2XMM J083026+524133 The most X-ray luminous cluster at redshift 1

cESO2008󰀆

2XMMJ083026+524133:

ThemostX-rayluminousclusteratredshift1

G.Lamer1,M.Hoeft1,J.Kohnert1,A.Schwope1,andJ.Storm1

arXiv:0805.3817v2 [astro-ph] 27 May 2008AstrophysikalischesInstitutPotsdam,AnderSternwarte16,D-14482Potsdam,Germanye-mail:glamer@aip.deReceived;accepted

ABSTRACT

Aims.InthedistantuniverseX-rayluminousclustersofgalaxiesarerareobjects.Largeareasurveysarethereforeneededtoprobethehighluminosityendoftheclusterpopulationatredshiftsz󰀁1.

Methods.WecorrelatedextendedX-raysourcesfromthesecondXMM-Newtonsourcecatalogue(2XMM)withtheSDSSinordertoidentifynewclustersofgalaxies.DistantclustercandidatesinemptySDSSfieldswereimagedintheRandzbandswiththeLargeBinocularTelescope.WeextractedtheX-rayspectraoftheclustercandidatesandfittedthermalplasmamodelstothedata.

Results.Wedeterminedtheredshift0.99±0.03for2XMMJ083026.2+524133fromitsX-rayspectrum.Withabolometricluminosityof1.3·1045erg/secthisisthemostX-rayluminousclusteratredshiftsz≥1.Wemeasuredagastemperatureof7.8±0.8keVandfindtentativeevidenceforacoolcore.Theopticalimagingrevealedarichclusterofgalaxies.Keywords.Galaxies:clusters:individual:2XMMJ083026.2+524133–X-rays:galaxies:clusters

1.Introduction

Clustersofgalaxiesplayanimportantroleascosmologi-calprobesandaslaboratoriesfortheevolutionofgalax-ies.ThereisobservationalevidencethatthecosmologicalevolutionoftheclusterX-rayluminosityfunctionmakesverymassive,X-rayluminousclustersrareobjectsathighredshifts(Henryetal.,1992;Rosati,Borgani&Norman,2002,e.g.).Atthesametime,simulationsshowthatthenumbercountsofthemostmassivedistantclustersdependstronglyonthecosmologicalparameters,makingthemthemostinterestingclustersampleforcosmologicalstudies(Borgani&Guzzo,2001;Romeretal.,2001,e.g.).

AftertheidentificationoftheROSATsurveysonlyahandfulofclustersatredshiftsbeyondz=1wereknown,sinceonlyfewROSATobservationsweredeepenoughtoreachthenecessaryfluxlimits.Theidentifi-cationofserendipitouslydetectedXMM-NewtonsourcespushedtheredshiftlimitforX-rayselectedclustersbe-yondz=1.4(Mullisetal.,2005;Stanfordetal.,2006).Clustersatredshiftsz∼1arenowroutinelyidentifiedfromserendipitousXMM-Newtonsourcesorindedicatedsurveyfields(e.g.Pierreetal.,2006;Finoguenovetal.,2007).However,themajorityoftheseclustershaveluminositiesofaonlyfewtimes1043erg/sorless,placingtheminthetran-sitionregimebetweenclustersandgroupsofgalaxies.Inordertoprobethehighluminositypartofthedistantclus-terpopulation,largeareasurveysareneededtofindhighluminosity,distantclusters.TodatethelargestmediumsensitivityX-raysurveyistheserendipitousXMM-Newtonsurvey,i.e.thesourcesfoundintheEPICfieldsoftheaccumulatedXMM-Newtonobservations.

2G.Lameretal.:2XMMJ083026.2+524133

XMMU J083026.2+524133

5 arcmin

Fig.1.EPICMOS1+PNimageofobservation0092800201inthe0.2-4.5keVenergyband.

Thecluster2XMMJ083026.2+524133wasthebrightestX-raysourceamongthese“emptyfield”candidates.

3.X-raydata

2XMMJ083026.2+524133hasbeenserendipitouslyde-tectedinXMMEPICobservationsofthequasarAPM

08279+5255(ObservationIDs0092800101,0092800201)andenteredthe2XMMsourcecatalogueasanextendedX-raysource.Theextendednaturewasclearlyconfirmedbyourvisualscreening(seeFig.1).Somecatalogueparam-etersofthesource2XMM0830aresummarizedinTable3.

Withmorethan4000detectedsourcecountsintheEPICPNcamerathesourceisbrightenoughtoattemptaredshiftdeterminationbasedonitsX-rayspectrum.SincethesourcepositionfallsintoagapbetweentwoCCDsoftheEPICMOS2camera,weonlyusedatafromtheEPICPNandMOS1camerasfortheanalysisdescribedhere.WeextractedthesourcecountsandcreatedthedetectorresponsematrixandeffectiveareafilesusingXMMSASversion7.1.Thesourceextractionregionwasacirclewitharadiusof52′′,beyondwhichtheS/N-ratioistoolow.

ObsID00928001010092800201

Table1.2XMMJ083026.2+524133:2XMMcataloguesourceparameters

data and folded model

10.01−Vek 1−s3 −s0tn1uoc dezil4a−m0r1on5−010.5125Energy (keV)

Fig.2.EPICMOS1(lowerspectruminred)+PN(upperspectrum)datawithbestfittingMEKALmodel.

30

100300kpc

0.5

G

GGG]G

sGtiGG

nGuG

.b0.2

G

GGGra[G

GGGG

yGtisGGoni0.1

GGGmGulGGGG Geccore radius

a󰀁

GGGGGGGfruGGGGGGGGs0.05

GGGGG

GGGGGGGG3β+12G

Ir0=

󰀂

1+

󰀄

r+C

c

󰀁2󰀃−aperture

󰀁

G

r500

󰀁

25

1020

50100

arc sec

Fig.4.Sphericallyaveragedsurfaceluminosityprofileused

toextrapolatetheX-rayluminositytor500.

WeusedXSPECversion12.0tofitaMEKALplasmamodelandtosubtractthebackgrounddata.TheGalacticneutralhydrogencolumndensityinthedirectionof2XMM0830isNH=4.0×1020cm−2,asobtainedfromradiodata(Dickey&Lockman,1990).TheFeK-lineisclearlyvisibleinbothspectraatkredshiftof2XMMBT∼3keV,seeFig.2.Hence,the0830isunam-biguouslydeterminedbytheX-raydata.Thebest-fitredshift,temperature,andmetallicityarez=0.99±0.03,kBT=7.8±0.8keV,andZ/Z⊙=0.35±0.1,seeFig.3.ThespectralfitprovidesalsothebackgroundsubtractedbolometricX-rayfluxFbol=1.9×10−13ergs−1cm−2withintheapertureof52′′.Usingtheconcordancecosmologicalmodelallowstocalculatetheintrinsicluminosityofthecluster.However,theusedaperturecorrespondstoaradiussignificantlysmallerthanrmissingfluxbyapproximating500,thereforeweestimatetheaβ-model.Fig.4showsthesphericallyaveragedsurfaceluminosity.Wedeterminerbythetemperature-radiusrelationgiveninO’Haraetal.500(2007).Fortheredshiftandtemperatureof2XMM0830

G.Lameretal.:2XMMJ083026.2+524133

1.23

1.1 redshift+

Abundance10.40.60.80.9+

0.867

8

kT (keV)

910

0.267

8

kT (keV)

910

Fig.3.Left:ConfidencecontoursforredshiftandtemperaturedeterminedfromtheX-rayspectra.Startingfromthein-nermostcontour,thelevelsindicate68%,90%,and99%confidence.Right:Sameconfidencecontoursformetalabundanceandtemperature.

10 9temperature [keV] 8 7 6 5 4

0 5 10

15 20 25 30radius [arcsec]

35 40

Fig.5.ThefittedX-raytemperaturesof2XMM0830intworegions,namelytheinnermost14′′(110kpc)andre-mainingperiphery.

derivedaboveweobtainr500=800±30kpc,correspondingto100′′.Withtheresultsoftheβ-profilefitwefindfortheintrinsicluminosityof2XMM0830Lbol(Themassofaclustercanbeestimatedbyassuminghydrostaticalequilibriumandsphericalsymmetry.Ifade-terminationofthetemperatureprofileisnotpossible,asfor2XMM0830,onehastoaddisothermality.Asaresult,thetotalmassisgivenby(seee.g,Hicksetal.(2007))M500∼

3β

µmp

(r/rc)2

4G.Lameretal.:2XMMJ083026.2+524133

1 arcmin

Fig.6.SDSSi-bandimagewithEPICMOS1X-raycon-toursof2XMMJ083026.2+524133overlaid.

LBCcameraandthez-bandimagewiththeredcamera.Bothbandshavetotalexposuretimesof18minutes(9×2minutes)andweretakenonMay10th,2008.Thedatare-ductionwasaccomplishedwithapipelinesoftwarebasedontheGarching-BonnDeepSurvey(GaBoDS)pipeline,whichhasbeenmodifiedatAIPtoprocesstheLBCdata.Theas-trometryandphotometryofthestackedimageshasbeentiedtotheSDSS.TheimagequalityofthereducedimagesisFWHM=1.1arcsecinthez-bandimageand1.5arcsecintheR-band.AcolourcompositeofthetwoimagesisshowninFig.7.TheLBTimagesrevealaclearoverdensityofredgalaxieswithintheX-raycontours.ThebrightestclustergalaxyislocatedclosetothepeakoftheX-rayemission.Itsz-bandmagnitudeof19.44andcolourr−z=2.15areconsistentwiththeX-rayredshiftz=1.Adetailedanal-ysisoftheLBTdatawillbepresentedinaforthcomingpaper.

5.Summary

BasedontheX-raydataweshowthattheclusterofgalaxies2XMMJ083026.2+52413345isamassiveclusteratz∼1.Itsluminosityof1.3·10erg/secandtemperatureof7.8±0.8keVareunsurpassedatitsdistanceandbeyond.TheonlycomparableknownclusterisCLJ1226.9+3332(Ebelingetal.,2001)whichislessdistantatz=0.89,butevenmoreluminousandhotter(L.4keVMaughanetal.,2007).

bol=5×1045ergs/sec,kT=10Theluminosityof2XMMJ083026.2+524133ispossiblyenhancedbyacoolcore.SinceitsX-rayfluxmakes2XMMJ083026.2+524133anidealtargetformoredetailedinves-tigationsinX-rays,wehaveappliedforadeepChandraobservationinordertoinvestigatethesuspectedcoolcoreoftheclusterandtoestablishanimprovedmassmodel.Howlikelyisittofindaclusterlike2XMMJ083026.2+524133intheexistingXMM-Newtonserendipi-toussurveydata?AccordingtoRomeretal.(2001)anX-rayallskysurveyjustsensitiveenoughforthe0.5-2.0keVflux7.5×1014erg/(seccm2)of2XMMJ083026.2+524133wouldyield∼10clusterswithtemperatureskT>6keV

Fig.7.LBTrandzbandcolourcompositeimagewithXMMEPICcontours(green).ThebluegalaxyjustsouthoftheX-raycentreisprobablyintheforeground.atredshiftsz>1,whenacosmologywithΩ7isassumed.InthisframeworkthediscoveryM=0.3andΩΛ=0.ofthisclusterinasurveyareaoflessthan1%oftheskyseemstoberatherlucky.WemighthavetoawaittheeROSITAall-skysurvey(Predehletal.,2006)tofindmoreclustersofthistype.

Acknowledgements.GLandMHacknowledgesupportbytheDeutschesZentrumf¨urLuft-undRaumfahrt(DLR)undercontractno.FKZ50OX0201.JKissupportedbytheDFGprioritypro-grammeSTP1177(grantno.SCHW563/23-1).ThisworkisbasedonobservationsobtainedwithXMM-NewtonandondataacquiredusingtheLargeBinocularTelescope(LBT).

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