The cluster environments of radio loud quasars

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We have carried out multi-colour imaging of the fields of a statistically complete sample of low-frequency selected radio loud quasars at 0.6

aSesto2001:TracingCosmicEvolutionwithGalaxyClustersASPConferenceSeriesTheclusterenvironmentsofradioloudquasarsJ.M.Barr,M.N.BremerDepartmentofPhysics,UniversityofBristol,TyndallAve,BristolBS81TL,U.K.J.C.BakerAstronomyDepartment,UniversityofCalifornia,Berkeley,U.S.A.M.D.LehnertMax-Planck-Institutf¨urextraterrestrischePhysik,Garching,GermanyAbstract.Wehavecarriedoutmulti-colourimagingofthe eldsofastatisticallycompletesampleoflow-frequencyselectedradioloudquasarsat0.6<z<1.1,inordertodeterminethecharacteristicsoftheirenvironments.Thelargestradiosourcesarelocatedinthe eld,andsmallersteep-spectrumsourcesaremorelikelytobefoundinricherenvironments,fromcompactgroupsthroughtoclusters.Thisradio-basedselection(includingsourcesize)ofhighredshiftgroupsandclustersisahighlye cientmethodofdetectingrichenvironmentsattheseredshifts.Althoughoursingle lterclusteringmeasuresagreewiththoseofotherworkers,weshowthatthesestatisticscannotbeusedreliablyon eldsindividually,colourinformationisrequiredforthis.1.IntroductionPowerfulradiosourcesatlowredshift,(CygA,3C295),areknowntoresideinmassiveellipticalgalaxiessuchasthosecommonlyfoundatthecentreofclusters.AGNatlowredshifthavebeenshowntoexistinenvironmentsofaboveaveragegalacticdensity(Yee&Green1987)andstudiesextendingthesemeasurementstohigherredshiftshownoevolutioninthistrend(Ellingson,Yee,&Green1991;Hill&Lilly1991;Yee&Ellingson1993;Woldetal.2000).Itisthereforereasonabletoexpectthatathighredshiftwemaybeableto ndclustersbylookingattheenvironmentsofAGN.Thisisfurthersubstantiatedbytheoreticalargumentsregardingtheformationofextendedlobe-dominatedradiostructures(e.g.Miley1980).Suchmodelsrequireacon ningmediumtoexistoverhundredsofkpc,thescalelengthsandpressuresbeingcomparablewiththosefoundinclustersofgalaxies.Anadvantageofinstigatingasearchinthismannerisprovidedbythefactthataquasarwillpinpointtheredshiftofinterestandthroughcarefulchoiceof ltersfacilitatetheidenti cationofassociatedgalaxiesbywayoftheircolours.

Thesereasonsmakeitpossibletobuildasampleofdistantgroupsandclustersfromastudyofradioloudquasars(RLQs),althoughcaremustbetakentounderstandtheselectionfunction.WithreasonableestimatesregardingthelifetimeoftheAGNanditse ectontheclusterthiscanbeachieved.Inanycasetheopportunitytostudyclustersandtheirconstituentsisnotlost.Arelatedbene tisthatwemay ndandstudysmallandcompactgroupsathighredshift.CurrentopticalandX-raymethodswillbeinsensitivetosuchlowmass/luminositysystemsandtheenvironmentsofRLQsmay

We have carried out multi-colour imaging of the fields of a statistically complete sample of low-frequency selected radio loud quasars at 0.6
2Barr,Bremer,Baker,
Lehnert
Figure1.Valuesofthespatialcrosscorrelationamplitudeasafunctionofredshiftforradioloudquasar elds.Filledsquares,thiswork.Diamonds,Woldetal.(2000).Triangles,Yee&Ellingson(1993).Squares,Ellingsonetal.(1991).Crosses,Yee&Green(1987).Asterisks,McClure&Dunlop(2000).ThehorizontallinesrepresenttheAbellclassesquotedinMcClure&Dunlop.
o erthebestmethodof ndingsigni cantnumbersofthesesystemsathighredshiftforsometimetocome.
2.Sampleandobservations
ThequasarsusedforthisstudyaredrawnfromtheMolongloquasarsample(MQS,Kapahietal.1998)oflowfrequencyselectedradioloudquasars.Thisisacompletesampleof111RLQswithS408>0.95Jyinacontiguousareaofsouthernsky( 30 >δ> 20 ,butexcludingtheR.A.range14h03m-20h03m).
Withintherange0h0m<RA<14h0mthereare30MQSobjectswhichhave0.6<z<1.1,22forwhichwehavedata.Eightsourceswiththesecriteriawererandomlyexcludedduetoobservingtimerestrictions.Thissamplehasmanymerits.Firstlyithasasingleselectioncriterion,thatoflowfrequencyradioluminosity.Thusitdoesnotattemptto“match”cataloguesofwhatmaybeintrinsicallydi erentobjects.Secondly,ofallobjectsinourchosenredshiftrangenoneareexcludedbasedonopticalorradiobias.Theremainingobjectscanthereforebethoughtofasarepresentativesample.Also,thelowfrequencyselectionavoidspreferentiallyselectingcoredominatedobjects,ie:thosewhichmaybeofintrinsicallylowerluminositybuthaveDopplerboostedemissionduetotheirorientationtothelineofsight.TheMRQisthususefulasa“completeandunbiased”sampleonwhichtostudytheenvironmentalpropertiesofRLQs.
Wehaveobservedourobjectsin2ormore lterswhichstraddletheredshifted4000 Abreakinordertomaximisethecontrastforearlytypegalaxies.Inseveralcaseswealsohavenearinfrared(NIR)JandK′ lters.ThetelescopesusedweretheCTIO4m,theESO2.2mand3.6mandtheAAT.
3.Single lteranalysis
Previousinvestigatorshavesoughttoquantifytheexcessofobjectsaroundquasars,(orradiogalaxies,oranyotherobject),bycountingsourceswithinacertainradiusinasingle lter(0.5Mpcinmostcases).Weinitiallyanalyseclusteringaroundquasarswithreferencetosuchameasure,thegalaxy-quasarspatialcrosscorrelationamplitude,

We have carried out multi-colour imaging of the fields of a statistically complete sample of low-frequency selected radio loud quasars at 0.6
Theclusterenvironmentsofradioloudquasars
3
Thesurfacedensityofobjectsasafunctionofpositioninthe eldsof
threeofourquasars,binnedaccordingtocolourrange.Clustercoresattheredshiftofthequasarwouldproducepeaksinthedensitymapsindicatedbythearrow.Whileallthree eldsproducehighvaluesofBgq,wecanseethatinthesecondcasetheoverdensityisatthewrongcolourforthequasarredshift.Thepositionofthequasarismarkedbythecircle.Figure2.
Bgq.AdetailedderivationcanbefoundinYee&Green(1984)andLongair&Seldner(1979).
Theresultsofapplyingthismethodtoourdatacanbeseenin gure1,aswellasthoseforotherinvestigators.Ourresultscon rmthatthequasarsoccupyavarietyofenvironmentsandshownoevidenceforevolutiontoaredshiftof1.
Thereare,however,reasonswhythesemethodsarenotnecessarilythemostaccu-ratetouseathighredshift.Firstly,theyassumethattheAGNisatthecentreofany(roughlycircular)overdensity.Asweprogresstohigherredshifts,clustersarelesslikelytobeinrelaxedspheroidalsystemsandmayexhibitsigni cantsubstructurewhichcanextendfurtherthan0.5Mpcfromthequasar.Secondly,thenumberdensityofobjectsatthemagnitudelevelsofinterestdisplaysvariationonthescale-lengthsofdistantclus-tersmakingitdi culttoestimatethebackgroundtosubtractfromanyoverdensity.Thirdly,evenwhenanobviousoverdensityisdiscovered,itmayconsistofobjectswhichhavethewrongcolourstobegalaxiesattheredshiftofthequasar(see g.2b).Inthiscasethesestatisticalanalyseswillnotonlybewrong,theymaybiastheaverageresultsystematicallyandindoingsorenderanymeasureoftheaveragepropertiesofasampleincorrect.
4.Multi-colourresults
Bychoosingoptical lterswhichstraddlethe4000 Abreak,wecanselectthepassivelyevolvingellipticalgalaxiesattheredshiftofthequasar.Figure2.showsthedensityofobjectsaroundthreeofourquasarsseparatedaccordingtocolour.Wecanidentifyagglomerationsashavingthepotentialtobeatthequasarsredshiftbyusingthesesortofplots.FurtheranalysislookingatthecolourdispersionorforaredsequenceorusingNIR lterscanthenquantifyaccuratelywhichgalaxiesbelongtotheclustercoreandwhichdonot.ExamplesofthesetechniquesaretobefoundinBakeretal.(2001)andBremeretal.(2001).

We have carried out multi-colour imaging of the fields of a statistically complete sample of low-frequency selected radio loud quasars at 0.6
4Barr,Bremer,Baker,
Lehnert
Figure3.Oursampledividedintosmallandlargesteepspectrumsources.Clusteredenvironmentsaremoreoftenassociatedwithsmallerradiostructures.
Becauseoftheargumentsregardingtheformationoftheradiolobesbeingdepen-dentonalargescalemedium,wewouldexpecttoseeacorrelationbetweenthesizeofthesourceandtheclusteringaroundit.Weseejustsuchacorrelationin gure3.Oursampleisseparatedintothosesourceswitharadiosize<20′′andthosewith>20′′.Unresolved atspectrumsources,(α<0.6),arerejectedinordertopreventbeamedsourceswithintrinsicallylowerluminosityfrombeingincluded.Theremainingsampleof19steepspectrumobjectsisthussplitapproximatelyinhalf.Ifwelooselyde neourclustersasthosewithmorethan10associatedcoremembers,weseethat veoutofoursixclustersareassociatedwithsmallerradiostructures.Wewouldn’texpecttoseea1:1relationshipbetweensizeandclustering.Theageofthesourcemayhaveallowedittoexpanddespiteaclusteredenvironment,orconversely,perhapsweseeayoungsourcewhichisexpandinguncon ned.Projectione ectsmayalsohidethetruesizeofthesource.
5.Summary
WehavefoundthatRLQsexistinavarietyofenvironmentsfromthe eldthroughcompactgroupstorichclusters.Inordertocorrectlyquantifytheenvironmentsofthesesources,multicolourinformationmustbeused.Thereisevidenceforaradiosize-environmentrelationwithsmallersourceslocatedinrichersystems.Byselectingsmall,powerfulRLQswecane cientlycreateasampleofdistantgroupsandclusters.Wealsohavetheabilityto“dialup”clustersataspeci credshiftbylookingatAGN.References
Baker,J.C.,Hunstead,R.W.,Bremer,M.N.,Bland-Hawthorn,J.,Athreya,R.M.,Barr,J.2001,AJ,121,1821
Bremer,M.N.,Baker,J.C.,Lehnert,M.D.2001,MNRAS,inpress
Ellingson,E.,Yee,H.K.C.,Green,R.F.1991,ApJ,371,49
Hill,G.J.,Lilly,S.J.1991,ApJ,367,1
Kapahi,V.K.,Athreya,R.M.,Subrahmanya,C.R.,Baker,J.C.,Hunstead,R.W.,McCarthy,P.J.,vanBreugel,W.1998,ApJS,118,327
Longair,M.S,Seldner,M.1979,MNRAS,189,433
McClure,R.J.,Dunlop,J.S.2001,MNRAS,321,515

We have carried out multi-colour imaging of the fields of a statistically complete sample of low-frequency selected radio loud quasars at 0.6
Theclusterenvironmentsofradioloudquasars
Miley,G.1980,ARAA,18,165
Wold,M.,Lacy,M.,Lilje,P.B.,Serjeant,S.2000,MNRAS,316,267Yee,H.K.C.,Green,R.F.1984,ApJ,280,79
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