X-ray Clusters at High Redshift

I.M.Gioia1

IstitutodiRadioastronomiadelCNR,40129Bologna,Italy,InstituteforAstronomy,Honolulu,Hawaii,96822USA

Abstract.Asthelargestgravitationallyboundstructuresknown,clus-tersprovideclearconstraintsontheformationofstructureandonthecompositionoftheuniverse.Despitetheirextremeimportanceforcos-mologythenumberofclustersathighredshift(z>0.75)israthersmall.ThereareonlyafewX-rayemittingexamplesreportedandahandfulofoptically-selectedones.Theseclusterscanprovidestringentconstraintsontheoriesoflargescalestructureformation,iftheyaremassiveenough.IwillreviewthestatusofthesedistantX-rayselectedclusters.TheseobjectsareofspecialimportancebecausetheirX-rayemissionandpres-enceofgravitationalarcsimplythattheyaremassive,comparabletolowredshiftexamples,andtheirexistenceisproblematicforsometheoriesofstructureformation.

1.Introduction

Theveryexistenceofamassiveclusteratz>0.5isproblematicforstandardCDMtheoriesofhierarchicalstructureformation(Evrard19;Peeblesetal.19).Whilethisproblemhasbeenrecognizedforsometime,ithasnotbeentakentooseriouslybecauseofthelackofconclusiveevidenceoftheexistenceofsuchmassivesystemsathighredshift.

Naturallythestatisticsontheabundanceofhighzclustersarepoorbecausethenumberofsuchclustersknownatpresentissmall.Furthermore,onemightarguethatmanyoftheseclustersmaynotbeasmassiveastheyappearduetoinflationofthemeasuredvelocitydispersionfromfieldgalaxycontaminationandprojectioneffects(seeamongothersFrenketal.1990).However,thenumberofdistantclustersissteadilyincreasingduetothecombineduseoftelescopessuchasHST(HubbleSpaceTelescope)andKeck,aswehaveheardfrommanycontributorsatthismeeting.

X-rayobservationsofdistant(z>0.7)opticallyselectedclustershaveshownthatonlyafewclustersaredetected(seeamongothersCastanderetal.1994),

andthosefewarerelativelymodestX-rayemitters(Lx∼1044ergs−1),withLxmuchlowerthanthatofpresentdayclusters.WhilethisfactwouldseemtoimplythatextremelyrichsystemswithlargeamountsofX-rayemittinggasdonotexistatthesehighredshifts,Iwillshowseveralexamplesthatindicatethatthisisnotthecase.InTable1,fiveexamplesofEMSS(EinsteinMediumSensitivitySurvey;Gioiaetal.1990,Stockeetal.1991)clusterswithaveragez=0.66,andzmax=0.826arelisted.InadditiontotheirhighX-rayluminosity,thereisothercompellingevidencethattheseclustersaregenuinelymassive.Threeofthez∼0.6-0.7clusterscontainlargelensedarcswhichallowacrudeestimateoftheprojectedmassintheclustercores.Forfourofthemweaklensingstudieshavebeenperformed.Theletter“s”inthethirdcolumnofTable1indicatesthepresenceofgiantarcsintheclustercore,whiletheletter“w”indicatesthatweaklensinganalyseshavebeenperformed.Lxisgivenin

2−1inthe0.3−3.5keVband,Tismeasuredinthe2−10unitsof1044h−x50ergs

keVbandandN0.5isthecentralrichnessofthecluster(definedinBahcall1981;noticethatinthesameunitsComahasN0.5=28).

Table1.

Name

EMSSClusterswithz>0.5

redshift

lensing

Lx(ergs−1)

Tx(keV)

N0.5

σ(kms−1)

Dressler&Gunn1992Gioia&Luppino1994c

Furuzawaetal.1994d

Luppino&Gioia1995e

Maccacaroetal.1994f

Donahue1996g

Carlbergetal.1996h

Donahueetal.1997i

Luppino&Kaiser1997

b

a

2.EMSSz>0.5clusters

SinceitsdiscoveryMS0015.9+16(aliasCL0016+16,Koo1981)hasbeenthearchetypalrich,X-rayluminous,distantcluster.ItispartoftheEMSSsinceitwas“rediscovered”inanobservationpointedatanothertarget.TheclusterisveryrichandhasalinearstructureelongatedintheNE-SWdirection.Smailetal.(1994)reporteddetectionofweakshear,whichhasbeenreconfirmedbytheanalysisperformedbyLuppinoetal.(1996)usingdifferentdeepimageswithalargerfieldofviewandadifferenttechnique(KaiserandSquires,1993;Squires

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andKaiser,1996).TwohoursofRbandKeckdataareinhandbuttheimageshavenotbeenassembledyet(Clowe,privatecommunication).

MS0451−03withanLxofalmost2×1045ergs−1isthemostX-rayluminousclusterintheEMSSandamongthebrightestclustersknown.ASCAdatabyDonahue(1996)showahotclusterat10.4Kevwithironabundanceof15%

1+3.814M⊙.Theshearsolarandatotalmass(within1h−50Mpc)of9.7−2.2×10

signaldetection,performedbyLuppinoetal.(1996)ona7200sRbandimagetakenwiththeUH2.2mtelescopeisat>5σ.TwohoursofRbandKeckdataareinhandalsoforthisclusterbuttheanalysishasnotbeenperformedyet.MS10−03isthemostdistantclusteroftheEMSSandamongthemostdistantX-rayselectedclustersknown.TheclusterhasafilamentarymorphologywiththeX-raycomingfromthecenterandelongatedinthesameE−Wdirectionastheopticalgalaxies(Donahueetal.1997).MS10−03isextremelyrichand

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quitehotat14.7+4−3.5keVasobtainedbyASCA(Donahueetal.1997).Astrongshearsignalat6σlevelisdetected(Luppino&Kaiser,1997).Thetotalmass

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(within1Mpc)fromX-raysandfromweaklensingareconsistent(2-6×1014h−50

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M⊙vs3-30×1014h−50M⊙).DeepimagingofMS1137+66withKeckandwiththeUH2.2mhavebeencollected.Fromthereducedimagesalargearchasbeendiscoveredclosetotheclustercenter(Cloweetal.1997).DifferentlyfromMS10−03andMS0015.9+16thisclusteriscompactandconcentratedwithnofilamentarystructure.Aweaklensinganalysis,performedbyCloweetal.ona8700secondR-bandexposureofKeck,usingtheIband(7500s)2.2mdataasacolorselectiontoremoveclustergalaxies,findsanicecentrallyconcentratedmasspeakfallingexactlyonthebrightestclustergalaxy.Themassfromweaklensingcomesouttobe2.9×1014h−1M⊙at500h−1kpc(assumingthebackgroundgalaxieslieatz=2).

AlsoforMS2053-04twohoursofRbandKeckdataareinhandbuttheimageshavenotbeenassembled.ArecentobservationofthisclusterhasbeenperformedwiththeItalian-DutchBeppoSAXsatellitebyScaramellaetal.(inpreparation).Thedatahavenotbeenfullyreducedyet.ApreliminaryanalysisshowsthisclustertobemuchcoolerthanMS10−03orMS0451−03.3.

TheROSATNEPSurvey

TheNorthEclipticPole(NEP)regionoftheRosatAll-SkySurvey(RASS;Tr¨umper1991)hasthelargestexposuretime(approaching10ks)oftheallRASS.TheNEPregioncoversa9◦×9◦field,andcontainsatotalof465X-raysourcesdetectedat>4σinthe0.1−2.4keV(Mullisetal.1998).Weareidentifyingallsourcesinthefield.TheprincipalderivativeisastatisticallycompletesampleofgalaxyclustersappropriateforabettercharacterizationoftheX-rayluminosityfunctionevolution.WehavediscoveredaverydistantclusterintheNEPatz=0.81.RXJ1716+66(Henryetal.1997)isamongthemostdistantX-rayselectedclusterstogetherwithMS10−03andtheX-ray

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clustersdetectedbyRosati(1997)intheRDCS(ROSATDeepClusterSurvey)orEbelingetal.(1997)intheWARPS(WideAngleROSATPointedSurvey).3.1.

RXJ1716+66

AswithMS10−03,itisnotlikelythatRXJ1716+66isinvirialequilibrium.ThegalaxiesinRXJ1716+66areinaninvertedS-shapedfilamentrunningnorth-easttosouthwest(Fig.1).ClustermembersextendallalongtheS.ThedistancefromthetoptothebottomoftheSisabout1.5Mpc(H0=50kms−1Mpc−1,q0=0.5).X-rayfollow-upobservationswithASCA(100ks)andwiththeROSAT

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HRI(171ks)provideatemperaturein2−10kevofkT=6.7+3−1.8(68%con-fidence)andafluxin0.5−2.0keVof1.16±0.33×10−13ergcm−2s−1.ItisintriguingthatthemorphologyofRXJ1716+66andMS10−03isfilamentarywiththeX-rayscomingfromthecenter.Wenotethissincetheinitialformationofprotoclustersisoftendescribedasmatterflowingalongfilaments(Bondetal.1996)withtheX-raysgeneratedattheimpactpointofthetwocollidingstreamsofmatter(Henryetal.1997).

Fig.1-A1024×1024subarrayimageofRXJ1716+66extractedfromthecenterofa4500sexposureintheI-bandtakenbyG.LuppinoandM.MetzgerwiththeUH8K×8KCCDmosaiccameraontheCFHTprimefocusinSept1995.NorthisupandEasttotheleft.Thisimagespans3.′6×3.′6(0.9h−1Mpcatz=0.81)atascaleof0.′′21/pixel.

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BothCFHT8K×8KmosaicCCDdeepimagesandKeckR-band(7500s)imagesweretaken.TheweaklensinganalysiswasperformedontheR-bandKeckimage,using2.2mI-banddata(26,100s)forcolorterms(Cloweetal.1997).ThemasspeakisjusteastoftheBCG,andanarmofmasstotheNE,neatlyfollowingthelineofgalaxiesNEoftheclustercoreisdetected.DougClowe’analysisgivesaverystrongsignalof4×1014h−1M⊙(backgroundgalaxiesatz=1.5).TheM/LVisequalto210h.Boththeopticallightmapandweaklensingmassmaphavetwospatiallydistinctmassivesub-clusters,aswellasalongfilamentarystructure.4.

Conclusions

Thereisclearevidencefromtheexistingdatathatlarge,densemassconcen-trationsexistedatanearlyepoch.AnotherpossiblesearchstrategytofinddistantX-rayclustersistolookforclustersaroundpowerfulradiogalaxies.Highzmassivestructureshavebeenfoundaroundseveral3Cradiogalaxies(i.e.:3C184,atz=0.99,Deltornetal.1997;3C324,atz=1.2,Smail&Dick-inson,1995,justtoquoteafew).ClusterabundanceshasbeencitedasoneofthestrongestevidenceagainstthestandardCDMmodelasnormalizedtoreproducethemicrowavebackgroundanisotropiesseenbyCOBEsatellite.Thenumberofhigh-zmassiveclusterspredictedbystandardCDM,orothermixedDarkMattermodels,istoolowwithrespecttothenumberofclustersobserved.Inlow-densityuniversesthereislessevolutionandnearlyall1015M⊙clustersformedbyz=0.5.Inahigh-densityuniverse,instead,only5%ofthepresentday1015M⊙clustershaveformedbythesameredshift.Thusmassiveclustersshouldbemuchrareratepochsearlierthan0.5ifΩ=1,contrarytotheobser-vations.AlternativeshavebeensuggestedwithlowΩmodels(Viana&Liddle,1996;Ekeetal.1996).TheopenCDM(Ω=0.3)andΛ-dominatedCDMmodelsarepreferredbecausetheyarecompatiblewiththedata.ItmaybepossibletoestimateΩfromforthcomingobservationsofintermediate-distantclusters(uptoz∼1).

Acknowledgments.IamgratefultoP.Henry,C.Mullis,N.Kaiser,G.Luppino,M.Donahue,D.Cloweforstimulation,helpandadvice.ThisworkhasreceivedpartialfinacialsupportfromNASA-STScIgrantGO-02.01-93AandGO-05987.02-94A,fromNSFAST95-00515,fromNASAgrantNAG5-2594andASIgrantsARS-94-10andARS-96-13.References

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