The Effect of the Interstellar Model on Star Forma

arXiv:0709.1972v1[astro-ph]12Sep2007DraftversionFebruary1,2008PreprinttypesetusingLATEXstyleemulateapjv.10/09/06THEEFFECTOFTHEINTERSTELLARMODELONSTARFORMATIONPROPERTIESINGALACTICDISKSElizabethJ.Tasker1,2andGregL.Bryan2DraftversionFebruary1,2008ABSTRACTWestudiedtheeffectofinterstellargasconditionsonglobalgalaxysimulationsbyconsideringthreedifferentmodelsfortheISM.Ourfirstmodelincludedonlyradiativecoolingdownto300K,oursecondmodeladdedanadditionalbackgroundheatingtermduetophotoelectricheating,andourthirdmodelusesanisothermalequationofstatewithatemperatureof104Kandnoexplicitheatingorcooling.Twocommonprescriptionsforstarformationareimplementedineachcase.Thefirstisbasedoncosmologicalsimulationswithalowthresholdforstarformationbutalsoalowefficiency.Thesecondassumesstarsformonlyinhighdensityregionsbutwithahigherefficiency.WealsoexploretheeffectsofincludingfeedbackfromtypeIIsupernovae.WefindthatthedifferentISMtypesproducemarkeddifferencesinthestructureofthediskandtemperaturephasespresentinthegas,althoughinclusionoffeedbacklargelydominatestheseeffects.Inparticular,sizeofthestar-formingclumpswasincreasedbothbybackgroundheatingandbyenforcinganisothermalISM.WealsolookedattheonedimensionalprofilesandfoundthatalognormalPDFprovidesagoodfitforalloursimulationsoverseveralordersofmagnitudeindensity.Overall,despitenoticeablestructuraldifferences,thestarformationpropertiesinthediskarelargelyinsensitivetoISMtypeandagreereasonablywellwithobservations.Subjectheadings:galaxies:spiral,galaxies:ISM,galaxies:evolution,methods:numerical,ISM:structure1.INTRODUCTIONStarformationisoneofthemostperplexingprocessesinthegalaxyduetoitsimmensecomplexityontheverysmallscalesanditsapparentsimplicityontheverylarge.Whatisdifficulttoknowistowhatextentthesmallscalephysicscanbeignoredwhenconsideringtheglobalevolutionofgalaxies.Observationally,weknowthereisasimplerelationshipbetweenthesurfacedensityofgasinadiskgalaxyandthesurfacedensityofstarformation(Kennicutt1998,1989;Schmidt1959).Thisshouldimplythatstarforma-tionisastraightforward,universalprocessdependingonaveragepropertiesofthegasovermanykiloparsecs.The(perhapsnaive)pictureisoneofgravityactingtocollapsethegasintogiantmolecularcloudsoutofwhichstarsform,aprocesswhichishinderedbyrotationalshear,thermalpressure,turbulence,magneticfields,cosmicraypressureandenergyinjectedfromsupernovae(MacLow2004).Yetwhenwestartlookingatstarformationonsmall,parsec,scales,wefindthegasoutofwhichstarsformtobeaturbulent,multiphasemedium,stronglyaf-fectedbylocalconditionsandnotatallindicativeofagloballaw.Socomplexisthisinterstellarmedium(ISM)thatsim-ulationswhichmodelithavebeenforcedtoconsideronlysmallsectionsofthegalaxytoachievetherequiredres-olution(e.g.Slyzetal.2005;Joung&MacLow2005;deAvillez&Breitschwerdt2004).Untilrecently,sim-ulationswhichmodeltheentiregalaxyhavebeenforcedtosimplifythestructureoftheISMtoanisothermal1DepartmentofAstronomy,UniversityofFlorida,Gainesville,FL326112DepartmentofAstronomy,ColumbiaUniversity,NewYork,NY10027orfixedtwo-orthree-phasebodyforthecalculationtobeatallfeasible(e.g.Harfstetal.2006;Lietal.2005;Robertson,Yoshida,Springel,&Hernquist2004;Semelin&Combes2002).NotthatsuchassumptionsaboutthestructureoftheISMaregroundless.Analyti-calcalculationsperformedbyMcKee&Ostriker(1977)puttogetherthenowtraditionalpictureofathree-phaseISM,largelyfilledbyhotgasfromsupernovaeexplo-sions.Othershavesuggestedmodifications(e.g.Cox2005;Norman&Ikeuchi1989)butthisbasicscenarioisstillwithus.Yetwhilebothoftheseapproacheshavebeenhighlyinformativeabouttheevolutionandstructureofgalaxiestheyeachhavedisadvantages.Thesmallboxsimulationsareunabletomodelglobalpropertiessuchasstarforma-tionhistories,diskstructureandtheSchmidtlaw,result-ingintheirmaincomparisonpointsbeingrestrictedtogalaxieswhereweareabletoobserveandmeasuretheISM.Globalmodels,ontheotherhand,havebeenun-abletocomputetheevolutionoftheinterstellarmediumandtheeffectofthissimplificationonthediskgalaxy’sownevolutionisunknown.Recently,however,simulationshavebeendevelopedthatarebridgingthisgap.Whilestillshortoftheresolu-tionattainedinkiloparsec-sizedsimulationboxes,thesemodelsdoachievetherefinementneededtomodelacompletegalaxydiskthatincludesamultiphaseISM.ThispapercontinuesourworkbeganinTasker&Bryan(2006a)(hereafterTB06)whichexaminedathree-dimensionalisolatedgalacticdiskusinganadaptive-meshrefinementtechnique.Inthatpaper,wefoundthatamultiphasemediumwithalargevarietyoftempera-turesanddensities,ingoodagreementwithsimulationsperformedintwo-dimensionsbyWada&Norman(2001)andsmallerscaledthree-dimensionalrunsbyWadaetal.2TASKERANDBRYAN(2002).Theseresultssupportedthesmallboxsimula-tionsinimplyingthatthetraditionalviewoftheinter-stellarmediumasathree-phasestructureinstrictpres-sureequilibriumhassomevaliditybutisasignificantover-simplification.Whatislessclearishowimportantthisis.Theex-istenceoftheSchmidtlawmightimplythatthede-tailsoftheISMarenotnecessarytoachieveaccuratemodelinginglobalgalaxysimulations.Andindeed,theSchmidtlawandtheobservedstarformationthresh-oldindiskgalaxies(Kennicutt1989)hasbeensuc-cessful