeitherthetaperingfractionforaspecicwavelengthrange,orthewavelengthrangeforaspecictaperingfraction.Wefoundthatthemoststablesolutionsoccurredforataperingfractionof0.25andwhenwetrimmedthespectraattheedges(seeFigures 2-7 2-8 2-9 and 2-10 ).Wechosetotrimthespectrabypreservingthelargestrestframewavelengthrangepossible:41505400A. 33

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Spectralregionstestsfortheoutskirtsgalaxies.TheaxesarethesameasinFigure 2-7 matchedwiththedominatingstellarpopulationofearly-typegalaxies.AsexplainedinSection 2.4.2 ,theoptimaltemplateisaspectrumofalinearcombinationoftemplatestarsoptimizedtoteachgalaxyspectrum.Wethenfoundapolynomialt(wechosethe7thorderpolynomial)tothegalaxy'sandthetemplate'sblackbodycurve.Thisallowedustocalculatetheresidualsbetweenthegalaxyspectrumandthe\modelgalaxy"inthefollowingway: R=GT PTPG(2{5) whereRrepresentstheresiduals,Gthegalaxyspectrum,Ttheoptimaltemplatespectrum,PGthepolynomialttothegalaxyspectrum,andPTthepolynomialttothetemplate.Thequantityintheparenthesesisthe\modelgalaxy",aspectrumwiththeexactshapeofthegalaxyandhighS/Nfeaturesoftheoptimaltemplate.Rreferstotheresidualnoisebetweenthegalaxyspectrumandthemodelgalaxy. 34

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TaperingfractiontestsforcentralComagalaxies.Thisgureshowshowthemeasuredvalueofatagivenspectralregionchangesdependingonthetaperingfraction.Thex-axisrepresentsthefourdierenttaperingfractions:1/16,1/8,1/4and1/2. 35

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Taperingfractiontestsforoutskirtsregiongalaxies.TheaxesarethesameasinFigure 2-9 However,tobuildatrueerrorspectrumwemusttakeintoaccounttheactualnoiseofthegalaxytogetherwiththeuncertaintyduetothetemplatemismatch.Wedidthisbytakingthesquarerootofthegalaxyandscalingthisspectrumtotheaveragenumberofitscounts.Then,wemultipliedthatquantitybytheamountofnoisefromtheresiduals: E=p 36

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Nowthatwehaveobtainedtheerrorspectrum,wecanderivetheuncertaintiesintheradialvelocityandmeasurementsbynumericalsimulations.ThisisdonewithintheMOVELandOPTEMAalgorithmswhereadierentoptimaltemplateiscomputedineachsimulation.TheerrorintheVRandmeasurementsisthencomputedasthestandarddeviationofthedierentsolutions.Foreachgalaxyweran1000simulations.Thevelocitydispersionmeasurementsfor74early-typegalaxiesinthecenteroftheComacluster,and44intheoutskirtsarepresentedinTables 2-1 and 2-2 .Inthesetablesweusethegalaxynumbers,theRightAscension,Declination,andthebjmagnitudeaccordingto Godwinetal. ( 1983 ). GMPGalaxybjVrVrSNRhms000magkm/skm/skm/skm/spix1

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GMPGalaxybjVrVrSNRhms000magkm/skm/skm/skm/spix1

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GMPGalaxybjVrVrSNRhms000magkm/skm/skm/skm/spix1 Table2-2.Outerregionmeasurements GMPGalaxybjVrVrSNRhms000magkm/skm/skm/skm/spix1

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GMPGalaxybjVrVrSNRhms000magkm/skm/skm/skm/spix1 ,wherewealsoshowhowonendstheresiduals(Equation 2{5 )betweenthegalaxyspectrumandthemodelgalaxy.WedeneanaveragevalueofS/Nperpixelofeachgalaxyas: S/N=hGi wherehGiistheaveragenumberofcountsofthegalaxy,andRrepresentstheresidualnoisebetweenthegalaxyspectrumandthemodelgalaxy.ThevalueofS/Nratiocalculatedinthiswayisanaveragevalueperpixelforeachgalaxyanditincludesthemismatchbetweentheactualgalaxyspectrumandthatofthemodelgalaxy. Inthisstudy,weonlyconsiderthegalaxieswhoseaveragesignal-to-noiseratioS/N>15sincebelowthisvaluethetofthemodelspectratothegalaxywasuncertain.Thereare74galaxiesinourcentralComaclustersamplewhichsatisfythisconditionand44galaxiesintheoutskirtssample. 40

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Jrgensenetal. ( 1995 ).WeselectedatemplatestarthatbesttsatypicalgalaxywithahighSNR,andbroadeneditbyconvolvingitbyGaussianswithrangingfrom35to100kms1.DierentamountsofnoisewereaddedtoeachspectrasotheSNRwouldyieldvaluesranging10{50.Theunalteredspectraofthestarwasusedasatemplate,whilewemeasuredthevelocitydispersionofthebroadenedandlowerSNRspectra.Thenalwasderivedafter1000simulationsusingtheboot-strappingmethod.Figure 2-11 showsthepercentagedierencebetweenthe\galaxy"spectra(templatespectrabroadenedto50kms1)andtheobserved,i.e.thesamebroadenedgalaxywithdierentamountofnoise. Alloursimulatedspectrahaveaslightlyoverestimatedmeasurementofthevelocitydispersion.Theeectislargerforgalaxieswithasmallervelocitydispersion.Forexample,agalaxywith=35kms1andSNRof15hasoverestimatedby6percent. Jrgensenetal. ( 1995 )ndthatagalaxywith=65kms1isoverestimatedby4percent,andagalaxywith=100kms1by1percentwhichisingoodagreementwithourmeasurements.Wechosenottocorrectforthissystematiceectsinceitissignicantlysmallerthantheuncertaintiesinthemeasurementsformajorityofthegalaxiesinoursample. Mooreetal. 2002 ; Smithetal. 2004 ,hereafterMLKC02andNFPSrespectively).Figure 2-12 A)showsthecomparisonofmeasurementsandincludestheuncertaintyinthe=Lit.Thisplotshowsnosystematicosetsbetweenourandthetwoliteraturesamples.Furthermore,themedianofthecomparison,1.01,andthermsscatterofthepoints,0.10whichsuggeststhattheerrorinthemeasurementsare 41

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SystematiceectsonmeasurementsdependingontheSNRofthegalaxy.They-axisshowsthedierencebetweenthespectrawithagivenvelocitydispersion,gal,andtheobservedvelocitydispersion,obs,i.e.thesamespectrawithdierentamountofnoise.Thespectrawerederivedfromsimulations.Thedottedlineisforaspectrumof35kms1,whiledash{dotlineisfor65kms1.Theotherlinesaremarkedaccordingly.WealsoincludedthedashedlineatSNRof15aswechosethisvalueasthelowerlimitforreliablemeasurementsinthispaper.[Reproducedfrom Matkovic&Guzman ( 2005 ).] small7%.WeconcludethatourmeasurementsareingoodagreementwiththoseofbothNFPS(opentriangles)andMLKC02(solidtriangles).Wealsonotethatsomegalaxiesseemtobeinconsistentwiththeaveragevaluewhentakingtheiruncertaintiesintoaccount.Thiswouldimplythattheuncertaintiesinthemeasurementsarepossiblyunderestimated.WecheckthisinpanelB). PanelBofFigure 2-12 showsthedierencebetweenmeasurementsweightedbytheiruncertainty.Whenthedierenceinvalueshereandintheliteratureissmallandtheuncertaintieslarge,theweighteddierenceislessthan1.0implyingthattheoveralluncertaintyinmeasurementsisoverestimated.Ontheotherhand,ifthedierenceislarge,anduncertaintiessmall(weighteddierence>1),themeasurements 42

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Externalcheckofmeasurements.A)showsourmeasurementscomparedtoNFPS(opentriangles)andMLKC02(solidtriangles).PanelB)showsthedierencebetweenmeasurementsweightedbytheiruncertainty.[Reproducedfrom Matkovic&Guzman ( 2005 ).] areinconsistentandtheiruncertaintiesunderestimated.Eventhoughitisnotpossibletodistinguishwhichsampleontheplothastheoverestimatedorunderestimateduncertaintiescomparedtotheother,bylookingatthemedianvalueofthedistributionwecangetanoverallsenseforhowwelltheerrorweightedmeasurementsagree.ThemedianvalueforobjectsinpanelB)is0.91,whichindicatesthattheuncertaintiesinthemeasurementsmaybeslightlyoverestimatedasjLitj=p 43

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Bursteinetal. 1984 ; Gorgasetal. 1993 ; Wortheyetal. 1994 ; Trageretal. 1998 )asitcoversawiderangeinopticalabsorption-linesamongwhicharesomeofthemostprominentfeaturesofearly-typegalaxies.Unfortunately,tomatchtheLick/IDSsystemwemustdegradethespectralresolutionofourgalaxies.Althoughwesacricetheinformationavailableinourhighresolutionspectra,stellarpopulationmodelswithhighspectralresolutiondonotexistasofyet.TheLick/IDSsystemisactuallyabasisforanextensivecollectionofstellarsynthesismodels,allowingonetoderiveagesandmetallicitiesofgalaxies. Wemeasuredtheline-strengthsfortheComacenterandoutskirtsearly-typegalaxieswiththesoftwareREDUCEME( Cardiel 1999 ),taskINDEX.ThissoftwareallowsforcarefuldeterminationofuncertaintiesintheindexmeasurementsasitusesMonteCarlosimulations(seeSection 2.4.3 formoredetails).LimitedbytheS/NofgalaxiesinoursamplewewereabletomeasurethefollowingLick/IDSindices:Ca4227,G4300,Fe4383,Ca4455,Fe4531,C24668,H,Fe5015,Mg1,Mg2,Mgb,Fe5270,andFe5335( Trageretal. 1998 );andtheirextensionstohigherorderBalmerlinesHAandHF( Worthey&Ottaviani 1997 )(seeTable 2-3 ).Wealsoincludethe[MgFe]0(asdenedbyTMB03)andhFei( Gonzalez 1993 )indicessincetheycloselymeasuremetallicityandarecommonintheliteratureallowingforaneasycomparison.Weusethesetwoindiceswithstellarpopulationmodelstopredictluminosityweightedages,metallicitiesandthe[/Fe]-ratios. 44

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Linestrengthindexdenitions IndexNameBlueBandpassCentralBandpassRedBandpassUnits Ca42274211:0004219:7504222:2504234:7504241:0004251:000AG43004266:3754282:6254281:3754316:3754318:8754335:125AFe43834359:1254370:3754369:1254420:3754442:8754455:375ACa44554445:8754454:6254452:1254474:6254477:1254492:125AFe45314504:2504514:2504514:2504559:2504560:5004579:250AFe46684611:5004630:2504634:0004720:2504742:7504756:500AH4827:8754847:8754847:8754876:6254876:6254891:625AFe50154946:5004977:7504977:7505054:0005054:0005065:250AMg14895:1254957:6255069:1255134:1255301:1255366:125magMg24895:1254957:6255154:1255196:6255301:1255366:125magMgb5142:6255161:3755160:1255192:6255191:3755206:375AFe52705233:1505248:1505245:6505285:6505285:6505318:150AFe53355304:6255315:8755312:1255352:1255353:3755363:375A dierencesbetweentheindicesofstandardstarswhichwerealsoobservedbyLick.Additionallyoneneedstocorrectthenebularemission,andapplyvelocitydispersionandaperturecorrections.WewerenotabletofullytransformourdataintotheLick/IDSsystembecausewedidnotobservestarsincommonwiththeoriginalstellarlibrary(seebelow).Below,wedescribetheprocedurewefollowed: wherebroadistheamounttobroadenthegalaxyby(inkms1),LickisLick/IDSresolutionforaparticularindex(seeTable5of Sanchez-Blazquezetal. 2006a ),galthevelocitydispersionofthegalaxyandInstrrepresentstheinstrumentalresolutionofourdata.Notethat2obs=2gal+2Instr. Sanchez-Blazquezetal. ( 2006a )whohave8galaxiesincommonwithoursample.First,weobtainedtheresponsecurvesforthe8matchinggalaxiesbydividingeachgalaxyspectrumbythatoftheux-calibratedgalaxy.Then,wettedapolynomialfunctiontoeachoftheresponsecurvesandcreatedameanuxcalibrationcurve.Thiscurveisusedtouxcalibratetheline-strengths,whiletheindividualresponsecurvesarelaterusedtocalculatetheuxrelateduncertaintiesintheindexmeasurements.Theoutskirtssamplewasux 45

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Trageretal. 1998 )whichisinsucientfordetermininganyosetsbetweenourdataandtheLick/IDSsystem.Weare,however,abletocompareourdatatootherdatasetsintheliterature(seeSection 2.5.3 ). Graham&Guzman 2003 ).Thus,wefoundnoneedforaperturecorrections.However,wedorecognizethatafewmassivegalaxiesinthissamplearelikelytobelargerthantheHYDRA-spectrographbersinwhichcasewearesamplingtheircentralregionsonly. Hammeretal. ( 2001 )and Kuntschneretal. ( 2002 )todeterminewhetheranygalaxiesinoursamplecontainedemission.Fromeachgalaxyspectrumwesubtractedit'soptimaltemplate(aspectrumofalinearcombinationoftemplatestarsoptimizedtoteachgalaxyspectrum,seePaperIformoredetails).Thismethodrevealedthat2galaxiesinoursamplehadHandOIIIemission.Weexcludethesetwogalaxies,GMP3733andGMP2516,fromouranalysis. ThemeasurementsofthecentralsamplearepresentedinTables 2-4 and 2-5 ,whiletheouterregionindexmeasurementsarepresentedinTables 2-6 and 2-7 .TheseindicesareattheLick/IDSresolution,sotheycanbeeasilycomparedtoothersourcesintheliterature.Theerrorineachindexmeasurementincludesuncertaintiesassociatedtotheuxcalibrationandtothephotonnoise. 46

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GMPCa42274227G43004300HAHAHFHFFe43834383Ca44554455Fe45314531C24668C24668HH

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GMPCa42274227G43004300HAHAHFHFFe43834383Ca44554455Fe45314531C24668C24668HH

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GMPFe50155015Mg1Mg1Mg2Mg2MgbMgbFe52705270Fe53355335

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GMPFe50155015Mg1Mg1Mg2Mg2MgbMgbFe52705270Fe53355335

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GMPCa42274227G43004300HAHAHFHFFe43834383Ca44554455Fe45314531C24668C24668HH

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GMPCa42274227G43004300HAHAHFHFFe43834383Ca44554455Fe45314531C24668C24668HH

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GMPFe50155015Mg1Mg1Mg2Mg2MgbMgbFe52705270Fe53355335 2.4.3 .ThetaskINDEXprocessesthegalaxyandtheerrorspectrainparallelandtheerrorsintheindexmeasurementsarethenderivedviabootstrapping. 53

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Poggiantietal. ( 2001a ,hereafterP01),NFPS( Nelanetal. 2005 ,N05)and Smithetal. ( 2008a ).Althoughthesestudiescontainasignicantlylargernumberofgalaxies,ourdataiscomplementarytothesesamplesasitincludesalargernumberoflowmassgalaxies(.100kms1)withvelocitydispersionmeasurementsreachingaslowas30kms1inComa. ThecomparisonbetweenthethreeliteraturesamplesandourdataareshowninFigures 2-13 2-14 ,and 2-15 andtheosetsarepresentedinTable 2-8 .ThemeanosetsaredenedasadierencebetweentheindexmeasurementinthispaperandtheindexmeasurementinP01,S08,orNFPS(N05)hIi=IhereIother.Wealsocalculatedtheerrorinthemeanoset,standarddeviation,andthestandarddeviationexpectedbytheerrors. Theosetsbetweenoursampleandthe Smithetal. ( 2008a )aresmallandnotsignicantforthemajorityoftheindices.TheindicesforwhichtheosetsaresignicantareMg1,Mg2andFe5270withmeanosetsof0:0140:003,0:0180:004and0:3340:127,respectively.However,thescatterbetweenthe Smithetal. ( 2008a )sampleandoursislargeandcannotbeexplainedbytheerrors.TheindicesCa4227,G4300,HA,HF,andCa4455eachhavescatter3timeslargerthanthescatterexpectedbythe 54

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Comparisonwith Smithetal. ( 2008a ).Thelineshaveaslopeof1.0andindicateaone-to-oneagreementbetweenthetwodatasets.Galaxieswhichdonottonthemodelgrids,the\o-grid"galaxies,arelabelledasredopencirclesandarediscussedinSection 4.2 .[Reproducedfrom Matkovicetal. ( 2008 ).] errors,whilethescatteris2timeslargerthanthescatterduetotheerrorsformajorityoftheotherindices. Thelargescatterinthecomparisonbetweenthe Smithetal. ( 2008a )indexmeasurementsandoursisdominatedbyanumberofgalaxieswhicharesystematicallyosetintheHplot(redopencirclesinFigure 2-13 ).BecauseourHand[MgFe]0indicesforthesegalaxiescannotbereproducedbythestellarpopulationmodels,weexcludethemfromfurtheranalysisandrevisitthisissueinx Wendthatthemeanosetsbetweenourindexmeasurementsandthoseof Poggiantietal. ( 2001a )aresmallandinmostcasesinsignicant.However,Ca4455,Fe5015,andMg1havenon-negligibleosets,0:2590:116,0:6520:208and0:0120:006,respectively.WenotethatsomeindicesmaybeosetbecauseourindicesarenotfullytransformedtotheLick/IDSsystemlikethe Poggiantietal. ( 2001a )sample. 55

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Comparisonofindexmeasurementsbetweenoursampleandthatof Poggiantietal. ( 2001a ).ThesymbolsandlinesarethesameasinFigure 2-13 .[Reproducedfrom Matkovicetal. ( 2008 ).] Similarlytothe Smithetal. ( 2008a )data,thescatterintheindiceswhencomparingourdatawith Poggiantietal. ( 2001a )islargeformostindices,exceptforMg1andMg2.Furthermore,thelargescatterbetweenthesetwodatasetscannotbeexplainedbytheerrors.ThediscrepancybetweenthemeasuredscatterandthescatterduetotheerrorsisthelargestforG4300,HA,HF,andMg1. Ontheotherhand,theNFPSindexmeasurementsareingoodagreementwithours.AmongtheatomicindicesonlyCa4227,Fe4531andFe5015havesignicantosets:0:2070:047,0:1520:066and0:3130:102,respectively,whiletherestoftheindicesareconsistentbetweenthetwostudieswithasmallscatter. Mostnoticeably,ourandtheNFPSindicesareinverygoodagreementintheMg-indicesafterasystematicosetinMg1of0:0230:002andinMg2of0:0170:002isapplied.TheNFPSdataarenotuxcalibratedwhichwouldmostlyaectthemolecularindicesliketheMg1andMg2andmayexplainwhythereisanosetbetweenthedatasetsforthesetwoindices.However,thescatterinthesetwomolecularindices 56

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Comparisonofindexmeasurementsto Nelanetal. ( 2005 ).ThesymbolsandlinesarethesameasinFigure 2-14 .[Reproducedfrom Matkovicetal. ( 2008 ).] isremarkablysmall(0.01magforboth).Thescatterintheremainingindicesisalsoconsistentwithintheerrorsinbothstudies.WeconcludethatourindexmeasurementsareingoodagreementwiththoseofNFPS. InthefollowingsectionsweonlyincludetheNFPSdatasetforcomparisonpurposes,sincethe Poggiantietal. ( 2001a )and Smithetal. ( 2008a )donotincludevelocitydispersions.Additionally,ourindexmeasurementsareinbetteragreementwithNFPS.TheNFPSdatawerenotuxcalibrated,andforfurtheranalysisweapplyanosettotheNFPSdatatobeconsistentwithoursaccordingtotheaveragevalueslistedinTable 2-8 57

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Linestrengthcomparisonwithotherstudies. IndexRef.hIistdexp G4300P010.2170:2641.5190.567N050.0200:0940.4710.368S080.1460:2341.0460.328 HAP010.2120:3041.7450.626N050.1050:1140.5690.508S080.3590:2251.0040.368 HFP010.0400:1620.9450.397N050.0370:0640.3200.242S080.1460:1460.6530.207 Fe4383P010.9700:2751.6040.789N050.1540:1390.6970.466S080.1130:2050.9160.420 Ca4455P010.2590:1160.6770.395N050.0010:0380.1890.194S080.0890:1220.5470.194 Fe4531P010.1930:1530.8900.573N050.1520:0660.3280.303S080.2060:1490.6680.318 C24668P010.2360:3452.0100.928N050.1540:1390.6970.626S080.1400:2321.0370.650 HP010.1290:1250.7290.349N050.0710:0420.2100.163S080.1180:0950.4240.205 Fe5015P010.6520:2081.2150.717N050.3130:1020.4770.421S080.2430:2461.0980.805 Mg1P010.0120:0060.0370.014N050.0230:0020.0100.014S080.0140:0030.0140.013 Mg2P010.0040:0070.0410.017N050.0170:0020.0100.017S080.0180:0040.0180.015 MgbP010.0150:1190.6920.339N050.0510:0410.1930.173S080.0150:1150.5130.213 Fe5270P010.1780:1120.6520.380N050.0180:0510.2410.196S080.3340:1270.5670.240 Fe5335P010.0740:0980.5720.430N050.0650:0500.2330.226S080.0580:1460.6530.276 Comparisonoflinestrengthsmeasuredinthisandotherstudies.Ref.:referenceintheliteraturewhereP01isfrom Poggiantietal. ( 2001a ),N05isfrom Nelanetal. ( 2005 )andS07isfrom Smithetal. ( 2008a );hIi:meanosetbetweenourstudyandother(IhereIother)andtheerrorinthemean;stdstandarddeviationofthedierences;exp:standarddeviationexpectedfromtheerrors.[Reproducedfrom Matkovicetal. ( 2008 ).] 58

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InFigure 3-1 A)weshowtheL{relationincludinggalaxieswithmeasurementsfromMLKC02, Hudsonetal. ( 2001 ),EFAR( Collessetal. 2001 ),andoursample.Thetotalnumberofgalaxiesis167,where24galaxiesareclassiedasgEs.Incaseofmultiplemeasurementsfromtheliterature,weusedaminimumvarianceweightedaverageofvelocitydispersionsandcalculatedtheerrorintheweightedaverage.Whenthosegalaxieswereincommonwithoursample,weusedthesameprocessbutadoptedthesymbolusedforoursample. WehavederivedthephotometryinMRfrom Gutierrezetal. ( 2004 )fortheMLKC02andoursample,whiletheothercatalogsincludedtheirownphotometry.Weaveragedtheactualluminositiesforgalaxieswithmultiplephotometry.Theuncertaintyintheaverageofthemagnitudeswas0.015mag,exceptfor3galaxies,whichweexcludedfromthesamplesincetheiruncertaintywaslargeandmostlikelyduetoacatalogmismatch.Outofourcentralsampleof72galaxieswithmeasurements,17werenotintheGutierrez 59

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Luminosity{relationfor:A)AlltheComagalaxiesinoursampleandthosefromtheliterature(seegurelegend).ThedashedlineisthemostrecentFJfromliterature,L/3:92( Forbes&Ponman 1999 );thedash-dotlineisaleastsquarestwhenminimizingtheresidualsinMR;thedash-dot-dotwhenminimizinginlog;andthesolidlineisthebisectort.B)Includingonlygalaxiesforwhich Gutierrezetal. ( 2004 )hadthebulge-to-totalratio(B/T)measurements.Thesolidlineistheleastsquaresbisectortforbulge-dominated(solidtriangles),thedashedlineforbulge+singleexponentialcomponent(opensquares),andthedottedlineforsingleexponentialcomponent(opencircles)galaxies.[Reproducedfrom Matkovic&Guzman ( 2005 ).] list.InthiscasewehavederivedR(Johnsonlter)byusingthebjmagnitudeslistedintheGMPcatalog.WeusedaleastsquaresttoobjectsthathadbothbjandRandobtainedthetransformationR=(1:0500:030)bj2:4220:529betweenthesetwomagnitudes. TheL{relationinFigure 3-1 A)exhibitsacurvatureorachangeofslope.Toallowforacomparisonwithearlierstudiesweperformleast-squareststothegEsandfaintearly{typegalaxiesseparately.WepartitionthegEsfromtheotherearly{typegalaxies 60

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Luminosity{least-squarests GalaxyTypeRegressionInterceptSlopermsn(1)(2)(3)(4)(5)(6) allgalaxiesMRjlog-10.9490.375-4.5510.1800.510mag1.820.07allgalaxieslogjMR-8.8460.444-5.5850.2100.565mag2.230.08allgalaxiesBisector-10.0010.376-5.0170.1790.521mag2.010.07 Bulge{dominatedBisector-9.1940.565-5.3700.2690.653mag2.150.11Bulge+Exp.Comp.Bisector-11.3740.608-4.3700.3060.464mag1.750.12ExponentialComp.Bisector-12.0210.687-4.1010.3190.628mag1.640.13 Notes.{Col.(1).{Galaxytypeinregression;Col.(2).{Regressionorder:MRjlog,meansminimizinginMRonlog.IncaseofSingleExponentialComponent,Bulge+SingleExponentialComponentandBulge{dominatedgalaxiesweonlyshowthebisectorvalue(Figure 3-1 ,rightpanel);Col.(3).{Interceptanduncertaintyinlinearregression;Col.(4).{Slopeanduncertaintyinlinearregression;Col.(5).{rmsofpointsaroundthelineart;Col.(6).{ThepowernofL/n.[Reproducedfrom Matkovic&Guzman ( 2005 ).] withthedottedlineatMR>22:17mag.ThedashedlinerepresentsamostrecentL/nfromtheliterature( Forbes&Ponman 1999 ),wherelog=0:102MB+0:243,correspondington=3:92.Weobtainedtheordinaryleastsquarest(OLS,asdescribedby Feigelson&Babu ( 1992 )forallthegalaxiesexcludinggEs.Thedash-dotlinerepresentsthetwhichminimizestheresidualsinMR,thedash-dot-dotminimizationinlog,andthesolidlineisthebisectorline.ThedetailsofthetsaresummarizedinTable 3-1 .OnlythegalaxywithaverylargeobservationalerrorfromMLKC02(seeFigure 3-1 )isexcludedfromthelinearregression. Faber&Jackson ( 1976 )showedthatluminosityofgEscorrelateswellwithvelocitydispersionforthesegalaxies.TheL{,orFaber-Jackson,relationcanbeexpressedasL/n,wherenwasoriginally4( Faber&Jackson 1976 ; Sargentetal. 1977 ; Schechter&Gunn 1979 ; Schechter 1980 ; Tonry&Davis 1981 ; Terlevichetal. 1981 ). Tonry ( 1981 )wasthersttonoteaslightchangeofslopeintheL{relationsuggestingthatn4formoreluminousobjectswhilen3forfaintergalaxies.Thisresultwasconrmedby Daviesetal. ( 1983 )whofoundn=4:20:9forgalaxiesbrighterthanMB=20(MR<21:67)andn=2:40:9forthosefainterthanthismagnitude,and Heldetal. ( 1992 )whofoundn=2:5fordEs.Unfortunately,thedatasamples 61

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Tonry ( 1981 ); Daviesetal. ( 1983 ); Heldetal. ( 1992 )onlyincludedadozenofthefaintearly{typegalaxies.TofurtherinvestigatetheLrelationforawiderangeofluminositieswepresentasampleof143galaxieswith22.MR.17:5mag. TheL{relation(Figure 3-1 )derivedforthislargesampleexhibitsachangeofslope;theslopeoffaintearly{typegalaxiesisshallowerthanthatofgiantellipticalgalaxies.Followingtheresultsofpreviousstudiesandincludingtheseinourdatasetwendthatthevalueofn4tsthebrightEendofthediagram.IncontrasttogEs,weobtainL/2:010:36forfaintearly{typegalaxies(adoptingthebisectort).Thisrelationspansarangeof4.5magnitudesfainterthanMR=22:17mag,thelowerlimitofgEs,andisthelargestsampleofthefaintearly{typegalaxiesinasingleclusterthusfar.Ourresultderivedfor143galaxiesisconsistentwithL/2:40:9derivedby Daviesetal. ( 1983 )fortheir14faintellipticalsandthatof deRijckeetal. ( 2005 ),anditisinconsistentwiththestandardFaber-Jacksonrelation.ThisraisesintriguingquestionsconcerningthephysicalprocessesresponsibleforthechangeofslopeintheL{relation. WenotethatourgalaxiesexhibitasmallsystematicosetdependingontheirSNR.However,ifcorrectedforthiseectof6percentatthelowestandlowestSNRgalaxies,theslopeofthefaintearly-typegalaxieswouldbeevenshallowerthanderivedhere. AfeasibleexplanationofthedierentslopebetweengEsandfaintearly{typegalaxiesmaybeduetothepresenceofothertypesofgalaxiesatlowerluminosities.Weusedthephotometryof Gutierrezetal. ( 2004 )toclassifygalaxiesfromthepresentcombinedsample.Incaseswherethebulge-to-total(B/T)luminosityratiosfrom Gutierrezetal. ( 2004 )datawereunreliable,weusedtheNASA/IPACExtragalacticDatabase(NED).WedenedgalaxieswithB=T=1:0asbulge-dominated,0:5
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3-1 ,right)indicatesthatthereisaslightdierenceintheslopesforeachtype.AlthoughadierentL{relationcanbederivedforeachgalaxytype,theindividualrelationsarestillwithin3standarddeviationsofeachother(Table 3-1 ).Itisnotsurprisingthatasignicantnumberofsingleexponentialcomponentgalaxiesappeartobepresentinthissamplesincethislightprolebestdescribesthedwarfellipticalgalaxies.However,classifyingtheselow-luminositygalaxiesisdicultwithoutresolvedphotometryandwealsonotediscrepanciesinclassicationdependingontheliteraturesource.Nonetheless,theslopesofallthreegalaxytypes,singleexponentialcomponent,bulge+exponentialcomponentandbulge-dominatedgalaxies,arestillindisagreementwiththeFJrelation,butconsistentwithL2:010:36asderivedearlierforthelow-luminosityearly{typegalaxies. Tonry ( 1981 )attributedthechangeofslopeintheL{relationtolessluminoussystemshavingsignicantrotation.Subsequently, Daviesetal. ( 1983 )investigatedrotationalpropertiesofaboutadozenfaintEswith20:5
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b1 Thisexpressionyieldstheamountofrotationafaintearly{typegalaxywouldneedtohaveinordertofollowthesamerelationbetweenluminosityandhv2ifollowedbygEs.Undertheseassumptions,theobservedchangeofslopeintheL{diagramwouldsimplybetheresultofnotincludingtherotationalcomponentinthekinematicenergyoffaintearly{typesystems. UsingtheexpressionabovewecancalculatetheexpectedVrot=forgalaxiesatdierentluminositiesandvelocitydispersions,andcomparethemtotheobservedvalues.Weshowthiscomparisonthroughasetofgraphs(seeFigure 3-2 ).TheleftpanelshowsVrot=vs.MR,andtherightpanelVrot=vs..ThesolidlineinbothpanelsrepresentsthepredictedvalueforVrot=,whilethepointsarethemostrecentdatafromtheliterature( Daviesetal. 1983 ; Simien&Prugniel 2002 ; Pedrazetal. 2002 ; Geha,Guhathakurta,&vanderMarel 2003 ; vanZee,Skillman,&Haynes 2004b ). AccordingtothepredictedrelationbetweentheVrot/andMR,Vrot/wouldhavetoincreasesteadilytowardthefaintend(leftpanelofFigure 3-2 ).Forexample,afaintearly{typegalaxywithMR=18magand=39kms1wouldhaveVrot==3:2,orVrot=123kms1.Thisvalueseemsunreasonablylargewhencomparedtoobserved 64

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RotationaleectsonMR{.ComparisonbetweenobservedandpredictedrelationVrot/A)withMRandB)with.InbothpanelsthesolidlinerepresentsthepredictedVrot/agalaxywouldhavetohaveinorderforthefaintearly{typegalaxiestofollowtheL/4relationdenedforgEs.Thesymbolsareasshowninthegurelegend.[Reproducedfrom Matkovic&Guzman ( 2005 ).] Vrot/valuesfrom Gehaetal. ( 2003 ),whichrangefromaslowas0.01to0:5.Thediscrepancyisworseforthefaintestgalaxies.WeconcludethatthepredictedVrot/isinconsistentwithobservationsofearly{typegalaxiesfainterthanMR=20:5mag.Therefore,itisimplausiblethatrotationissolelyresponsibleforthedierenceintheL{slopebetweenfaintearly{typeandgiantellipticalgalaxies. AnindependentconrmationofL/2hasrecentlybeenprovidedby deRijckeetal. ( 2005 ).TheyinvestigatehowwelldierentgalaxyformationscenariosreproducethisslopedierencebetweenthegiantellipticalsandbulgesofspiralsanddEs.Thesemi-analyticalmodelswhichincludequiescentstarformation,post-mergerstar-burstsandgas-losstriggeredbysupernovawindsseemtodescribethiseectwell. 65

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Forbes&Ponman ( 1999 ).Wedonothavedirectagemeasurementsfortheseobjectsyet.However,inthefollowingsectionweplotthecolorrelationforourgalaxies.Underassumptionsdescribedinthefollowingsectionweareabletoinvestigatetheeectsageandmetallicityhaveonthisrelation. Inconclusion,faintearly{typegalaxiesfollowawell-denedL/2:010:36relation.ThisrelationisdistinctfromthetraditionalFaber-JacksonrelationdenedforgiantEgalaxiesandmightindicatethatgEsshouldnolongerbeviewedascanonicalearly{typegalaxies.Wealsoconcludethatrotationinthefaintearly{typegalaxiesisnotresponsibleforthechangeinthesloperelativetothatderivedforgEs. 3.2.1DerivingtheColor{Relation 3-3 showstheJohnsonB{Rmag(fromTrentham,unpublisheddata)vs.logforgalaxiesinoursample,includingsomegalaxieswithmeasurementsfromtheliterature.Incaseofthegalaxiesincommonwiththeliterature,weperformedaweightedaverageonthevelocitydispersionsandtheirerrors.WeplotonlygalaxiesthathaveB{Rmeasurements,exceptinthecaseofthe Hudsonetal. ( 2001 )samplewherewehavederivedthecolorsfromtheGMPcatalog.ThetransformationbetweentheGMPb{rcolorsandB{Rofoursamplewas:BR=(1:1280:098)(br)j0:5350:178.The 66

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Color{relationforearly{typegalaxies.Inbothpanelsthedash-dottedlinerepresentsaleastsquareslineartwhenminimizingtheresidualsinB{R,dash-dot-dotwhenminimizinginlog,andthesolidlineisthebisectorline.PanelA)showstheC{relationforthegalaxiesintheComacluster,whileB)includesgalaxiesfromalltheclustersstudiedby Faberetal. ( 1989 ).[Reproducedfrom Matkovic&Guzman ( 2005 ).] Table3-2. Color{leastsquarests.[Reproducedfrom Matkovic&Guzman ( 2005 ).] PanelRegressionInterceptSloperms A)B{Rjlog0.9040.0450.3230.0230.066maglogjB{R0.4350.1620.5550.0810.066magBisector0.6800.0890.4340.0450.071mag B)B{Rjlog1.0070.0260.2790.0110.049maglogjB{R0.5950.0800.4630.0360.049magBisector0.8070.0410.3690.0180.052mag derivedC{relationsminimizingresidualsineitherlog,B{R,orusingthebisectorforallthegalaxiesinthegurearefoundinTable 3-2 .Theright-handpanelincludesgalaxiesfromclustersobservedby Faberetal. ( 1989 )whereweusedthecolortransformation(BR)=(BV)+0:71from Fukugitaetal. ( 1995 ). 67

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Terlevichetal. 2001 ,andreferencestherein). Caldwell ( 1983b )and Prugnieletal. ( 1993 )foundthatfaintearly{typegalaxiesroughlyfollowtheCMRforgiantEs.Asimilar,distance-independent,relationisthecorrelationbetweencolorand,C{. InFigure 3-3 ,weshowtheC{relationforfaintearly{typegalaxiesandgiantellipticals.Allofthegalaxiesseemtofollowthesamerelation,althoughwenotethelackofgEsintheplotcontainingonlyComagalaxies(panelA).WeconrmedtheuniformityoftheC{relationbycheckingourresultwiththeU{Vcolorsof Terlevichetal. ( 2001 ).PanelB)includesgalaxiesfromdierentclusters( Faberetal. 1989 )andindicatesthatbothfaintandgiantearly{typegalaxiesfollowthesameC{relation. Sinceluminosityandarerelated,theC{relationisequivalenttotheCMR,whichinturnsuggestsamorefundamentalrelationbetweengalaxymetallicityandmass.Althoughcolorsdependbothonmetallicityandagechangesinthestellarpopulations,theevidencesofarsupportsthatmetallicitychangesareresponsiblefortheslopeoftheCMR,whileagedierencescontributetothescatterobservedaroundthatrelation.InthefollowinganalysisweassumethatthesameappliestotheC{relation.Notehowever,thatitislikelythatbothageandmetallicityaecttheslopeandthescatter. Bernardietal. ( 2005 )showthatgalaxieswithlargevelocitydispersiontendtobeolder.Theyalsoshowthatataspecic,galaxieshaveawiderangeinbothageandmetallicityinawaythattheoldergalaxieshavesmallermetallicitiesandtheyoungergalaxieslargermetallicities.AssumingthattheintrinsicscatterintheC{relationispredominantlydue 68

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Boweretal. ( 1992 ,hereafterBLE92)showedthatitispossibletodetermineminimumagesforgalaxieswithdierentformationscenariosbyimplementingevolutionarystellarpopulationsynthesismodelsandtheintrinsicscatterintheCMRofthesegalaxies.WecloselyfollowtheirmethodbutusetheintrinsicscatterderivedfromtheC{relationforourgalaxiesinstead.Sincetheuncertaintyintheobservedparametersis0.024mag,theintrinsicscatteris0.067. Weusedtheevolutionarystellarpopulationsynthesismodelsof Bruzual&Charlot ( 2003 )tosimulateagalaxywithanexponentiallydecliningstar-burst,=1Gyr,andaChabrierIMF.InFigure 3-4 ,toppanel,weshowhowtheB{RcolorofagalaxywithmetallicityZ=1or0:4Zevolveswithtime.Hence,wendhowtherateofchangeofcolorvarieswiththeageofthegalaxy(middlepanel).Thecolorchangecanberelatedtotheintrinsicscatterinthecolorandtherangeofepochsformajorstar-formationeventsbytherelation: where(BR)isthescatterinB{RcolorandSFEistherangeinthestarformationepoch(BLE92).ThisisinaccordancewiththeassumptionthatthescatterinC{isonlyduetoagevariation,orinthiscase,tothescatterinthestarformationepoch. Afterndingthed(B{R)/dtatdierentagesofthegalaxy,andusingtheintrinsicscatterof0.067intheC{relation,wederivevaluesforthemaximumrangeinthestarformationepoch.InFigure 3-4 (panelC)weshowthemaximumrangeinthestarformationepochsasafunctionoftheformationtime,constrainedbytheintrinsicscatterinC{foroursample.Iftheaverageageofourgalaxysampleis10Gyrold,forexample,themaximumrangeinitsstarformationepochwillbe3Gyr.However,inordertodeterminetheupperlimitonvariationsintheagesofgalaxieswemusttakeintoaccountthatthescatterintheSFepochwillalsodependonhowthegalaxieswereformed. 69

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A)B{Rcolorvs.galaxyage,assumingexponentialburst.ThesolidlinerepresentsagalaxywithZ,whilethedottedlineis0:4Z.B)TherateofchangeofB{Rcolorwithtimeofformation.Thedashedlinesrepresentdierentparameterswhichdependonhowsynchronizedthegalaxyformationisassumedtobe.of1.0correspondstonocoordination,while=0:1isforstrongcoordination.C)Scatterinthestarformationepochvs.galaxyage.Assumingstrongcoordinationingalaxyformation,thegalaxyageof6Gyrs(frommiddlepanel)wouldimplythatitsscatterinSFepochis1Gyr.[Reproducedfrom Matkovic&Guzman ( 2005 ).] 70

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3-4 ,panelB)withascatterinthestarformationepochof1Gyr(wheretH=13:7Gyr,M=0:3and=0:7).Asareference,agalaxythatis6Gyroldmusthaveformedatredshiftz0:7.NotethatwecanonlyputalowerlimitontheagesofgalaxiesandanupperlimitonthescatterintheirSFepoch,providedthatweknowthelevelofcoordinationofgalaxiesduringtheirformation. AlthoughweinitiallyassumedthatthescatteroftheC{relationisduetoanagespreadaroundtheformationepochinasingleburst,secondaryburstsofstarformationwillalsocontributetothescatter.Infact,observationsofgalaxiesinclustersatredshiftsz0:5pointtoapossibilityofsecondarystar-bursts( Butcher&Oemler 1978 1984 )inwhatmaybecometoday'spopulationoffaintearly{typegalaxiesinclusters.Bymodellingthesecondarystar-bursts,wecanplaceupperlimitsonthestar-burststrengths. Assumingthatallgalaxiesformedatt>10Gyrsandhadasecondarystarburst5Gyrsago,weagainusethemodelsof Bruzual&Charlot ( 2003 )withtwoexponentiallydecliningburststomakethisconstraint.ForthispurposewealsofollowthediscussionofBLE92.UsingthescatterintheB{Rcolorforvariousuniformlydistributedburststrengths,solarmetallicityandaChabrierIMF,wendthetypicalrmsburststrengthrtyp=(BR)=0:17(theratiobetweenthestellarmassofthesecondarybursttothatoftheinitialburst).Ourobservedscatterof(BR)rms=0:07placesanupperlimitonthesecondaryburstof40percentbystellarmassoftherstburst.Unfortunately,this 71

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Inconclusion,wehaveshownthatthereisawelldenedrelationbetweencolorandforfaintearly{typesystems.Assumingthatmetallicitychangesareresponsiblefortheslopeofthiscorrelationwhileagevariationsarethemaincontributortothescatter,itispossibletoconstraintheagerangeofmajorstarformationeventsforagivenformationepoch.However,itisdiculttodecoupletheeectsofageandmetallicityusingcolors.InChapter4westudythedetailedstellarpopulationpropertiesoffaintearly{typegalaxies,bothageandmetallicity,usinglinestrengthindicesandstellarpopulationsynthesismodels.Furthermore,wewilltestifthegalaxiesinthecenteroftheclusteraremoremetal-richthanthoseintheoutskirts,sincethisispredictedbythegalaxyharassmentmodel( Moore,Lake,&Katz 1998 ). 3.1.1 and 3.1.2 )wecharacterizedtheinternalkinematicsoflow-massearly{typegalaxies.ThecharacterizationwasbasedondE/dS0galaxieswhichliewithinthevirialcoreoftheComacluster,alsothedensestregionofthecluster.InthissectionwecomparethekinematicsofthedE/dS0galaxiesinthecenterandtheclusteroutskirts(justoutsidethevirialcore).WerstinvestigatetheL{relationinSection 3.3.1 ,followedbytheC{relationinSection 3.3.2 WederivetheL{relationforouroutskirtssampleoffaintearly{typegalaxiesinFigure 3-5 .WendthattheslopesoftheL{relationsforthecenterandtheoutskirts 72

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ComparisonofL{relations ClusterRegionInterceptSlopermsn Center10:0010:3765:0170:1790.521mag2:010:07Outskirts11:2530:3234:4810:1630.605mag1:790:07 regionsoftheComaclusterareconsistentwithin2standarddeviations.ThelinearregressionresultsarepresentedinTable 3-3 Wealsoderivedthermsscatteraroundthettedlinesforbothsamples.Theoutskirtsgalaxiesshowalargerscatter0.605dexthanthegalaxiesinthecenterwhosescatteris0.521dex.Theobservedscatterfortheoutskirtsdatais0.28magcomparedto0.22maginthecenter.Therefore,theintrinsicscatterisalsolargerfortheoutskirtsgalaxies. Althoughtheslopesofthetworegionsofdierentdensitywithintheclusterareconsistentwithoneanother,wedonoteatendencyfortheouterComagalaxiestohaveslightlyhigherluminositiesatagiven.Thereisapossibilitythatthegalaxiesintheouterregionoftheclusterareonaverageyoungerthanthegalaxiesintheclustercenterasyoungerageswouldmakethemmoreluminous.Furthermore,theintrinsicscatteroftheoutergalaxiesisalsolargerthanthescatterforgalaxiesinthecenter.ThismayimplythatthedEs/dS0shaveawiderrangeinages.WeinvestigatethishypothesisinSection 3.3.2 withcolor-andcolor-magnitudediagrams.However,weaddresstheageestimatesforthesegalaxiesviatheirspectrallinestrengthsinChapter4. 73

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Luminosity{plotswithoutskirts.Theredlineisttothecentraldata,whilethebluelinerepresentsthettotheoutskirtsdata. WeretrievedthecolorsfromtheDataRelease6oftheSloanDigitalSkySurvey(SDSS),whichobservedtheComacluster.Figure 3-6 showstheC{relationusingtheSDSSg-rcolor(panelA).Wealsoshowthecolor-magnitudediagramforourtwosamples.InChapter4wedeterminedthatanumberofgalaxiesintheoutskirtssamplehaveemissionandwemarkedtheminFigure 3-6 withpurplediamondsontheplot. Thereisnosignicantdierenceintheslopesnorthedistributionofthecentralandoutskirtsgalaxiesinthecolor{relation.However,theoutskirtsdataexhibitalargerintrinsicandrmsscatteraboutthelineart.Thismayindicatesmalldierencesintheagesofgalaxiesforthetwoenvironmentswithinthecluster,ifweassumethatthescatterofthecolor{relationismostlyduetoage.WealsonotethatthereisalargernumberofgalaxieswithbluecolorsfortheoutskirtsdE/dS0s,thanforthecenter.Weplotahistogramofg-rcolorsinFigure 3-7 tocheckforanydierencesinthecolor-distribution.Thereisaslighttendencyfortheoutskirtsgalaxiestowardbluercolors. 74

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EnvironmentaleectsonC{relation.TheredcirclesrepresentourcentralComasample,whilethebluesquaresdepicttheoutskirtsgalaxies.ColorswerederivedfromSDSS(DR6).Thepurplediamondsmarkthegalaxieswithemissionfeatures.Thesegalaxieswerenotincludedinthelinearts.Theredsolidlineandthebluedashedlinerepresentthelinearregressionlinesforgalaxiesinthecenterandtheoutskirts,respectively. Table3-4. Color{andcolor-magnituderelations RelationRegionInterceptSlopeIntrms g-rvs.Center0:4170:0280:1700:0140.0290.030Outskirts0:3760:0620:1940:0350.0520.054 g-rvs.rCenter1:2370:0540:0310:0030.0350.035Outskirts1:2280:1630:0320:0100.0650.065 ThecolumnmarkedbyIntrepresentstheintrinsicscatter,whilethermscolumnisthescatterofgalaxiesaroundthelinert. Infact,bothtendenciesofseeingbluercolorsandalargerscatteroftheouterdEs/dS0sseemstobemagniedinthecolor-magnitudediagram(panelB).Thismayhinttowardmoreuniformagesand/ormetallicitiesforthesegalaxiesinthecenteroftheclusterthanfortheoutskirts.Sincebluercolorseitherindicateyoungeragesorlowermetallicities,wecanexpecttondthatthegalaxiesintheoutskirtsareeitheronaverageyoungerorhavelowermetallicitiesthanthegalaxiesinthecoreofthecluster.WefurtherinvestigatethisinChapter4onceweusespectroscopicinformationtoderivetheagesandmetallicitiesforthesegalaxies. 75

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Histogramofg-rcolorforcenterandouterComagalaxies. 76

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4-1 .Inthisplot,weincludegalaxiesfromoursampleandgalaxiesintheComaclusterfrom Nelanetal. ( 2005 ).Wecombinedthetwodatasetsandbinnedindexmeasurementsforgalaxiesofsimilarvelocitydispersion.Allthebinshavethesamenarrowintervalinlogof0.118dex.Thereddiamondsrepresenttheaveragevalueofeachbinweightedbytheuncertaintiesintheindexmeasurements. Dierenttypesofbehaviorsemergebetweentheindicesandvelocitydispersionsfortheseearly-typegalaxiesintheComacluster(Figure 4-1 ).TheI{relationscannotallbedescribedbyalineart.Tomoreeasilydescribethedata,wegroupedtheI{trendsaccordingtothesteepnessoftheirslopeandthebehaviourofthelow-galaxieswithrespecttothehigh-ones.WeplottedtheBalmerindicesinaseparatecolumninthegure. Thethreetypesofrelationsare: 77

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IndexN<100N>100I<100I>100std<100std>100probKS<100>100 StatisticsforgroupIofI{plots.N:numberofgalaxieswith<100kms1andwith>100kms1excludingthe\o-gridgalaxies";I:averagevalueoftheindexforlow-andhigh-galaxies;std<100andstd>100:standarddeviationofthepointsforthelow-andhigh-galaxies;probKS:theKSprobabilitythatthetwosetsaredrawnfromthesamedistribution;<100and>100:SpearmanRankcorrelationcoecientsforlow-andhigh-galaxies.[Reproducedfrom Matkovicetal. ( 2008 ).] WeplotthesethreeinthedierentcolumnsofFigure 4-1 .TomoreeasilydescribethedierencebetweenthegroupsofI{trends,theplotsalsoincludeseparatelineartstolow-andhigh-galaxies(dashedanddash-dotedlinesrespectively),eventhoughthecorrelationcoecientsforthesetsarelow. Itisimportanttonotethatthisgurecontains13low-galaxies,markedasyellowcircles,whichlieoutsidethemodelgrids,theirHmeasurementsaresystematicallyosetfromthe Smithetal. ( 2008b )data,andtheypotentiallyhavespuriousmeasurementsforsomeindices(discussedinSection 4.2.1 ).Weexcludethesegalaxiesfromfurtheranalysisandrefertothemas\o-grid"galaxies. 4-1 )exhibitsnon-linearI{relations.Galaxieswith>100kms1showaatrelationwithindicesinthisgroup,whilethelow-galaxiesexhibitawiderangeoftheindexvalueinquestionandmuchsteeperslopes.Thelow-galaxiesalsohavealargerscatterandalowermeanvaluethantheirmoremassivecounterparts. InTable 4-1 ,wecalculatedthestandarddeviationofthelow-andhigh-galaxiesandweshowthisgraphicallyinthegure.Thescatterofindexvaluesis2timeslargerforthelow-galaxiesthanitisforthehigh-ones.Additionally,themeanvalueofanindexdiersforlow-andhigh-galaxiesby7{27%. 78

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Index{logplots.The\o-grid"galaxiesarenotincludedinbinsandslopecalculations.IncludesdatafromtheNFPSsample(blueopencircles),andoursample(blacklledcircles).Webinnedthedatabyvelocitydispersionwhereeachbincontainsanequalnumberofobjects.Thereddiamondsareweightedaveragevaluesofeachbin.Theyellowcirclesrepresentthegalaxieswhichdonottonmodelgrids.Theverticaldottedlinecorrespondstolog=2:0kms1.ThefourcolumnsofthisgurearemarkedasgroupsI,II,IIIandBalmerlines.Theycorrespondtothedierent`types'ofrelationsbetweentheindicesandvelocitydispersionasdiscussedinSection 4.1 .InthebottomrightcornerofI{plotsingroupIweshowthe1-sigmascatterforlow-(log<2:0)ontheleftandhigh-galaxiesontheright.InthecaseofgroupIIandtheBalmerLinesgroup,theselinescorrespondtotheintrinsicscatterforlow-andhigh-respectively.ForgroupIII,weshowtheintrinsicandobservedscatter,respectively,inthebottomrightcorner.TheblacklinesrepresentlineartsforallthegalaxiesintheindividualI{gures,whilethegreendashedandlightbluedash-dottedlinemarklineartstolow-andhigh-galaxiesrespectively.[Reproducedfrom Matkovicetal. ( 2008 ).] 79

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IndexNInterceptSlopeInt<100Err<100Int>100Err>100probKS StatisticsforgroupIIofI{plots.N:numberofgalaxies;InterceptandSlope:zeropointandtheslopeofthelineartwithuncertainties,respectively;Int:intrinsicscatterforthetwosub-samples,i.e.standarddeviationofresidualsbetweenthepointsandthelineartforeachsub-sampleofgalaxies(lowandhigh-);Err:scatterduetotheerrors;probKS:theKSprobabilitythatthetwosetsaredrawnfromthesamedistribution;:Spearman-Rankcorrelationcoecient.[Reproducedfrom Matkovicetal. ( 2008 ).] Table4-3. IndexNInterceptSlopeIntErr StatisticsforgroupIIIofI{plots.N:numberofgalaxies;Interceptandslopeofthelineartwhichtakesintoaccountboththeuncertaintiesintheindexandvelocitydispersion;Int:intrinsicscatter;Err:scatterduetotheerrors;:Spearman-Rankcorrelationcoecient.[Reproducedfrom Matkovicetal. ( 2008 ).] WeperformedtheKolmogorov-Smirnovtest(KS)todeterminewhetherthetwogroupsofgalaxiescomefromthesamedistribution.InTable 4-1 weshowtheKSprobabilitythatthetwosetsaredrawnfromthesamedistribution.SincethevaluesoftheKSprobabilityarequitesmall,weconcludethatlow-andhigh-galaxiesmayindeedcomefromdierentpopulations. 4-1 showsindiceswhichexhibitweaklinearrelationswith,haveshallowslopes,andnotablylargerscatteratthelow-end.Forthisgroupofindicesweperformalinearttakingintoaccountboththeerrorsintheindexand,andcalculatetheintrinsicscatter(Intr)andthescatterpredictedbytheerrors(Err)forbothlow-andhigh-galaxies(Table 4-2 ).Theintrinsicscatterforthelow-andhigh-sub-samples,isshowninthelowerright-handcornerofFigure 4-1 .WealsoperformedtheKStestontheresidualsbetweentheindicesandtheirrespectivelinearts 80

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Galaxieswith<100kms1havealargerintrinsicscatter Ca4227istheonlyindexforwhichtheintrinsicscatterissmallerthanthescatterduetotheerrorsforbothlow-andhigh-galaxies.Theremainingindicesdisplayalargerintrinsicscatterthanthescatterduetoerrors(1:22:9times)forbothlow-andhigh-sub-samples,exceptforFe5015forwhichtheintrinsicscatteriszero. Althoughlineartrendsemergefromvisualinspectionforthisgroupofindices,theircorrelationcoecients,,arelow(lowerthan0.5).ThisindicatesweakI{correlations,especiallyforCa4227.AccordingtotheKStest,onlyFe5335andhFeishowahighenoughprobabilitythatthelow-andhigh-galaxiesaredrawnfromthesamepopulation. 4-1 weshowthemetallicindiceswhichdisplayatightrelationwith.ThisisquantiedinTable 4-3 ,whereweperformedtheSpearman-Rankordertest,andweincludetheslopesandinterceptsoflineartstotheserelations. Weshowtheintrinsicscatterandthescatterexpectedbytheerrorsinthebottomright-handcornerofFigure 4-1 foreachoftheseindices.Forallindicesinthisgrouptheintrinsicscatterislargerthanthescatterduetoerrors,althoughthetwoareverycloseinvalueforMg1andMg2.Moreover,thecorrelationcoecientsforalltheindicesinthisgroupimplyarobustrelationwith. Pni=1(Indexi(intercept+slopei))2

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Nelanetal. ( 2005 )within1standarddeviation(Table 4-3 ). 4-3 ). IncaseofH,thescatterforlow-galaxiesisasymmetricwithmoregalaxieshavingalowerHindex.TogetherourandtheNFPSsamplehaveastandarddeviationfromthelineartof0.306with23galaxieshavingalowervalueoftheirHindex,comparedtothescatterof0.243for18galaxieswithahigherHindex. Terlevichetal. 1981 ; Dressler 1984 ; Guzmanetal. 1992 ; Benderetal. 1993 ; Jorgensenetal. 1996 ; Kuntschneretal. 2001 ).Recentstudies,however,showthattheserelationsaremorecomplexthanoriginallythought.Forinstance,Mg2maybesignicantlydependentonbothage(15%)andrelativeabundancesofheavyelements(2030%)( Mehlertetal. 2003 ; Thomasetal. 2005 ).WhileHmainlydependsonage,italsochangeswithmetallicityandchemicalcomposition( Sanchez-Blazquezetal. 2006a ). Inthisstudywepresent,forthersttime,theMg2,HandrelationsbetweenotherLickindiceswithdownto30kms1forahomogeneoussampleofearly-typegalaxiesinoneofthedensestenvironmentinthenearbyuniverse:thecenteroftheComacluster.WeshowthattheMg2relationspanstheentirerange30{260kms1withasmallscatter.WealsoinvestigaterelationsbetweentheH,Handthemetallicity 82

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Fisher,Franx,&Illingworth 1995 ; Jorgensen 1997 ; Trageretal. 1998 ; Kuntschner 2000 ; Caldwell,Rose,&Concannon 2003 ; Nelanetal. 2005 ; Sanchez-Blazquezetal. 2006a )andisusuallyinterpretedasaninterplaybetweenageandmass.However,recentevidencethattheHrelationisweakoratintheComacluster( Mehlertetal. 2003 ; Sanchez-Blazquezetal. 2006a )mayalludetowardsitsdependenceontheenvironment.Furthermore,thereissomeevidencethattheagevariationofgalaxiesinclustersisnotlargeandtheHrelationmaymostlybedrivenbymetallicity Kuntschner&Davies (inFornax 1998 )orbyboth,variationsinglobalmetallicityandrelativeabundanceofdierentheavyelementsforthegalaxiesintheComacluster( Sanchez-Blazquezetal. 2006a ). Weconrmthattheearly-typegalaxiesinthecoreoftheComaclustershowaweakanti-correlationbetweentheirHlinestrengthandvelocitydispersions.Ourresultissimilartothatof Caldwelletal. ( 2003 ,hereafterCRC03)whoalsondalargeasymmetricscatterforthelow-galaxies.However,theCRC03datashowanoppositeeectinasymmetrywheremoregalaxieshavehigherBalmerline-strengths,whilewendthatmoregalaxieshavelowervalueoftheirHindex.Itispossiblethatthiseectisenvironmental,sinceCRC03sampleincludesgalaxieslowerdensityenvironments(Virgo,theeldandlowerdensityenvironments)thanours. Ineithercase,weextendtheHrelationtolow-galaxiesdownto30kms1in,orby0.2dexwhencomparedtoCRC03.WeconrmthatHandareanti-correlated,andthatthisrelationhasanasymmetricscatterinthelow-regime. 83

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Terlevichetal. 1981 ; Gorgas,Efstathiou,&AragonSalamanca 1990 ; Guzmanetal. 1992 ; Bender,Burstein,&Faber 1993 ; Bernardietal. 1998 ; Collessetal. 1999 ; Jrgensen 1999 ; Concannon,Rose,&Caldwell 2000 ; Kuntschner 2000 ; Poggiantietal. 2001a ; Proctor&Sansom 2002 ; Worthey&Collobert 2003 ; Mehlertetal. 2003 ; Sanchez-Blazquezetal. 2006a ).ThetightrelationbetweenMg2andhasbeeninterpretedasevidencethatallellipticalgalaxieshavealowdispersioninage( Benderetal. 1993 ; Bernardietal. 1998 ).Furthermore,theparameterdrivingthisrelationhasbeenundermuchdebate.Originally,studiesarguedthattheMg2relationdependedmostlyonmetallicity( Forbesetal. 1998 ; Terlevichetal. 1999 ),whileageandrelativeabundancesofdierentheavyelementshaverecentlybeenproposedtoalsoinuencethisrelation( Trageretal. 1998 ; Jrgensen 1999 ; Trageretal. 2000a ; Kuntschneretal. 2001 ; Poggiantietal. 2001b ; Mehlertetal. 2003 ; Caldwell,Rose,&Concannon 2003 ; Thomasetal. 2005 ; Sanchez-Blazquezetal. 2006a ). OurMg2relationisconsistentwithotherstudiesinboththeslope(Table 4-3 )andthelowintrinsicscatter.Although,wenoteaslightlylargerdispersionaroundthelineforlow-galaxies,forthersttime,weconrmthatthisrelationisrobustfortheentirerange(30{250kms1)in. ThedierentshapesofI{trendsfoundinthisworkareadirectconsequenceofthedramaticallylargerscatterforlow-galaxies. Concannon,Rose,&Caldwell 84

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2000 )alsofoundthatthescatterinindex-relationsislargerforlowmassgalaxies.TheirinterpretationofthisresultfortheHrelationisthatthelowmassgalaxieshaveexperiencedamorevariedstarformationhistoryandhavealargerspreadinage.Similarly, Sanchez-Blazquezetal. ( 2006a )showthatthescatterintheI{relationsismainlyaconsequenceoftheelementabundancesvaryingwithage.WeinvestigatedwhethertheshapesoftheI{relationsarerelatedtothevariationsinindividualelementabundancesusing Tripicco&Bell ( 1995 ), Thomasetal. ( 2003 )and Kornetal. ( 2005 ). Indicesintherstcolumn(groupI)ofFigure 4-1 havestrongFe-dependenceincommon,exceptforFe4531andG4300whichmostlydependonTiandtoalesserextentonFe.Thesecondcolumn(groupII)containsindiceswhichdependonboth,-peakelementsandFe.Inthethirdcolumn(groupIII)weagainndamixtureofelementsdrivingtheindices.Carbon,and-peakelements,MgandOinparticular,doappeartoinuencemostindicesinthiscolumn,withtheexceptionof[MgFe]0whichdoesnotdependmuchonthe[/Fe]ratio( Thomas,Maraston,&Bender 2003 ).Finally,HA;Findicesdependonthe[/Fe]ratioalthoughthisdependencydiminisheswithincreasingmetallicity( Kornetal. 2005 ),whiletheHindexismoderatelyinuencedbyelementalabundanceratios.IftheabundanceratiosarewhatdrivestherelationbetweenthehigherorderBalmerlinesand,thenthereisapossibilitythatthelargerscatteroftheseindicestowardthelow-galaxiesiscausedbythedecreasedmetallicity.Inconclusion,wedonotndanyclearcorrelationsbetweentheshapeoftheI{relationswiththeelementabundancedrivingtheindices,neitherwith-norFe-peakelements. Poggiantietal. ( 2001a )ndthattheslopesoftheindex-magnituderelationscanbeexplainedbytrendsbetweentheageandmetallicitywithluminosity.TheserelationsareanalternateformoftheI{relations,sincemagnitudeandarerelatedviatheFaber-Jacksonrelation. Sanchez-Blazquezetal. ( 2006a )ndthatvariationsinthesetwoparametersarenotsucienttoexplaintheI{slopes.TheyconcludethatalikelyexplanationforthedierentI{relationscouldbetherelativeabundanceofelements 85

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Worthey,Faber,&Gonzalez 1992 ; Greggio 1997 ; Jorgensen 1997 ; Kuntschner 2000 ; Trageretal. 2000a ; Thomas,Maraston,&Bender 2003 ; Mehlertetal. 2003 ; Thomasetal. 2005 ).Morespecically, Sanchez-Blazquezetal. ( 2006a )showthattheI{slopesarebestreproducedwhenthe-peakelementschangemorethantheFe-peakelementsand,the[Mg/Fe]and[N/Fe]ratioschangemorethantherestofthealphaelementswith.Wendnoclearevidenceinsupportoftheseresults. TheanswertothedierentshapesofI{relationsmaylieinthendingthatC24668,Mg1,Mg2andMgb,allingroupIII(exhibitingrobustlinearrelations),areindependentofthemicro-turbulentvelocityofstellaratmospheres( Tripicco&Bell 1995 ).Thisisunusual,sincemostotherindicesdependonthisparameter.Infact,accordingto Tripicco&Bell ( 1995 ),changingthemicro-turbulentvelocityofstellaratmospheresjustby1kms1causeschangesintheindiceswhicharemoresignicantthanifoneweretodoublethemetalabundance.Hence,theshapeand/ortightnessoftheI{relationsformetalliclinesmaybedeterminedbyhowmuchanindexdependsonthemicro-turbulentvelocityoftheunderlyingstellaratmospheres. Thomas,Maraston,&Korn ( 2004 ,hereafterTMK04),anextensionofTMB03,toderivetheages,metallicitiesandabundanceratiosforoursampleofgalaxies. Themodelsthatweusearebasedontheevolutionarypopulationsynthesiscodefrom Maraston ( 1998 ).Theyaccountforelementratiochangesbasedontheresponsefunctionsfrom Kornetal. ( 2005 )viaamethodsimilartotheoneintroducedby Trageretal. ( 2000b ).TheTMB03modelsspanarangeinagebetween1and15Gyr,totalmetallicity, 86

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Binsinandmodelparametersforcentralgalaxies.[Reproducedfrom Matkovicetal. ( 2008 ).] BinNlogRangehlogihlogAgeih[Z=H]ih[=Fe]i [Z/H],from2:25to0.65,andthe-ratiovaluesof0:00:5.Usinganage-sensitiveindexvs.ametallicity-sensitiveindexwiththemodelsallowsforaderivationofages,metallicitiesand-ratiosofgalaxies. WeuseHasthemainindicatorofage.Thisline-strengthisonlymarginallysensitivetothe/Feratio,whilethehigherorderBalmerlinesaresignicantlyaectedby[/Fe]atsuper-solarmetallicities(TMK04).Furthermore,Hisaprominentfeatureinthespectraofourgalaxies.Asametallicitygaugeweusethe[MgFe]0indexasdenedbyTMB03,albeititisalsodependentonage.Thisindexissensitivetotheoverallmetallicityand,similarlytoH,dependslittleonthe[/Fe]ratio. WeuseacombinationoftheH{[MgFe]0,andhFei{Mgbindicestodeterminetheages,metallicitiesandthe-ratiosforourComagalaxies.However,beforeinvestigatingtherelationsbetweenage,metallicityand[/Fe]wenotethat,duetothetiltofthemodelgridsandthegivenerrorsinindividualindices,theerrorsinderivedagesandmetallicitiesarelikelytobecorrelated( Kuntschneretal. 2001 ; Terlevich&Forbes 2002 ).Inordertoreducetheerrorintheline-strengthindices(andthereforethecorrelatederrorsinthederivedparameters)wehaveobtainedanaveragevalueofeachindexforgalaxieswithsimilarvelocitydispersions(forbins,seeTable 4-4 ).Theseaveragevaluesbinnedbyvelocitydispersionrepresent\average"or\binned"galaxies. Asaprecursorystep,weplotourgalaxiesontopofthemodelgridsinFigure 4-2 .IntheHvs.[MgFe]0plot,wexedthe-ratiotothesolarvalueaccordingtothendingsof Gorgasetal. ( 1997 ),sothatwecandeterminetheagesandmetallicities.WhileinthehFei{Mgbplot,theageissetto6Gyr,asthisisanaverageageofourlow-galaxiesand 87

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Matkovic&Guzman ( 2005 ).Oncetheageisataxedvalue,wecandeterminethemetallicitiesandthe[/Fe].Wemarkedthelow-andhigh-galaxieswithdierentsymbolsandalsoincludedthe\averagegalaxies"forwhichtheindicesarebinnedbyvelocitydispersion. TheComaclustergalaxiesinoursampleexhibitawiderangeinboththeiragesandmetallicities.Further,themoremassivegalaxieshave,onaverage,metallicitiesequaltoorlargerthansolar,whilethelow-galaxies(thethreesmallestdiamonds)haveonaverage,subsolarmetallicitiesandyoungerages.SimilarresultsofwideageandmetallicityrangeshavealreadybeennotedbyotherauthorsintheliteratureforboththeComacluster( Jrgensen 1999 ; Poggiantietal. 2001a ; Mehlertetal. 2003 ; Nelanetal. 2005 ; Sanchez-Blazquezetal. 2006b ),andforthelowerdensityenvironments( Caldwelletal. 1993 ; Jorgensen 1997 ; Trageretal. 1998 2000b ; Nelanetal. 2005 ; Sanchez-Blazquezetal. 2006b ; Bernardietal. 2006 ). Oursampleseemstosplitaround[MgFe]03,ormorepreciselyaroundthesolarmetallicity.Thisisinagreementwith Poggiantietal. ( 2001a )whondthattheirfaintComaclustergalaxiesaredividedintotwogroups,onebeingmetal-richandtheotheronemetal-poor.Inoursample,galaxieswiththesuper-solarmetallicitiesarepredominantlyhigh-early-types.Agroupoflow-galaxiesisalsopresentinthisregimeandthesegalaxiesareonaverageyoungerthanthehigh-galaxies.Incontrast,alltheotherlow-galaxiesexhibitsub-solarmetallicities.Thisresultmayimplytwodierentformationmechanismsforlow-early-typegalaxieswithintheComacluster.Alternatively,galaxiesenteringtheclusterenvironmentatdierentepochswouldbestrippedfromtheirgasatdierentevolutionarystages,possiblyexplainingthemetallicitydierences. Ontheotherhand, Mehlertetal. ( 2003 )and Thomasetal. ( 2005 )ndthattheirsamplesofearly-typegalaxiessplitintotwosubclassesatH2AwheretheyoungersubclasshassolarorhighermetallicitiesontheH{[MgFe]0plot. Mehlertetal. ( 2003 )ndthatthe`youngclump'(theirFigure4)isdominatedbyS0galaxies,ratherthanEs. 88

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Plotsof[MgFe]0vs.HandMgbvs.hFei.Thelow-andhigh-galaxiesarerepresentedbythebluesolidandgreenopencircles,respectively.WemarkthenucleateddEs( Graham&Guzman 2003 )withpurpleopensquares.Thelargeredsoliddiamondsdenotethegalaxiesbinnedbyvelocitydispersionwhereeachbinisofanequalintervalinlog.Thelargerdiamondsrepresentthemoremassivegalaxieswithlarger.Thebinsdonotincludegalaxieswhichlieoutsidethemodelgrids.Forcomparison,wealsoincludeGlobularClustersfrom Cenarroetal. ( 2007 )asthegreytriangles.[Reproducedfrom Matkovicetal. ( 2008 ).] 89

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( 2005 )attributethisdivisiontoeitheryoungerstellarpopulationsorbluehorizontalbranchstars.Heretoo,wecanarguethatsuchadivisionexistsforthelow-galaxiesinoursample,butnotforthemoremassiveEs.Wedenoteagroupofgalaxieswitholdagesandlowmetallicities,whiletheremaininglow-galaxiesinoursamplehaveintermediateagesandalargerangeinmetallicity.Thissuggeststhatsomelowmassearly-typegalaxiesharboryoungerstellarpopulations,whiletheothersareoldandmetalpoor.Wealsofoundnocorrelationwithmorphologyforthisresult. ThebottompanelofFigure 4-2 showsarelationoftheMgbandhFeiindicesoverlaidwithmodels.Whentheageisxed,itispossibletoderivethe[/Fe]ratiosforthesegalaxies.Similartothe[MgFe]0vs.Hplot,thereisadivisioninthesamplebetweengalaxiesaroundthesolarmetallicityinthisgure.Majorityofgalaxieswithsuper-solarmetallicitiesarehigh-galaxies.Theyclusteraround[/Fe]=0:3whichisconsistentwiththewell-knownoverabundanceofMgamongEs,i.e.,adepressionofFewithrespecttothesolarvalues( Trageretal. 2000a ).Althoughthelow-galaxiesshowawiderrangein{ratios(0.0{0.5)thantheirmoremassivecounterparts,themajorityoflow-galaxies,withtheexceptionofafewobjects,havelow[/Fe].0:2.Thisresultindicatesthatthelow-massgalaxieshavehadamoreextendedstarformationhistory( Gorgasetal. 1997 ) 4-2 ).Allofthesegalaxiesarelowmasswith306<70kms1.WeperformedanumberofteststodeterminewhetherthesegalaxiestrulyhavesuchlowvaluesoftheHindex. First,wecheckedforapossibilityofnebularemissionintheHfeature,asitwouldmakethislinestrengthappearweaker,i.e.yieldingolderages.Asidefromthetestthatwehavealreadyperformedbydividingeachspectrawithit'soptimaltemplate(seeSection 2.5.1 ),wealsostackedthespectraofthesegalaxiestogether(sincetheyhaveasmallrangein)attheoriginalresolution(FWHM=1.9A)andcheckedforanypossibleemission 90

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PlotofHfortheo-gridgalaxiescombined.Westackedthespectraof13galaxieswhichdidnottonthemodelsfor[MgFe]0vs.Hplot.[Reproducedfrom Matkovicetal. ( 2008 ).] intheHabsorptionfeature(Figure 4-3 )whichwasnotdetectedpreviously.Atthisresolution,wedonotndanycontaminationfortheo-gridgalaxiesbynebularemission. Second,wecheckedwhethertheseo-gridgalaxieswereconsistentwiththepositionofglobularclusters(hereafterGCs)onthe[MgFe]0vs.Hplot.Ifthemodelswouldextendtothesegalaxies,theywouldcorrespondtoveryoldandverymetal-poorobjectssimilartoGCsortheycouldalsobe\primordial"assuggestedby Rakos&Schombert ( 2004 ).Figure 4-2 showsapossibilitythatsomefortheo-gridgalaxiesareconsistentwithGCs,whileanumberofthesegalaxieslieinaregionevenolderandmoremetalpoorthanGC. WealsocheckedwhethertheH/HA;Fratiosareconsistentfortheo-gridgalaxieswiththeothergalaxiesinoursampleandGCsfrom Cenarroetal. ( 2007 ).ThisisshowninFigure 4-4 .Theo-gridgalaxiesdeviatenoticeablyfromtheHAandHFwithHplotswhencomparedtootherComagalaxiesinoursampleandtheGCs.Interestingly,theo-gridgalaxiesshownodeviationsinthe[Mg/Fe]plot.ThispointstowardapossibilityofsomeproblemsinthemeasurementsoftheBalmerlinesfortheseo-gridgalaxies,whichisnotnecessarilytruefortheotherindices. 91

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Testingtheo-gridgalaxies.A)andB)showHindexvs.thehigherorderBalmerlinesHAandHF.C)andD)investigatewhetherthereareanyinconsistenciesinthe[Mg/Fe]betweentheo-gridgalaxieswiththeothergalaxiesinthesampleandGCs( Cenarroetal. 2007 ).TheblacklledcirclesrepresenttheComagalaxiesfromoursamplewherethebluediamondsareo-gridgalaxies.NucleateddEsaremarkedbypurpleopensquaresandtheGCsbygreytriangles.[Reproducedfrom Matkovicetal. ( 2008 ).] Thenaltestfortheo-gridgalaxieswastocompareourindexmeasurementsandspectratothatofSmithet.al(2008,inpreparation;privatecommunicationwithRussellSmith).Indeed,theo-gridgalaxiesclearlydeviateintheplotofHmeasuredhereandinSmithetal.(2008).Wehavebeenunabletondthecauseforsuchdierences.Conservatively,weexcludethesegalaxiesfromfurtheranalysis. 92

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Thomasetal. ( 2005 )toderivetheages,metallicitiesand-ratiosforoursampleofearly-typegalaxiesintheComacluster.Thisprocedureconsistsof,rst,determiningtheageandmetallicityofeachgalaxybyinterpolatingthemodelgridsforthe[MgFe]0Hplot,atagiven-ratio.Thisparticularcombinationofindiceshaslowsensitivitytoabundanceratiosandis,thereforewellsuitedfordeterminingtheothertwoSPMparameters,ageandmetallicity.Then,wextheageasitwasderivedintherststep,andwederivethe[/Fe]andmetallicitywithhFeiMgbindexcombination.Thistwo-stepprocedureisrepeateduntilthemetallicitiesderivedfrom[MgFe]0HandhFeiMgbmatchwell(i.e.betterthan15%dierence). GMPAgeAge[Z=H][Z=H][=Fe][=Fe]

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GMPAgeAge[Z=H][Z=H][=Fe][=Fe] 94

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Modelparametersvs.relations InterceptSlope logAgevs.0.220:340:310:18[Z=H]vs.1.000:260:530:13[=H]vs.0.130:130:170:07 respectiveerrorsareshowninTable 4-5 ,whiletheseparametersforthebinnedgalaxiesareinTable 4-4 Ourdataofearly-typeComagalaxiesspanawiderangeinallSPMparameters.Thelow-galaxieshaveonaverage:lowerages,6:10:1vs.8:90:1Gyrforhigh-galaxies;lowermetallicities,0:0500:003vs.1:1310:004dex;andslightlylower[/Fe],0:1730:002vs.0:2380:002,closertothesolarvalueof0.0.Here,weexcludedthegalaxieswhichlieothemodelgridsandwediscussthesegalaxiesinx 4-5 ).Wealsoincludetheindividualgalaxiesintheseplotsalthoughweperformthelinearleast-squaresregression(seeTable 4-6 )forthebinneddataonly.Thebinvaluesfortheage,metallicityand[/Fe]werecalculatedbyaveragingtheseparametersforindividualgalaxieswithineachbin.Theerrorsinthemodel-derivedaverageparametersweredeterminedbytakingthestandarddeviationoftheindividualparametervalueswithineachbinandthendividingbythesquarerootofthenumberofgalaxiesinthebin.Althoughnotstatisticallysignicant,trendsemergebetweentheage,metallicityand[/Fe]with,andwecomparethemtothesamerelationsfrom Nelanetal. ( 2005 ). 2 95

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Velocitydispersionvs.age,metallicityand[=Fe].Thesmalllledcirclesareindividualgalaxiesinourdataset,reddiamondsrepresent\average"galaxieswhoseindiceswerebinnedbyvelocitydispersionpriortoderivingtheSPMparameters,andtheblueopencirclesrepresenttheNFPSdatawithoutanyosetsapplied. 96

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4-5 showstherelationbetweenlogageandlog.Thelineartbetweenthesetwoparametersisuncertainduetothelargeerrorsinage.However,boththebinnedandtheindividualgalaxiesinthisgureprovideclearevidenceforatrendbetweenageandwherethelow-galaxiesdisplayyoungerages. Withintheerrors,ourage{logtrendisconsistentwith Nelanetal. ( 2005 ),althoughtherearesomedierences. Nelanetal. ( 2005 )ndthattheage{logrelationsteepensforthelow-massgalaxies.Wedonotndthiseect,sincetheage{logforthesegalaxieslevelsoat4Gyrinourcase. Whethertheage-relationexistsforearly-typegalaxiesornotisstillanunresolvedissueintheliterature. Jrgensen ( 1999 ), Kuntschneretal. ( 2001 ), Mehlertetal. ( 2003 ), Thomasetal. ( 2005 ),and Sanchez-Blazquezetal. ( 2006b )donotndarelationbetweentheseparameters,althoughresultsfrom Sanchez-Blazquezetal. ( 2006b )yieldarelationforgalaxiesinlowdensityenvironments.However,atleastatrendbetweenageandisfoundinthesamplesof Concannon,Rose,&Caldwell ( 2000 ), Poggiantietal. ( 2001a ), Caldwell,Rose,&Concannon ( 2003 ), Proctoretal. ( 2004 ), Nelanetal. ( 2005 ),and Bernardietal. ( 2006 ).Additionally, Nelanetal. ( 2005 )deriveage-relationsforothersourcesintheliteratureandndthemtobeinagreementwiththeirdata. Althoughwendanage-trendforoursampleofComaearly-typegalaxies,theuncertaintiesintheagemeasurementsarelargeandwecannotconrmarelationbetweenthesetwoparameters.Nonetheless,weobservethatthelow-galaxiesexhibit,onaverage,youngeragesthantheirmoremassivecounterparts. InChapter 3 ,weusedthescatterintheColor-relationandtheevolutionarystellarpopulationsynthesismodelsof Bruzual&Charlot ( 2003 )toestimatetheformationepochforourComagalaxies.Wefoundthat,ifweassumeastrongcoordinationintheformationepochofgalaxiesintheComacluster,mostofthesegalaxieswouldhaveformedabout6Gyragoandwithinascatterof1Gyr.TheresultsfromChapter 3 areconsistent 97

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4-5 .Wendthattheaverageageoflow-galaxiesis6Gyrandthescatterintheloglogagerelationimpliesascatterinformationepochof1:5Gyr. 4-5 ).Thehigh-early-typegalaxiestendtobemoremetal-richthanthelow-galaxieswhichalsoexhibitalargerrangeintheirmetallicities. Ourderivedslope,[Z/H]/0:530:13,isinagreementwithanumberofstudieswhichndafairlyrobustmetallicity{relation( Kuntschneretal. 2001 ; Mehlertetal. 2003 ; Nelanetal. 2005 ; Thomasetal. 2005 ; Sanchez-Blazquezetal. 2006b ; Bernardietal. 2006 ).Furthermore,ourmetallicity{trendisingoodagreementwiththatof Nelanetal. ( 2005 )whichisalsoshowninFigure 4-5 .Weextendthistrendtogalaxieswith=30kms1withnoevidenceforachangeofslopeoroset. 4-5 ).Thelowmassearly-typegalaxiesexhibit-ratiosclosertothevaluesinthesolarneighborhood,althoughthetrendissuggestiveof[/Fe]>0evenatthelowest.Thehighermassgalaxieshaveanoverabundanceof[/Fe]. Arelationbetween[/Fe]andhasalreadybeennotedbyanumberofauthorsintheliterature( Trageretal. 2000b ; Kuntschneretal. 2001 ; Proctor&Sansom 2002 ; Mehlertetal. 2003 ; Thomasetal. 2005 ; Bernardietal. 2006 ),althoughconictingwith Proctoretal. ( 2004 ).Inaccordancetotheformerstudies,wendthatthe-ratioincreaseswithincreasingvelocitydispersion.However,duetolargeuncertainties,wecanonlyconrmatrendandnotacorrelationbetweentheseparameters. Our[/Fe]{slopeof0:170:07isshallowerthantheslopesderivedbyotherauthorswhond0:3( Trageretal. 2000b ; Thomasetal. 2005 ; Nelanetal. 2005 ).However,wenotethatthereare3objectsinthe[/Fe]{plotwith-ratiosthatarequitelarge.These 98

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Ingeneral,super-solar-ratiosdenotethatgalaxiesformedquickly,i.e.,onshortstarformationtime-scales,andathighredshifts( Matteucci 1994 ).Therefore,an[/Fe]{trendsuggeststhatthelow-galaxieshadmoreextendedstarformationhistoriesthantheirmassivecounterpartswherenewstarsformedfromalreadymetal-enrichedenvironment( Gorgasetal. 1997 ).Infact,acoupleofmechanismsexplainingtheextendedstarformationhistoriesofthelowmassgalaxiesalreadyexist.OneinvolvesUVbackgroundradiationwhichcanextendthedurationofstarformationbysuppressingcoolingmoreeectivelyinlowmassgalaxies( Kawata 2001 ).Whiletheotherusesacombinationofcooling,starformation,energyfeedback,andchemicalevolutiontoextendthestarformationhistoryofthesegalaxies( Chiosi&Carraro 2002 ). 4-6 weinvestigaterelationsbetweenmetallicityandage,[/Fe]andage,andmetallicityand[/Fe]foroursampleofComaclusterearly-typegalaxies.Wedonotndanyrelationsbetweenthemodelparametersforallthegalaxiesinoursample.However,whenweexaminethedierencebetweenthehigh-andlow-galaxies,wendtrendsintheage-metallicityandthemetallicity{[/Fe]plots. Wedonotndanycorrelationsbetweenageand[/Fe],norbetweenmetallicityand[/Fe]evenataxedvelocitydispersion.However,wedonoteaweaktendencyforthehigh-galaxiestohavehigher-ratiosandtobemoremetalrichthanthelow-galaxies.Thiseectisstrongerinthe Michielsenetal. ( 2007 )sample(theirFigure7)whosedataextendtolowermetallicitiesandareinlowerdensityenvironmentthanourComaclustergalaxies.Assumingthatourandthe Michielsenetal. ( 2007 )samplespanasimilarrange 99

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Relationsbetweenthemodelparameters(age,metallicityand[=Fe]).Thegreencirclesrepresentgalaxieswith1006<185km/s;theyellowtrianglesmarkthegalaxieswith506<100;andthebluetrianglesaregalaxieswith306<50.Thelineartbetweenageandmetallicityforthehigh-galaxiesismarkedwithasolidline,whilethettothelow-galaxies(<100kms1)ismarkedbyadashedline.Weexcludedthegalaxywiththelowestmetallicityfromthelinearregression(itismarkedwithacross).Thearrowsinthetoprightcorneroftheage-metallicityplotrepresenttheaveragecorrelatederrorellipseforageandmetallicity.Thearrowsintheage-metallicityplotrepresent1standarddeviationcorrelatederror. Table4-7. Relationsbetweenageandmetallicity [Z=H]=a+blogAgeInterceptSlope in,andthatthemetallicity-abundancetrendsarenotduetocorrelatederrors,thereisapossibilitythatthiseect,tooisduetotheenvironment. Therelationbetweentheageandmetallicityatagivenwasrstdiscussedby Trageretal. ( 2000a ).Westudythepossibilityofsuchrelationsforourlow-andhigh-galaxies.TheSpearmanRankcoecients(Table 4-7 )implythattherelationsbetweentheageandthemetallicityexistforthetwo-sub-samples,whilenocorrelationswerefoundforeithertheage{[/Fe]northemetallicity{[/Fe].Notethattheonegalaxy,markedbyacrossinFigure 4-6 ,whichweexcludedfromtheSpearmanRanktestandlinearregressionisagalaxywiththelowest=30kms1inoursample. Existenceofanage-metallicityrelationwheregalaxieswithyoungeragestendtobemoremetalrich,impliesthattheseyounggalaxieshavehadmultiplestarformation 100

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Trageretal. 1998 ; Jrgensen 1999 ; Kuntschner 2000 ; Kuntschneretal. 2001 ; Poggiantietal. 2001a ; Terlevich&Forbes 2002 ; Sanchez-Blazquezetal. 2006b ,tonameafew).Somestudies,however,suggestthatthisrelationisaconsequenceofcorrelatederrors( Trageretal. 1998 2000a ; Ferreras,Charlot,&Silk 1999 ; Kuntschneretal. 2001 ).While Poggiantietal. ( 2001a )observeanage-metallicityrelationatallmagnitudesintheComacluster, Sanchez-Blazquezetal. ( 2006b )donotndthisrelationinComaandarguethattheirapparenttrendisduetocorrelatederrors,althoughtheyalsonoteapossibilitythattheirsampleisbiasedtowardhigh-galaxies,makingtheage-metallicityappearat. High-galaxiesfollowthesamerelationastheage{metallicityrelationfoundby Trageretal. ( 2000a ).Toinvestigatethis,weestimatedanaveragecorrelatederrorforourgalaxiesfromtheerrorellipsederivedwiththeiterativeprocessasdescribedinx .ThedirectionandsizeofthiscorrelatederrorareshowninthetoprightcornerofFigure 4-6 .Asitcanbeseen,thedirectionofthecorrelatederrorscoincideswiththeslopeoftheage{metallicityrelation.Furthermore,thesizeoftheerrorsisconsistentwiththeextentofthedistributionofageandmetallicityvalues.Thisimpliesthatthetheage{metallicitycorrelationatagivenvelocitydispersionmaybesimplytheresultofcorrelatederrorsinbothparameters! Thelinearregressionbetweentheagesandmetallicitiesforthelow-andhigh-galaxiesshowsthattheslopesforthetwosub-samplesareeectivelythesame,butosetwithdierentzeropoints.Atagivenage,thehigh-galaxiesaremoremetalrichbyafactorof2thanthelow-galaxies.Similarly,atagivenmetallicity,thelow-galaxiesare3Gyryoungerthantheirmoremassivecounterparts.Thisresultalsocompareswellwith Michielsenetal. ( 2007 )whosedatasamplecontainsdEgalaxiesfromtheVirgoclusterandtheeld.BothdatasetsarewellanchoredtothesampleofmassiveEsfrom Sanchez-Blazquezetal. ( 2006b ).However,therearedierencesinthedistributionof 101

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Michielsenetal. ( 2007 )dEs,buttheyhaveasmallerrangeinmetallicity.Thispointstowardsanenvironmentaldependenceoftheage-metallicityrelationsincethecentralregionoftheComaclusterisoneofthedensestregionsinthelocaluniverse.Unfortunately,wecannotsaythiswithcertaintywithoutknowinghowlowinthe Michielsenetal. ( 2007 )samplegoes,sincetheonegalaxyinoursamplewithverylowmetallicityontheage-metallicityplotisagalaxywiththelowest=30kms1.Theeectofndingthewiderrangeinmetallicityinthelower-densityenvironmentsthaninthecoreoftheComacluster,thus,maybepurelyduetosamplinggalaxieswithlowervelocitydispersions. 4.5.1Index{RelationsforOutskirtsGalaxies Thecomparisonoftheindex-relationsbetweenthecenterandoutskirtssampleisshowninFigure 4-7 .Low-galaxiesintheoutskirtssamplehavealargerscatterformostindices,whencomparedtothecentralindices.GroupIIshowsthattheintrinsicscatterforlow-galaxiesintheoutskirtsislargerthanthatofthecentralgalaxies.Wealsotlinesthroughthelow-galaxiesonly,fortheoutskirtssample.However,theSpearmanRankcoecientsforthesegalaxiesarelow(0.3{0.5).WedonotethatmoregalaxieswithlowvaluesofFe5335lieintheouterthaninthecentralsample. AlltheindicesingroupIIIhaveconsistentslopesbetweenthetwosamples.Exceptfor[MgFe]index,whichshowsasteeperslopeforoutskirtsgalaxies.However,thisislikelyduetotheshapesofFe5270andFe5335indices. 102

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Index{logplotsincludingoutskirtsgalaxies.ThesymbolsforthecentraldataarethesameasinFigure 4-1 .ThedatafromtheNFPSsampleisrepresentedbyblueopencircles;ourcentralsamplebyblacklledcirclesandyellowlledcirclesfortheo-gridgalaxies;thereddiamondsareaverageindexvaluesforeachbininvelocitydispersion.Theoutskirtsdataarerepresentedbydarkredopentriangles,wherethedownwardfacinglightbluetrianglesdenotegalaxieswithemission.InthebottomrightcornerofI{plotsingroupIweshowthe1-sigmascatterforlow-galaxiesinthecenter(black)andoutskirts(darkred)ontheleftandhigh-galaxiesontheright.InthecaseofgroupIIandtheBalmerLinesgroup,theselinescorrespondtotheintrinsicscatterforlow-(centerandoutskirts)andhigh-(onlycenter)respectively.Wherethescatterfortheoutskirtswascalculatedaroundthelinedenedforthecentralgalaxies.ForgroupIII,weshowtheintrinsicscatterforthecenterandoutskirtsandtheobservedscatter,respectively,inthebottomrightcorner.Theyellowdashedlineshowslinearrelationsforthelow-galaxiesintheoutskirts. 103

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4-8 .ThelinearregressionlinesfortheoutskirtsdataarepresentedinTable 4-8 anddonotincludegalaxieswithemissionlines,eventhoughthesegalaxiesaredisplayedinthegure.Table 4-9 showsthevaluesforthemodelparametersbinnedby,wherethebinshavethesamerangeinasthecentraldata.Wedidnothaveenoughgalaxiesintheoutskirtstomatchthehighest-bin(bin1)ofthecentraldata. Wedonotseeasignicantosetinagebetweenthecenterandtheoutskirtssamples.However,forlow-galaxiesintheoutskirtsthereisahinttowardyoungerages.Whilethehigh-galaxiestendtobeolderintheouterregionwhencomparedtothecentraldata. Themetallicity-plotshowsatendencytowardslowermetallicitiesfortheouterComagalaxieswhencomparedtothegalaxiesinthecenter.Thelowestandsecondtohighest-binshowmetallicitiesconsistentwiththeonesinthecenterofthecluster.Although,bothofthesebinsmaybedrivenbyafewobjectswithquitehigh[Z/H]. Themostobviousdierenceinthetwosamplesisinthe[=Fe]{plot.Outskirtsgalaxieshavehigherratiosthanthegalaxiesinthecenterofthecluster.Quiteafewobjectsfromtheoutskirtsdataexhibitsupersolar-ratios.Thisisunexpected,sincemoststudiesintheliteraturendthatdwarfearly-typegalaxieshavesolarorsubsolarabundances. Todoublecheckthatourderivationsofages,metallicitiesandelementabundancesarenotexperiencinganysystematiceects,wecomparethesemeasurementstoadierent 104

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Environmentaleectsonvs.age,metallicityand[=Fe].ThesmallredlledcirclesandbluetrianglesrepresentindividualgalaxiesfromthecentralandouterComasamples,respectively.Theredandbluelleddiamondsaremodelparametersbinnedbyforcenterandouterdata,respectively.Theredsolidlinecorrespondstothelineartforthecentralbinnedgalaxies,andthebluedottedlinetotheouterdata.Thebinsareofidenticalsizein.Notethat,sincethereisonlyonegalaxyinthehighestregimefortheoutskirtssample,wedonothaveabinforwhichwouldcorrespondtothelargestgalaxiesinthecenter.ThegreenopencirclesaretheNFPSdatawithoutanyosetsapplied.Wealsoidentifygalaxieswhichfallintothreegroupswhichhavesupersolar-ratiosand1)areoldandhavelowmetallicities(purple),2)areyoungandhavehighmetallicities(black),and3)areyoungandhavelowmetallicities(orange).Wealsoincludegalaxieswithemissionwithyellowtriangleswithblackedges. 105

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Outersamplemodelparametersvs.relations InterceptSlope logAgevs.0890:53.0:900:30[Z=H]vs.0620:31.0:240:17[=H]vs.0090:27.0:100:15 Table4-9. Outergalaxybinsinandmodelparameters BinNlogRangehlogihlogAgeih[Z=H]ih[=Fe]i method.Ourcollaborator,Dr.PatriciaSanchez-Blazquez,hasindependentlymeasuredthemodelparametersusingthe2methodof Proctoretal. ( 2004 ).Thismethodutilizesalltheindicesbyobtainingthebesttby2minimizationtoallthegivenLickindicesinage,metallicityand-ratios.Figure 4-9 showsthecomparisonofourmeasurementstothosewiththe2method.Therearesmallsystematicosetsbetweenthetwomethods.Forexample,wederiveslightlyyoungerages,lowermetallicities,andhigher[=Fe]thanwhenweusethe2method.However,thesesmallshiftsarenotsignicantforthepurposesofndingoveralltrendswithorcomparingthestellarpopulationsofthecenterandtheoutskirtsgalaxies. Furthermore,therearepotentiallysomeproblemswiththe2methodasisrecognizedintheliterature.Forexample,someindiceshaveoverlappingpass-bandswhichmeansthattheyaregivendoubleweightsinthet.Currently,thepreferredmethodformeasuringthestellarpopulationparametersofunresolvedobjectsisthemethodof Thomasetal. ( 2003 ).AnotherreasonwhywechosethesemodelsistobeabletocompareourresultswiththeNFPSgroup,thelargestanddeepeststudyofstellarpopulationsinclusterstodate,inaconsistentway. Whentheresidualstothebesttbetweentheobservedvalueandthebest-ttingvalueobtainedbythe2minimizationarelarge,thisbecomesobvious.Thus,onecanchoosetoeliminate\spurious"indicesfromthet.Wetestwhetherexcludingsomeindicesthatweusedinourderivationofthestellarpopulationparameterswouldchange 106

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Comparisonofmodelmeasurementswithadierentmethod.Themethodweuseinthisworkby Thomasetal. ( 2003 )toderiveages,metallicitiesandabundancesofelementsiscomparedtothe2methodby Proctoretal. ( 2004 ).Theredlledcirclesandtheblueopencirclesrepresentourcentralandoutskirtsgalaxies,respectively.[ProducedbyPatriciaSanchez-Blazquez.] theresults.Figure 4-10 showssuchatestwhereweshowthecomparisonbetweenthemodelparametersderivedusingalltheindicesvs.a2twithouteitherFe5335orMgb.Wechosetochecktheseindicesbecausetheyareusedinthederivationofthe-ratiosintheiterativemethodof Thomasetal. ( 2003 ). WedonotndthattherearesignicantdierencesinthemeasurementsofthestellarpopulationparameterswhetherweexcludetheFe5335ortheMgbindices.The2methodshowsnodierenceinages,metallicitiesnorratioswhenFe5335isexcludedfromthet. 107

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Consistencycheckswiththe2method.Eachgurerepresentsacomparisonbetweenthe2twhenusingalltheindicesandwhenweexcludeeitherFe5335orMgb.[ProducedbyPatriciaSanchez-Blazquez.] 108

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Nowthatwehaveconrmedthetrendsinthestellarpopulationparameters,wecanexaminetheminmoredetail.InFigure 4-8 ,wedividedthegalaxieswithhigh[=Fe]intothreegroups.Onegroupshowsoldagesandlowmetallicities(markedbypurpleletters).Anothergroupisyoungandhashighmetallicities(black),whilethelastgrouphasyoungagesandlowmetallicities(orange).Atrstglancetheseresultsarequitesurprising,sinceonewouldnotexpecttondsuchvarietyofstellarpopulationpropertiesamongdEgalaxies.However,our\outskirts"regioncorrespondstoaregioninfallingintothecluster(seeFigure 4-11 ).ThismayexplainwhyweseesuchheterogeneityinstarformationhistoriesofdE/dS0galaxiesinthisregionoftheclusteraswemaybeobservingthesegalaxiesindierentphasesoftheirinfallintothecluster. Forexample,ifdIsweretheprogenitorsofdEs(e.g. vanZeeetal. 2004a )andhadashortstarburstwhichwasquicklyfollowedbytheirinfallintothecluster,mostoftheircoolgaswouldbelostthroughram-pressurestripping.Inthiscase,wewouldexpecttoobservehighabundancesofthelightelementsandlowmetallicitiessincetheywouldnothaveenoughtimetoincorporateFefromSNTypeIaintothenewgenerationofstars.Theagesoftheunderlyingstellarpopulationsofthesegalaxieswouldthendependonhowlongagothegalaxiesenteredthecluster.Iftheyenteredtheclusterrecently,theymaystillappearyoung,whileiftheyenteredtheclusterlongtimeago,theirageswouldbeold. Ontheotherhand,thegroupofgalaxieswitholdages,lowmetallicitiesandhigh-ratiosmaycorrespondtogalaxieswhichunderwentasimilarscenario,exceptthattheymayhaveenteredtheclusteratearliertimes.Thiswouldallowtheirstellarpopulationstoage.Whilethenewmetal-enrichedstarswouldneverformbecausethesegalaxieslostmostoftheircoolgaswhileenteringthecluster,orduringsubsequentinteractionswithothergalaxiesinthecluster( Mooreetal. 1996 ). 109

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Liskeretal. 2007 ).SomedEgalaxiesintheVirgoclusterexhibitbluecoreswhichimpliesthattheyareeitheryoungormetalpoor. Anotherpointwewouldliketoaddressisthefactthatwendalargerfractionofgalaxieswithemissionintheoutskirtsofthecluster,thanwedointhecenter.Wefoundthatonly2galaxiesinthecentralComaregionhademission,whilethisnumberwas9intheoutskirts.Oneofthepredictionsofthegalaxyharassmentscenarioisthatmorespiralremnantsshouldbefoundinoutskirtsofacluster( Mooreetal. 1998 ).Sincegalaxiesintheoutskirtswillhavelessinteractionsthantheonesinthecenter,theyaremorelikelytokeepsomeoftheirspiralstructure.Wecanonlyobservethiswithphotometry,howeveraspectroscopicsignatureofspiralarmsisthattheyhavestarformation.Itisthus,possiblethatthelargerfractionofearly-typegalaxieswithemissionthatweseeintheoutskirtsdatarelatestothispredictionofspiralremnants. 4.4 wedeterminedthatanage{metallicityanti-correlationatagivenforearly-typegalaxiesinthecenteroftheComaclusterismostlyduetocorrelatederrors.Here,weinvestigatewhetherthereareanytrendsbetweentheseparametersintheoutskirtsregionofthecluster.WealsocomparetheseresultswiththegalaxiesinthecenterofComa.Figure 4-12 showstherelationsbetweenageandmetallicity,ageand[=Fe],andbetween[=Fe]andmetallicity. Thelow-dEs/dS0sintheoutersampleoftheComaclustershownocorrelationbetweenageandmetallicity.Although,theirSpearman-Rankcorrelationcoecientisverylow,5%(seeTable 4-10 ),westillincludedthelinearttothelow-galaxiesinthis Table4-10. Outerregionrelationsbetweenageandmetallicity [Z=H]=a+blogAgeInterceptSlope 110

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Spatialdistributionofgalaxiesafteranalysis.Bluelledcirclesdenoteouroutskirtsgalaxies.Thepurplelledcirclesaregalaxieswhichhavehigh-ratios,oldages,andlowmetallicities.Theblackcirclesrepresentgalaxieswithsupersolarabundanceswhichhaveyoungagesandhighmetallicities,whilethesamecharacteristicsapplytotheorangecircles,exceptthatthisgrouphaslowmetallicities.Galaxieswhichhaveemissionaremarkedasyellowcircles. gure.Thisisbecauseitdemonstratesthatthegalaxiesintheouterregionoftheclusterareonaverageyoungerandhavelowermetallicitieswhencomparedtothecentralgalaxies. SimilarlytothecentraldE/dS0s,thesegalaxiesintheoutskirtsoftheclustershownocorrelationsbetweentheir[=Fe]andage,norbetweenthemetallicityand[=Fe].TheonlydierencebetweenthetwosamplesisatendencyofanumberofdEs/dS0sintheoutskirtstohaveyoungeragesandlowermetallicitiesthantheonesinthecenter.Thiswasalreadynotedintheprevioussection,wherewealsodiscusseddierentformationscenariosforthedE/dS0galaxiesinthecenterandtheinfallingregionoftheComacluster. 111

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Relationsbetweenthemodelparameters(age,metallicityand[=Fe]).Thelledredcirclesandbluesquaresaregalaxieswith>100kms1forthecenterandoutskirtsdata,respectively.Theorangeup-facingandlightbluedown-facingtrianglesaregalaxieswith506<100km/sforcenterandoutskirts,respectively;theyellowup-facingandgreendown-facingtrianglesmarkthegalaxieswith286<50.Thelineartbetweenageandmetallicityforthecentralhigh-galaxiesismarkedwithasolidline,whilethettothelow-galaxies(<100kms1)inthecenterismarkedbyadashedline.Thedottedbluelinerepresentsthettothelow-galaxiesfortheoutskirtssample.Forthecentralsampleweexcludedthegalaxywiththelowestmetallicityfromthelinearregression(itismarkedwithacross).Wedidnotincludegalaxieswithemissionfromtheoutskirts.Thearrowsinthetoprightcorneroftheage-metallicityplotrepresenttheaveragecorrelatederrorellipseforageandmetallicityandcorrespondto1standarddeviationcorrelatederror. 112

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Wehavepresentedmeasurementsofinternalkinematicsandderivedthepropertiesofunderlyingstellarpopulationsfordwarfearly-typegalaxiesintheComacluster.Ourstudyconsistedofcharacterizingthesepropertiesinoneofthedensestenvironmentsinthelocaluniverse,thecenterofComa.Wealsodeterminedthesepropertiesinalessdenseregionofthecluster,locatedSWoftheclustercore.ThetwosampleshavesignicantlyincreasedthenumberofdEs/dS0swithinternalkinematicsoutsideoftheLocalGroup.ThesevelocitydispersionmeasurementshavealsoprovideduswithauniqueopportunitytoassesstheunderlyingstellarpopulationsofdEs/dS0satagivenmass.Finally,wecomparedthestarformationhistoriesofthesegalaxiesinthetworegionsofdierentdensitywithintheComacluster. Wendthatdwarfearly{typegalaxiesfollowawell-denedC{relation.Byassumingthatthisrelationismostlydrivenbyanincreasedmetallicitywithincreasinggalaxymass,whilethescatterreectsagedierences,weinvestigatedhowwecanconstraineithertheages,therangeofstarformationepoch,orthestrengthofsecondaryburstsinthefaintearly{typegalaxiesforvariousgalaxyformationscenarios.However,we 113

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GalaxiesintheoutskirtsregionoftheclusterfollowthesameL{relationasthecentraldEs/dS0s.Theintrinsicscatterinthisrelationislargerfortheoutergalaxies.Wealsonoticeatendencyfortheoutergalaxiestohaveslightlyhigherluminositiesatagiven.Furthermore,wendthatthegalaxiesintheoutskirtshavealargerintrinsicscatterincolor{relation,andatendencytowardbluercolorsinthecolor{magnitudediagram.Weinterpretthesedierencesaspossiblyduetoeither,onaverage,youngergalaxiesorlowermetallicitiesintheoutskirtsofthecluster.Weconrmthishypothesiswithourstudyoftheunderlyingstellarpopulations. WealsondthattherelationsbetweentheBalmerlinesandhavenegativeslopesandarefairlyrobust,withHhavingtheweakestcorrelationcoecient.WendanasymmetricscatterintheHrelationwithmoregalaxieshavingalowerHindex.Sincetheasymmetryisintheoppositedirectionfromtheonefoundinthelowerdensityenvironments( Concannon,Rose,&Caldwell 2000 ),itispossiblethatthiseectdependsontheenvironment. 114

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TheresultofdierentshapesfortheI{relationsalsoholdsforthedE/dS0galaxiesintheoutskirtsoftheComacluster.However,thescatterinalltheI{relationsislargerfortheoutskirtsdata.Furthermore,ouroutskirtsdataconrmtheasymmetryintheHrelationtowardsmoregalaxieshavinglowervaluesoftheirHlinestrengthindex. Weusethestellarpopulationmodelstoderiveages,metallicitiesand[/Fe]forourComaclustergalaxies.WendawiderangeinalltheSPMparameterswherethegalaxieswithsuper-solarmetallicitiesaredominatedbythehigh-galaxies,whilethelow-galaxiesareonaverageyounger,havelowermetallicities,and-ratiosthatscatteraroundthesolarvalue.Thisimpliesthatthelow-galaxieshadsomeresidualstarformationintherecentpastandthattheirstarformationhistoriesaremoreextendedthantheyareforthehigh-galaxies.Theseresultsarealsoconrmedbythetrendswendbetweentheage,metallicityand[/Fe]with. Dwarfellipticalgalaxiesintheoutskirtsseemtobemorediversethanthoseinthecenterofthecluster.Galaxiesintheoutskirtsshowatendencytowardyoungerages,lowermetallicitiesandhigher[/Fe]whencomparedtothecentraldEs/dS0s.Thisissurprising,sincemostdEsstudiedsofarhavesubsolartosolarelementabundances.However,whenweexaminetheSPMparametersforindividualgalaxies,wendthattherearethreegroupsofgalaxieswithsupersolar[/Fe]ratios:agroupwitholdagesandlowmetallicities,anothergroupwithyoungagesandhighmetallicities,andthelastgroupwithyoungagesandlowmetallicities.Weinterpretthisresultbydierentformationscenariosforthesegalaxies.Furthermore,ouroutskirtsregionofComacorrespondstoa 115

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Wendthattheage-metallicityanti-correlationofearly-typegalaxiesinthecenteroftheComaclusterismostlikelyduetocorrelatederrors.TheouterComadEs/dS0salsoshownocorrelationbetweentheiragesandmetallicities.However,thereisatendencyoftheoutskirtsgalaxiestowardlowermetallicities,youngeragesandhigher-ratios.Wewereabletocompareourresultswiththoseof Michielsenetal. ( 2007 )whoobserveddEgalaxiesintheVirgoclusterandtheeld.OurComaclustergalaxiesseemtohaveasmallerrangeinmetallicitywhencomparedtothe Michielsenetal. ( 2007 )dataset.Therefore,thereisapossibilityofanenvironmentaleectonthemetallicityrangefordE/dS0galaxies,unlessthe Michielsenetal. ( 2007 )dataincludegalaxieswithlowervelocitydispersions. Anaturalextensionofthisworkistoextendoursampletoincludealargernumberofgalaxiesandreachdeeperlimits.Currently,Iamapartoftheground-basedspectroscopicstudytofollow-uponHSTimagesofdierentregionsinthecluster.Thisstudyisbasedondatafromthe10-mKeckTelescopeandthemulti-slitspectrographDEIMOS.ThespectraobtainedthroughthisstudywillprovideafaintersampleofdEgalaxies,withlowervelocitydispersions.Furthermorewewillbeabletostudygradientsinthestellarpopulationsandrotationsofthesegalaxies. CurrentlyfavoredformationmodelsdonoteasilyexplainwhysomedEsrotateandsomedonot.Itisplausiblethatdierentformationmechanismscouldexplainthis 116

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AnaMatkovicwasborninBelgrade,formerYugoslaviain1978.Shebecameinterestedinastronomyinhighschoolandattendedastronomyseminarsatthe\ScienceStationPetnica."TheseseminarsinspiredAnatopursuestudyingscience.ShecompletedherundergraduatestudiesfromMountUnionCollege,Ohiomajoringinphysics.InFallsemesterof2000,shestartedgraduatestudiesinastronomyattheUniversityofFlorida. 125