A Possible Origin of the Mass-Metallicity Relation of Galaxies

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v:astro-ph/0611723v3 4 Dec 2006Mon.Not.R.Astron.Soc.000,1–14(2006)Printed5February2008(MNLATEXstylefilev2.2)

APossibleOriginoftheMass–MetallicityRelationof

Galaxies

J.K¨oppen1,2,3⋆,C.Weidner4,5†andP.Kroupa5,6‡

1UMR7550,ObservatoireAstronomique,11ruedel’Universit´e,F-67000Strasbourg,France2Institutf¨urAstronomieundAstrophysikderUniversit¨atKiel,D-24098Kiel,Germany3InternationalSpaceUniversity,Parcd’Innovation,F-67400Illkirch,France4DepartementodeAstronom´ıayAstrof´ısica,PontificiaUniversidadCat´olicadeChile,Av.Vicu˜naMacKenna4860,Macul,Santiago,Chile5ArgelanderInstitutf¨urAstronomiederUniversit¨atBonn,AufdemH¨ugel71,D-53121Bonn,Germany6TheRhine-Stellar-DynamicalNetwork

Accepted5February2008.Received5February2008;inoriginalform5February2008

ABSTRACTObservationsshowthatgalaxiesfollowamass–metallicityrelationoverawiderangeof

masses.Onecurrentlyfavouredexplanationisthatlessmassivegalaxiesarelessable

toretainthegasandstellarejectaandthusmaylosethefreshlyproducedmetalsinthe

formofgalacticoutflows.Galaxieswithalowcurrentstarformationratehavebeen

foundtocontainstarclustersuptoalowermasslimit.Sincestarsarepredominately

borninclusters,andlessmassiveclustershavebeenfoundtobelesslikelytocontain

verymassivestars,thisimpliesthatinenvironmentsorattimesoflowstarformation,

thestellarinitialmassfunctiondoesnotextendtoashighmassesasduringhighstar

formationepochs.Itisfoundthattheoxygenyieldisreducedbyafactorofthirty

whenthestarformationrateisdecreasedby3to4ordersofmagnitude.Withthis

concept,chemicalevolutionmodelsforgalaxiesofarangeofmassesarecomputedand

showntoprovideanexcellentfittothemass–metallicityrelationderivedrecentlyby

Tremontietal.(2004).Furthermore,themodelsmatchtherelationbetweengalaxy

massandeffectiveyield.Thus,thescenarioofavariableintegratedstellarinitialmass

function,whichisbasedontheconceptofformationofstarsinclusters,mayoffer

anattractivealternativeorpartialexplanationofthemass–metallicityrelationin

galaxies.

Keywords:stars:massfunction–ISM:abundances–ISM:evolution–galaxies:

abundances–galaxies:evolution–galaxies:ISM

1INTRODUCTION

Ithasbeenknownforalongtimethatthereexistfairly

well-definedrelationsbetweenthemetallicityandmass(or

luminosity)amonggas-richgalaxies(Lequeuxetal.1979;

Skillmanetal.1989)aswellasamonggas-poorgalaxies

(Faber1973;Brodie&Huchra1991).Zaritskyetal.(1994)

pointoutthesimilarityoftheserelations,whichissuggestive

ofacommonphysicaloriginofthisphenomenon.Recently,

thewealthofdatafromtheSloanDigitalSkySurveyper-

mittedTremontietal.(2004)tostudythemass–metallicity

relationoverawiderangeofmassesandmetallicitiesfor

about53000star-forminggalaxies.

⋆e-mail:koppen@astro.u-strasbg.fr†e-mail:cweidner@astro.puc.cl‡e-mail:pavel@astro.uni-bonn.deTremontietal.(2004)interprettheirdatawithasce-

narioofgasoutflowswhicharestrongerindwarfgalaxies.

Moremassivegalaxiesareabletoretainthegaslongerand

moreeffectivelythanlowmassobjects,thusthemetallic-

itycanbuilduptohighervaluesbecausefollowinggenera-

tionsofstarswillbeformedinanenrichedgasenvironment,

whilethelow-massobjectslosetheirmetalsthroughgalac-

ticwinds.Differentamountsofdarkmattercanalsoassist

inthisprocess(Dekel&Silk1986).DeLuciaetal.(2004)

showthatthechemicalevolutioninhierarchicalgalaxy

formationmodelsareabletoexplaintheobservedmass–

metallicityrelationandotherrelationsliketheTully-Fisher

relation.Thisisachievedbyincludingfeedbackprocesses

likegalacticwindsintotheircosmologicalsimulations.The

formulationofthefeedbackincludesfreeparameterssuchas

thefeedbackefficiencyortheyield.

Whilegalacticoutflowsareobservedandthisscenario2J.K¨oppen,C.WeidnerandP.Kroupa

providesasatisfactorymatchoftheobservedrelation,one

shouldnotyetexcludeotherexplanationsorcontributions

fromotherprocesses.Thisisparticularlyimportantbe-

causethecurrentgalacticwindmodelsstillneedtobe

individually-fittedtothegalaxiesandarenoteasilyex-

plainedbyphysicalparametersofthegalaxy.Furthermore,recentworksbyLeeetal.(2006)andDalcanton(2006)show

thatsimpleoutflowsofISMgashavedifficultiesreproducingtheyieldsin

dwarf

galaxies.

Lee

etal.(2006)usedobserva-

tionswithSpitzertoderivethemass–metallicityrelation

fromanumberofdwarfirregulargalaxies.Theyconclude

thatthedispersionintherelationandthelargevariationsintheeffectiveyields“aredifficulttounderstandifgalacticsuperwindsoroutflowsareresponsibleforlowmetallicitiesatlowmassorluminosity.”InDalcanton(2006)aseriesof

closed-boxchemicalevolutionmodelsincludinginfalland

outflowispresented.Thereitisshownthatneithersimple

infallnoroutflowcanreproducetheobservedloweffective

yieldsinlowmassgalaxies,butmetal-enrichedoutflowscan

do.Thatis,effectivelythefreshlysynthesisedelementsneed

toberemovedfromthemattercycle,whichis,inprinciple,

equivalenttoreducingthenumberofmassivestars.