We study the effects of radiative cooling, star formation and stellar feedback on the properties and evolution of galaxy clusters using high-resolution Adaptive Mesh Refinement N-body+gasdynamics simulations of clusters forming in the LCDM universe. Coolin

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aOutskirtsofGalaxyClusters:intenselifeinthesuburbsProceedingsIAUColloquiumNo.195,2004 c2004InternationalAstronomicalUnionA.Diaferio,ed.DOI:00.0000/X000000000000000XSimulatingtheFormationofGalaxyClustersDaisukeNagaiandAndreyV.KravtsovDepartmentofAstronomyandAstrophysics,KavliInstituteforCosmologicalPhysics,TheUniversityofChicago,Chicago,IL60637;daisuke,andrey@oddjob.uchicago.eduAbstract.Westudythee ectsofradiativecooling,starformationandstellarfeedbackonthepropertiesandevolutionofgalaxyclustersusinghigh-resolutionAdaptiveMeshRe nementN-body+gasdynamicssimulationsofclustersformingintheΛCDMuniverse.Coolingleadstothecondensationofgasintheinnerregionsofclusters,whichinturnleadstosteepeningofthedarkmatterpro le.Thecoolinggasisreplacedbythehigher-entropygasfromtheouterregions,whichraisestheentropyandtemperatureofgasintheclustercore.Themagnitudeofthesee ectsislikelyoverestimatedinthecurrentsimulationsbecausetheysu erfromtheovercoolingproblem:amuchlargerfractionofbaryonsisintheformofcoldgasandstarsthanisobserved.We ndthatthethermalstellarfeedbackalonedoesnotremedythisproblem.Additionalad-hocpreheatingcanlowertheamountofcoldgasbutasimpleuniformpreheatingresultsinincorrectstarformationhistory,asitdelaysthebulkofstarformationuntilz<1.Ouranalysisshowsthattheovercoolinginaclusterasawholeisreallytheovercoolinginthecentralgalaxyanditsprogenitorsathighredshifts.Thisindicatesthatanadditionalheatingmechanismthatcancontinuouslyheatthegasintheclustercoreisrequiredtoreproducetheobservedclusterproperties.EnergyinjectionbytheActiveGalacticNuclei,whichmayprovidesuchheating,maythusbeanimportantmissingingredientinthecurrenttheoreticalmodelsofclusterformation.

We study the effects of radiative cooling, star formation and stellar feedback on the properties and evolution of galaxy clusters using high-resolution Adaptive Mesh Refinement N-body+gasdynamics simulations of clusters forming in the LCDM universe. Coolin

2DaisukeNagaiandAndrey

Kravtsov

Figure1.Thecoldgas+stellarmassfractionofthesimulatedclusterthatincludescooling,starformationandstellarfeedback.ThreelinesindicatetheCSFrunswithvaryingstarformation:thestarformationisallowedtoproceedonlyintheregionwherethelocalgasdensityislargerthan1.0(Red),0.1(Blue)and0.01cm 3(Green).The gureshowsthat35-55%ofbaryonswithinthevirialradius(indicatedbythedashedline)areintheformofcoldgasandstarsatz=0,whichissigni cantlylargerthantheobservedvalueof10-20%.

theSmoothedParticleHydrodynamics(SPH)simulations(e.g.,Suginohara&Ostriker1998;Lewiset.al.2000;Tornatoreet.al.2003).We ndthatthesameproblemexistsinourhigher-resolutionEuleriansimulationsthatincludemetallicity-dependentradiativecooling.Moreover,we ndthatthecoldandstellarmassfractionsdependonthedetailsofstarformationimplementation.However,wewerenotabletosolvetheovercoolingproblembyvaryingparametersofthermalstellarfeedbackwithinreasonablelimits,asshowninFigure1.

3.E ectofPreheating

Additionalpreheatingcanhelpresolvesomeofthediscrepanciesbetweenclustersim-ulationsandobservations.Suchpreheating,presumablyassociatedwiththeepochofvigorousstarformation,isoftenadvocatedasthesolutiontotheovercoolingproblemandthediscrepanciesbetweenthepredictedandobservedclusterscalingrelations(e.g.,Bialeket.al.2001;Muanwonget.al.2002,daSilvaet.al.2004).Ourresults,however,indi-catethattheadditionalpreheatingresultsinstarformationhistorysigni cantlydi erentfromobservations,asshowninFigure2.Ifthepreheatingoccursrelativelyearly(z～3)thegasmanagestocoolbackbyz～1,andtheresultingstarformationrateremainshigh(～1000M⊙yr 1)atz 0.5.Thisresultsinincorrectstarformationhistory:mostoftheclusterstarsforminthelast7-10billionyearsinstarkcontrasttoobservations,whichindicatethatlessthan30%ofclusterstarsformduringthisepoch.Ifpreheatingoccurslate,theamountoftherequiredenergyisunrealisticandtheexpectede ectontheintergalacticmediumisincontradictiontoobservationsofthelow-redshiftlymanalphaforest.Theseresultsprovideastrongargumentagainstthesimpleuniform“preheating”scenario.

We study the effects of radiative cooling, star formation and stellar feedback on the properties and evolution of galaxy clusters using high-resolution Adaptive Mesh Refinement N-body+gasdynamics simulations of clusters forming in the LCDM universe. Coolin

OutskirtsofGalaxyClusters:intenselifeinthesuburbs

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Figure2.Thestarformationrate(left)andthestellarmass(right)asafunctionofredshift.Thesestarformationhistorywasconstructedusingstellarparticlesinthelagrangianvolumeenclosedwithinthevirialradiusofthesimulatedclusteratz=0.Linesindicatethesimulationsthatincludecooling,starformationandstellarfeedback(solid),andthosewithadditionalpreheatingof0.3keV/particleatz=4.5(dotted),z=6(dashed)andz=9(long-dashed).

4.DiscussionsandConclusions

Theovercoolingproblemisaseriouschallengefortheclusterformationsimulations.Ouranalysisshowsthattheovercoolinginaclusterasawholeisreallytheovercoolinginitscore.Inotherwords,thecentralclustergalaxiesformedinsimulationsaretoomassiveandbrightcomparedtoobservations.Analysisofclusterevolutionindicatesthattheproblemsetsinearlyathighredshifts,andisnotlimitedtoasingleobjectorregion.Thisindicatesthatanadditionalheatingmechanismisnecessaryforreproducingtheobservedclusterproperties.TheseresultslendsupporttotheideathatfeedbackfromActiveGalacticNucleiisanimportantmissingingredientinthecurrentsimulationsofclusterformation.

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