A Revised Model for the Formation of Disk Galaxies

arXiv:astro-ph/0604553v29Jan2007THEASTROPHYSICALJOURNAL654:27-52,2007JANUARY1PreprinttypesetusingLATEXstyleemulateapjv.10/09/06AREVISEDMODELFORTHEFORMATIONOFDISKGALAXIES:LOWSPINANDDARK-HALOEXPANSIONAARONA.DUTTONDepartmentofPhysics,SwissFederalInstituteofTechnology(ETHZurich),CH-8093Zurich,SwitzerlandFRANKC.VANDENBOSCHMax-Planck-InstitutfürAstronomie,Königstuhl17,69117Heidelberg,GermanyAVISHAIDEKELRacahInstituteofPhysics,TheHebrewUniversity,Jerusalem,IsraelANDSTÉPHANECOURTEAUDepartmentofPhysics,Eng.Physics&Astronomy,Queen’sUniversity,Kingston,ONK7L3N6,CanadaTHEASTROPHYSICALJOURNAL654:27-52,2007January1ABSTRACTWeuseobservedrotationvelocity-luminosity(VL)andsize-luminosity(RL)relationstosingleoutaspecificscenariofordiskgalaxyformationintheΛCDMcosmology.Ourmodelinvolvesfourindependentlog-normalrandomvariables:dark-haloconcentrationc,diskspinλgal,diskmassfractionmgal,andstellarmass-to-lightratioΥI.AsimultaneousmatchoftheVLandRLzeropointswithadiabaticcontractionrequireslow-chalos,butthismodelhasV2.2∼1.8Vvir(whereV2.2andVvirarethecircularvelocityat2.2diskscalelengthsandthevirialradius,respectively)whichwillbeunabletomatchtheluminosityfunction(LF).SimilarlymodelswithoutadiabaticcontractionbutstandardcalsopredicthighvaluesofV2.2/Vvir.Modelsinwhichdiskformationinducesanexpansionratherthanthecommonlyassumedcontractionofthedark-matterhaloshaveV2.2∼1.2VvirwhichallowsasimultaneousfitoftheLF.Thismayresultfromnon-spherical,clumpygasaccretion,wheredynamicalfrictiontransfersenergyfromthegastothedarkmatter.Thismodelrequireslowλgalandmgalvalues,contrarytonaiveexpectations.However,thelowλgalisconsistentwiththenotionthatdiskgalaxiespredominantlysurviveinhaloswithaquietmergerhistory,whilealowmgalisalsoindicatedbygalaxy-galaxylensing.ThesmallerthanexpectedscatterintheRLrelation,andthelackofcorrelationbetweentheresidualsoftheVLandRLrelations,respectively,implythatthescatterinλgalandincneedtobesmallerthanpredictedforΛCDMhalos,againconsistentwiththeideathatdiskgalaxiespreferentiallyresideinhaloswithaquietmergerhistory.Subjectheadings:galaxies:formation—galaxies:fundamentalparameters—galaxies:spiral—galaxies:structure1.INTRODUCTIONInthestandard,colddarkmatter(CDM)basedmodelfordiskformation,setoutbyFall&Efstathiou(1980),disksformoutofgasthatslowlycoolsoutofahotgaseoushalo,associatedwiththedarkmatterpotentialwell,whilemain-tainingitsspecificangularmomentum.Duringthisprocessthedarkmatterhalocontractstoconserveitsadiabaticinvari-ants(Blumenthaletal.1986).Becauseofthecentrifugalbar-rierthegassettlesinarotationallysupporteddiskwhosesizeisproportionaltoboththesizeandangularmomentumofthedarkmatterhalo(Mo,Mao&White,1998;hereafterMMW).Consequently,thestructureanddynamicsofdiskgalaxiesareexpectedtobestronglycorrelatedwiththepropertiesoftheirdarkmatterhalos.Inparticular,thecorrelationsbetweentheobservable,structuralparametersofdiskgalaxies,rotationve-locity,V,size,R,andluminosity,L,areexpectedtobeare-flectionofthevirialpropertiesofdarkmatterhalos,whichscaleasVvir∝Rvir∝M1/3vir.Slightdeviationsfromthesescal-ingsareexpectedfromthefactthatmoremassivehalosare,onaverage,lessconcentrated.Anyfurtherdeviationsmusteitherreflectsomeaspectsofthebaryonicphysicsrelatedtogalaxyformation,orsignalafailureinthestandardpictureoutlinedabove.InwhatfollowswerefertotherelationsbetweentheglobaldiskparametersastheVL,RL,andRVrelations.Inthepast,theVLrelation,alsoknownastheTully-Fisherrelation(Tully&Fisher1977),hasreceivedmuchattentionasadistanceindicatorowingtotherelativelysmallobservedscatter.AlthoughnumerousstudieshaveaddressedtheoriginoftheVLrelation,noconsensushasbeenreached.Inpartic-ular,itiscurrentlystillunderdebatewhethertheoriginoftheVLrelationismainlygovernedbyinitialcosmologicalcon-ditions(e.g.,Eisenstein&Loeb1996;Avila-Reese,Firmani&Hernández1998),orbythedetailedprocessesgoverningstarformation(Silk1997;Heavens&Jiminez1999)and/orfeedback(e.g.,Kauffmann,White&Guiderdoni1993;Coleetal.1994;Elizondoetal.1999;Natarajan1999).Inthemostrecentmodels(vandenBosch2000;2002;Navarro&Stein-metz2000;Firmani&Avila-Reese2000,hereafterFA00)itistypicallyunderstoodthatbothinitialconditionsandbary-onicphysicsrelatedtostarformationandfeedbackmustplayanimportantrole.ReproducingtheVLzeropointhasalsobeenalongstandingproblemforCDMbasedgalaxyforma-tionmodels.Inparticularnomodelhasbeenabletosimul-taneouslymatchtheluminosityfunctionandVLzeropointusingstandardΛCDMparameters(Coleetal.2000;Benson2Duttonetal.2006etal.2003;Yang,Mo,&vandenBosch2003).Thisprob-lemcanbetracedtothehighvaluesofV/VvirexpectedforΛCDMhalosoncetheeffectsofthebaryons,suchasadia-baticcontraction(Blumenthaletal.1986),aretakenintoac-count.Allsolutionstothisproblemrequireachangetoeitherthestandardcosmologicalmodelorthestandardpictureofgalaxyformation.Anotherpotentialproblemfordiskformationmodelsistheformationoflargeenoughdisks.Therelativefragilityofdiskgalaxiesandthestronglyorderedmotionoftheirstarsandgasisgenerallyinterpretedasevidenceforarelativelysmoothformationhistorywithoutviolentmergerprocesses.Thesub-sampleofdarkmatterhaloswi