The cluster environments of radio loud quasars

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

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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

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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

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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

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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

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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

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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

Theclusterenvironmentsofradioloudquasars

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