M31, M33 and the Milky Way Similarities and Differences

The large galaxies in the Local Group, while all disk galaxies, have diverse stellar populations. A better understanding of these differences, and a physical understanding of the causes, requires more detailed study of the older populations. This presents

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arXiv:astro-ph/0204190v1 11 Apr 2002M31,M33andtheMilkyWay:SimilaritiesandDi erencesRosemaryF.G.Wyse1TheJohnsHopkinsUniversity,DepartmentofPhysicsandAstronomy,Baltimore,MD21218,USAAbstractThelargegalaxiesintheLocalGroup,whilealldiskgalaxies,havediversestellarpopu-lations.Abetterunderstandingofthesedi erences,andaphysicalunderstandingofthecauses,requiresmoredetailedstudyoftheolderpopulations.Thispresentsasigni cantchallengetoGAIAbutthescienti creturnsarealsosigni cant.11INTRODUCTIONStudyoftheresolvedstellarpopulationsofgalaxiesintheLocalGroupo ersgreatscienti creturnsinourunderstandingofhowgalaxiesformandevolve.TheLocalGroupmembergalaxies(cf.[1])includethethreediskgalaxiesM31,M33andtheMilkyWay,andnumerousdwarfcompanions,bothgas-richandgaspoor.Whatcausestheirsimilaritiesandalsotheirdiversity?Therearetwomainaspectstogalaxyformationandevolution,namelythehistoryofmassassemblyandre-arrangementandthehistoryofstarformation.Theoldstellarpopulationsplayaparticularroleindecipheringthesehistories,sinceoldstarsusuallyretainamemoryofcertainaspectsoftheirearlylife,suchasthesurfacechemicalabundances,andoftenorbitalangularmomentumandorbitalenergy.

Theimportantquestionsconcerningthemassassembly,apartfromitsrate,bothpastandpresent-day,include:whatwasthenatureofthemass?–sincecollisionlessdarkmatter,collisionlessstarsandcollisionalgasbehavedi erently;whatwasthedensitydistributionofanyandallcomponents?–sincephysicalprocessessuchastidalstrippingdependonrelativedensities,anddynamicalfrictiontimescaledependsonmassratios;whatarethespeci cangularmomentaandorbits?–sincethecouplinge cienciesofvariousprocessesdependonthese.Theimportantquestionsconcerningstarformationhistoryinclude:whatwastherateofstarformationandhowdid/doesitvaryasfunctionofspatiallocation?;whatwasandisthestellarInitialMassFunction?–thevisibilityofgalaxiesathighredshift,theircontribu-tionstobackgroundlightindi erentpassbands,theirchemicalenrichment,stellarfeedback,thesupernovarate,gasconsumptionrateetc.dependontheIMF;whatwas/isthemodeofstarformation?–whatfractionformedinsuperstarclusters?;andofcourse–whatistheconnectiontothehistoryofmassassemblyofthevariouscomponents.

Spiralgalaxiesareclearlydiverseintheirproperties,forexampleinbulge-to-diskratio,buttheoriesshouldbeabletoproducethegalaxypopulationintheLocalGrouprathernaturally,withoutappealtospecialconditions.ThustheLocalGroupmembersare‘typical’galaxiesin

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