Galaxy Interactions and Starbursts at High Redshift

Using high resolution N-body simulations with hydrodynamics and star formation, we investigate interactions and the resulting starbursts in galaxies with properties typical of $z\sim 3$. We apply spectral population models to produce mock-HST images, and d

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aGalaxyDynamics:fromtheEarlyUniversetothePresentASPConferenceSeries,Vol.3×108,bes,G.A.MamonandV.Charmandaris,eds.GalaxyInteractionsandStarburstsatHighRedshiftR.S.Somerville1,G.Rosenfeld1,T.S.Kolatt2,A.Dekel1,J.C.Mihos3andJ.R.Primack212RacahInstituteofPhysics,HebrewUniversity,Jerusalem3PhysicsDepartment,UniversityofCalifornia,SantaCruzDepartmentofAstronomy,inghighresolutionN-bodysimulationswithhydrody-namicsandstarformation,weinvestigateinteractionsandtheresultingstarburstsingalaxieswithpropertiestypicalofz～3.Weapplyspectralpopulationmodelstoproducemock-HSTimages,anddiscusstheob-servedmagnitude,color,andmorphologicalappearanceofoursimulatedgalaxiesinboththerest-UVandrest-visualbands.1.IntroductionAninterestingalaxyinteractionsathighredshiftcanbemotivatedfromseveraldirections.Ithasbeenshownthatmanyoftheobservedpropertiesofgalaxiesatz～3–4(e.g.,theirnumberdensity,luminosityfunction,colours,sizes,velocitydispersions,etc.)arewell-reproducedbyamodelinwhichthesegalaxiesarepredominantlystarburststriggeredbygalaxyinteractions(Somerville,Primack,&Faber1999;SPF).Inaddition,thecollisionrateofdarkmatterhalosinhigh-resolutioncosmologicalN-bodysimulationsshowsamarkedincreaseatearlierepochs,andtheclusteringpropertiesofthesecollidinghalosatz～3aresimilartothoseoftheobservedLyman-breakgalaxies(Kolattetal.1999).Previousnumericalinvestigationsofstarburstsininteractinggalaxies(e.g.Mihos&Hernquist1994a;1996)haveassumedinitialpropertiestypicaloflocalgalaxies.Manyoftheseproperties(e.g.,gascontent,surfacedensity,disk-to-

haloratio)maybequitedi ingthesamecodeasMihos&Hernquist,wearecarryingoutanongoingprogramofsimulationstomorefullyexploretheparameterspacewithanemphasisonhigh-redshiftgalaxies.AmorecompleteversionofthesepreliminaryresultswillbepresentedinRosenfeldetal.(inprep.).

2.Simulations

WeNusetheTREESPHcode,includingstarformationusingaSchmidtlaw(ρ˙ =Cρ

constantgas)asdescribedinMihos&Hernquist(1994b).Wesettheproportionality

inthisequationbyrequiringtheisolatedgalaxiestolieontherelationgivenbyKennicutt(1998).Massesandmassratiosoftypicalcollidingdarkmatterhalosatz=3weredeterminedusingthesimulationsdiscussedinKolattetal.(1999).Forallcalculationsrequiringtheassumptionofacosmology,we

1

Using high resolution N-body simulations with hydrodynamics and star formation, we investigate interactions and the resulting starbursts in galaxies with properties typical of $z\sim 3$. We apply spectral population models to produce mock-HST images, and d

GalaxyInteractionsandStarburstsatHighRedshift

2Figure1.Starformationrate(left)andobservedmagnitudeatz=3asafunctionoftimesincethestartofthesimulation.Intherightpanel,thebottomcurvesincludeanestimateofthee ectsofdust(seetext).assumethesamecosmologicalmodelasthosesimulations,namely 0=0.3, Λ=0.7,andH0=70km/s/Mpc.Themassandexponentialscaleradiusofthestarsandgasinthegalacticdiskinhabitingahaloofagivenmassatthedesiredredshiftareestimatedusingthesemi-analyticmodelsofSPF,andareconsistentwithknownpropertiesofLyman-breakgalaxiesatz～3.Thedisksareassumedtobestablebeforethestartoftheinteraction.Theresultspresentedhereareforaninteractionbetweenequalmass,bulgelessgalaxies,withhalomass7.1×1011M⊙,stellarmass7.2×109M⊙,stellarscaleradius1.7kpc,gasfraction0.5andgasscaleradius3.4kpc.Allotherproperties(relativeinclination,orbit,etc.),arethesameasthe ducialcaseofMihos&Hernquist(1996;MH96).

3.Results

Thestarformationrateoverthecourseoftheinteractionisshownin gure1(leftpanel).ThebehaviourisqualitativelysimilartotheresultsofMH96.Weconvolvethisstarformationhistorywithstellarpopulationmodels(GISSEL98,Bruzual&Charlotinprep.)toobtaintheapparentmagnitudeofthegalaxyatz～3( gure1,rightpanel).WehaveusedthesolarmetallicitymodelswithaSalpeterIMF.WeshowthemagnitudesintheWF/PCF606W(V606)andNICMOSF160W(H160) lters(ABsystem),whichprobetherest-frame～1500 Aand4000 Apartofthespectrumatz=3.Thetopsetofcurvesintherightpanelneglectsdustextinction;thebottomsetshowstheresultofincludingacorrectionofafactorof～5at1500 A,assuggestedbyrecentobservationalestimatesoftypicalextinctioncorrectionsinbrightLBGs(Meureretal.1999;Steideletal.1999),andassumingaCalzettiattenuationcurve(Calzettietal.1996).Withthislevelofdustextinction,theburstinggalaxywouldbevisibleatpresentspectroscopiclimitsforabout200Myr,whereasintheabsenceofthe

Using high resolution N-body simulations with hydrodynamics and star formation, we investigate interactions and the resulting starbursts in galaxies with properties typical of $z\sim 3$. We apply spectral population models to produce mock-HST images, and d

GalaxyInteractionsandStarburstsatHighRedshift3burstthegalaxywouldhavebeenwellbelowthedetectionlimit.Notethatintheabsenceofdustreddening,theV606 H160colorofthegalaxyduringtheburstisquiteblue(-0.5–0),butafterthedustcorrection,typicalvalues(0.5to1.0)arereasonablyconsistentwithobservations.

Figure2showshowtheinteractinggalaxywouldappearifobservedatz～3byHST(intheabsenceofnoiseorskybackground)atvarioustimesduringthemerger.LeftpanelsshowtheWF/PCV606 lterwithapixelsizeof0.04arcsec;therightpanelsshowtheNICMOSH160 lterwith0.08arcsecpixels.Fromtoptobottom,thegalaxiesareshownatthebeginningofthesimulation,duringthe rstinteractionofthegalaxies,afterthegalaxieshaveseparatedagain,and nallyafterthe nalmerger.Theseimagescanbematchedupwiththestarformationandtotalmagnitudecurvesshownin gure1bymultiplyingthesimulationtimeunitsshownonthe gurebyafactorof8.3toconverttoMyr.

Ifinfacttheobservedgalaxypopulationbecomesincreasinglydominatedbymergingstarburstsystemsathigherredshifts,onewouldexpecttoobservelargerfractionsofgalaxieswithdisturbedmorphologiesandsigni cantsubstruc-ture.Weplantousesynthesizedimagessuchas gure2todevelopstatisticstoquantifythesee ects,andeventuallytotestwhetherthemorphologicalproper-tiesofobservedhigh-redshiftgalaxiesareconsistentwiththecollisionalstarburstscenario.

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1999,ApJ,519,1

Using high resolution N-body simulations with hydrodynamics and star formation, we investigate interactions and the resulting starbursts in galaxies with properties typical of $z\sim 3$. We apply spectral population models to produce mock-HST images, and d

GalaxyInteractionsandStarburstsatHighRedshift

4Figure2.MockHSTimagesofsimulatedstarburstgalaxiesatz～3atdi erenttimesduringthemerger.LeftpanelsshowstheV606 lter(rest～1500 A)andrightpanelsshowtheNICMOSH160 lter(rest～4000 A).Thelightestgreycorrespondstoasurfacebrightnessof28andthedarkestto18magnitudesarcsec 2.

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