The Dark Matter Radial Profile in the Core of the Relaxed Cluster A2589

We present an analysis of a Chandra--ACIS observation of the galaxy cluster A2589 to constrain the radial distribution of the total gravitating matter and the dark matter in the core of the cluster. A2589 is especially well-suited for this analysis because

DraftversionFebruary2,2008

APreprinttypesetusingLTEXstyleemulateapjv.11/12/01

THEDARKMATTERRADIALPROFILEINTHECOREOFTHERELAXEDCLUSTERA2589

DepartmentofPhysicsandAstronomy,UniversityofCaliforniaatIrvine,4129FrederickReinesHall,Irvine,

CA92697-4575,buote@uci.edu,lewisa@uci.edu

ToAppearinTheAstrophysicalJournalMarch20Issue

DavidA.BuoteandAaronD.Lewis

arXiv:astro-ph/0312109v2 29 Jan 2004

ABSTRACT

WepresentananalysisofaChandra–ACISobservationofthegalaxyclusterA2589toconstraintheradialdistributionofthetotalgravitatingmatterandthedarkmatterinthecoreofthecluster.A2589isespeciallywell-suitedforthisanalysisbecausethehotgasinitscoreregion(r 0.1rvir)isundisturbedbyinteractionswithacentralradiosource.Fromthelargestradiusprobed(r=0.07rvir)downtor≈0.02rvirdarkmatterdominatesthegravitatingmass.Overthisregiontheradialpro lesofthegravitatinganddarkmatterare ttedwellbytheNFWandHernquistpro lespredictedbyCDM.Thedensitypro lesarealsodescribedwellbypowerlaws,ρ∝r α,whereα=1.37±0.14forthegravitatingmatterandα=1.35±0.21forthedarkmatter.Thesevaluesareconsistentwithpro lesofCDMhalosbutaresigni cantlylargerthanα≈0.5foundinLSBgalaxiesandexpectedfromself-interactingdarkmattermodels.

Subjectheadings:galaxies:clusters:individual(A2589)—darkmatter—intergalacticmedium—

X-ray:galaxies:clusters—cosmologicalparameters

1.introduction

Theverypreciseconstraintsthathavebeenplacedre-centlyonthecosmologicalworldmodelbyobservationsofthecosmicmicrowavebackground(e.g.,Spergeletal.2003)andhigh-redshiftsupernovae(e.g.,Perlmutteretal.1999)requirethatmostofthematterintheuniverseisnon-baryonic“darkmatter”andthatthelargestcontrib-utortotheenergydensityoftheuniverseisthe“darken-ergy”.Butthesetwomysteriousquantitiesthatdominatetheenergydensityoftheuniversestillcouldbemerely t-tingparameters–akintoPtolemaicepicycles–thatarenotphysicalquantities.Thereis,therefore,greaturgencytodiscoverthenatureofthedarkmatteranddarken-ergywhicharethefoundationsofthenewcosmologicalparadigm.

Thestructureofdarkmatter(DM)halosisasensitiveprobeofthepropertiesoftheDM.InthestandardΛCDMparadigm,N-bodysimulationsshowthattheradialden-sitypro lesofDMhalosarefairlyuniform,approximatelyparameterizedbytheNFWpro le,ρ(r)∝r 1(rs+r) 2(Navarroetal.1997).Atsmallradii(r rs)thedensitypro lesfollowapowerlaw,althoughtheprecisevalueofthepower-lawexponentremainscontroversial;i.e.,ρ(r)∝r α,withα=1accordingtoNFWandα=1.5accordingtothesimulationsofMooreetal.(1999).Ob-servationsoftherotationcurvesoflow-surfacebrightness(LSB)galaxies(e.g.,Swatersetal.2000)suggestapro- lefortheDMthatissubstantially atter(α≈0.5)thanΛCDMinthecentralregions.TheseobservationsinspiredSpergel&Steinhardt(2000)toproposetheexistenceof“self-interacting”DM(SIDM).IntheSIDMmodeltheDMparticlesareassumedtopossesssomecrosssectionforelasticcollisionswitheachother.DetailedCDMsimu-lationsincorporatingtheSIDMideahavecon rmedthattheDMpro lesofLSBgalaxiescanbe attenedasob-served(e.g.,Dav´eetal.2001).

LikeLSBgalaxies,galaxyclustersprovideexcellentvenuestostudyDMbecausetheyareDM-dominatedfrom

1

deepdownintotheircores(≈0.01rvir;e.g.,Dubinski1998)outtotheirvirialradii(1rvir).Moreover,severalpowerfultechniquesareavailableforprobingtheclustermatterdis-tributions:opticalstudiesofgalaxydynamicsandgravita-tionallensing,andX-rayobservationsofthehotgas.Eachofthesetechniqueshasadvantagesanddisadvantages.Forexample,someadvantagesofusingX-rayobservationsarethatthehotgasinclusterstracesthethree-dimensionalgravitationalpotentialwithanisotropicpressuretensor.ThequalityofdataforthehotgasislimitedonlybythesensitivityandresolutionoftheX-raydetectorsandnotbythe nitenumberofgalaxies.

ThemostimportantlimitationassociatedwithX-rayobservationsofclustermassdistributionsistheassump-tionofhydrostaticequilibrium.Alargefractionofnearbyclusters(z<0.2)dohaveregularimagemorphologiesandappeartoberelaxedon0.5-1Mpcscales(e.g.,Mohretal.1995;Buote&Tsai1996;Jones&Forman1999).Forsuchclusterswithregularmorphologies(i.e.,notcurrentlyex-periencingamajormerger)cosmologicalN-bodysimula-tionshavedeterminedthatmassescalculatedbyassumingperfecthydrostaticequilibriumaregenerallyquiteaccu-rate(e.g.,Tsaietal.1994;Evrardetal.1996;Mathiesenetal.1999).Butclustersthatareobservedtoberelaxedon0.5-1Mpcscalesaretypicallyassociatedwithcooling ows(e.g.,Buote&Tsai1996),andChandraobserva-tionshavedemonstratedthattheinnercoresofcooling owsarehighlydisturbedexhibitingholesand lamen-tarystructures(e.g.,Fabianetal.2000;Davidetal.2001;Ettorietal.2002)whichcertainlyraiseseriousquestionsabouttheassumptionofhydrostaticequilibrium.

Hence,forstudiesofDMitisimperativeto ndclustersthatarerelaxed(i.e.,havesmooth,regularX-rayimages)fromdeepwithintheircoresoutto~Mpcscales.OurrecentanalysisoftheChandraACIS-Sdataofonesuchcluster(A2029,Lewisetal.2002,2003)hasprovidedthestrongestconstraintonthecoreDMpro leforagalaxyclustertodate;i.e.,α=1.19±0.04,constraineddownto≈0.01rvir.ThecoreDMdensitypro leofA2029iscon-

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