The interplay between radio galaxies and cluster environment

A(MNLTEXstylefilev2.2)

Theinterplaybetweenradiogalaxiesandcluster

environment

arXiv:0705.0574v1 [astro-ph] 4 May 20071,2,34

ManuelaMagliocchetti&MarcusBr¨uggen1

234

INAF,OsservatorioAstronomicodiTrieste,ViaTiepolo11,34100,Trieste,ItalyESO,Karl-Schwarzschild-Str.2,D-85748,Garching,GermanyS.I.S.S.A.,viaBeirut2-4,34100,Trieste,Italy

JacobsUniversityBremen,CampusRing1,28759Bremen,Germany

1February2008

ABSTRACT

BycombiningtheREFLEXandNORASclusterdatasetswiththeNVSSradiocat-alogue,weobtainasampleof145,z<0.3,X-rayselectedclustersbrighterthan3·10−12ergs−1cm−2thatshowacentralradioemissionabove3mJy.Forvirial

14.5

massesMvir<∼10M⊙,11clustersoutof12(correspondingto92%ofthesystems)areinhabitedbyacentralradiosource.Thisfractiondecreaseswithhighermassesas

−0.4

.Ifthisdecreaseisaselectioneffect,itsuggeststhatthemajorityofX-ray∝Mvir

selectedclustershostintheircentrearadiosourcebrighterthan∼1020W/Hz/sr.Adivisionofthesampleintoclustersharbouringeitherpoint-likeoranextendedradio-loudAGNrevealsthatthesteepeningoftheLX−Trelationforlow-temperatureclustersisstronglyassociatedwiththepresenceofcentralradioobjectswithextendedjetsand/orlobestructures.Inthelattercase,LX∝T4whileforpoint-likesourcesonerecoversanapproximatelyself-similarrelationLX∝T2.3.MonteCarlosimula-tionsshowthatthesteepeningoftheLX−Trelationisnotcausedbyclustersbeingunder-luminousintheX-rayband,butratherbyoverheating,mostlikelycausedbytheinterplaybetweentheextendedradiostructuresandtheintraclustermedium.Inthecaseoflow-masssystems,wealsofindatightcorrelationbetweenradioluminosityandclustertemperature.Theeffectsofthecentralradiosourceonthethermalstateofaclusterbecomelessimportantwithincreasingclustermass.

Thepresenceofradiosourceswithextendedstructures(61,correspondingto∼42%ofthesample)isenhancedinX-rayluminousclusterswithrespectto’field’radio-loudAGN.Furthermore,wefindthattheluminositydistributionoftheclusterradiopopulationdiffersfromthatofallradiosources,asthereisadeficitoflow-luminosity

22

(LR<∼10W/Hz/sr)objects,whilethenumberofhigh-luminosityonesisboosted.Theneteffectontheradioluminosityfunctionofradiogalaxiesassociatedtocluster

24

centresisofaflatteningatallluminositiesLR<∼10W/Hz/sr.Keywords:cosmology:observations-galaxies:clusters:general-galaxies:active-galaxies:statistics-X-rays:galaxies:clusters-radiocontinuum:galaxies

1INTRODUCTION

Arguablyoneofthemostimportantproblemsinthefield

ofstructureformationisthequestionofhowcoolingofgas,whichleadstostarandgalaxyformation,iscontrolledbyvariousheatingprocesses.Thewidelyaccepted,hierarchicalpictureforgalaxyformationstatesthatbaryonicmatteraccretesontodarkmatterhalos.Thispictureimpliesthatintheabsenceofanyfeedbackprocess,thegalaxymassfunctionfollowsthedarkmatterhalomassfunction.However,ithasbeenknownforsometimethatthisisnottrue(seee.g.White&Frenk1991).Atthehigh-

andlow-massend,thereisadeficitofgalaxiescomparedwithdarkmatterhalos.Thesediscrepanciesindicatethatvariousheatingmechanismsmustbeinvolvedintheformationofgalaxies.GalaxyclusterspossessthelargestdarkmatterhalosfoundintheuniverseandcontaingasatX-ray-emittingtemperatures.Theintraclustermedium(ICM)ofgalaxyclustershasbeenstudiedintensivelyandX-rayselectedclustersofgalaxiesprovideanexcellentlaboratorytoexplorethecomplexinterplaybetweengaspropertiesandheatingmechanisms.

Galaxyclustersareknowntoobeyfairlytightscal-

2M.Magliocchetti&M.Br¨uggen

ingrelationsbetweenglobalpropertiessuchasmass,temperatureandX-rayluminosity.Theserelationsareimportantwhenusinggalaxyclusterstoestimatecosmo-logicalparameters.Itisalsoknownthatsimple,self-similarmodels(Kaiseretal.1986)thatcorrectlyreproducethescalingpropertiesofthedark-matterdistributionofgalaxyclusters(e.g.Borganietal.2001;Pointecouteauetal.2005;Vikhlininetal.2006)failindescribingtheobservedscalingrelationsofthehotbaryoniccomponent.Thisdiscrepancyincreasesasonemovesfromtheregimeofrichclustersofgalaxiestothatofpoorclustersandgroups,givingrisetotheso-calledentropy-floorproblem(e.g.Ponmanetal.1999;Ponmanetal.2003;Pratt&Arnaud2005;Piffarettietal.2005).Somesourceofextraheatinghastobein-vokedtoreconciletheoreticalexpectationsandobservations.Moreover,intheabsenceofnon-gravitationalheating,verydenseclustercoresinwhichthecoolingtimeisoftenlessthantheHubbletime(so-calledcoolcoreclusters)shouldcoolandaccretegasatratesofhundredsandmoresolarmassesperyear(seeFabianetal.2004forareview).However,thismodelconflictswithresultsfromX-rayspectroscopythathaveprovidedevidencethatthereisnomassivecoolingofgasinthecentralregionsofcoolcoreclusters.(e.g.Petersonetal.2001;2003;B¨ohringeretal.2002).Numericalsimulationsofclustersofgalaxiesthatincluderadiativecooling(e.g.Bryan2000;Voit&Brian2001;Wu&Xue2002)starformationandsupernovafeedbacks(e.g.Borganietal.2004;2005)arefoundtobeunabletoreproducetheobservedpropertiesoftheintraclustermedium.

Currently,themostpopularmodelthatisinvokedtoexplainthedearthofgasbelowabout1keV(Petersonetal.2003;Tamuraetal.2001)intheICMreliesontheheatingbyacentralAGN.Inprinciple,radiogalaxiesareenergeticenoughtohaltthecoolinginthecentresofgalaxyclustersandexplainthehigh-masstruncationofthegalaxylumi-nosityfunction(seee.g.Bestetal.2006).Radio-loudac-tivegalacticnucleidrivestrongoutflowsintheformofjetsthatinflatebubblesorlobes.Theselobesarefilledwithhotplasma,andcanheattheclustergasinanumberofways(e.g.Br¨uggen,Ruszkowski&Begelman2005;Ruszkowskietal.2004;Br¨uggen&Kaiser2002;Churazovetal.2001).

DirectevidenceforAGNheatingisgrowing.ImagestakenbyCHANDRAandXMM-NewtonhaveshownthatAGNsinthecentresofgroupsandclustersinflatebubblesinthesurroundingX-rayemittinggas.ObservationsofthePerseusCluster(Fabianetal.2003;2006)andoftheVirgoCluster(Formanetal.2005,Simionescuetal.2006)revealsoundwavesandweakshocksthatextendouttoseveraltensofKpc.SimilarresultsarereportedfrominvestigationsofHydraA(Nulsenetal.2005),MS0735.6+7421(McNamaraetal.2004)andAbel478(Sandersonetal.2004).Fromamorestatisticalpointofview,Crostonetal.(2005)con-sideredawell-definedandhomogeneoussampleofROSATgroupsandreportevidenceforthegaspropertiesofgroupscontainingradiogalaxiestodifferfromthoseassociatedtoradio-quietones,inthesensethatradio-loudgroupsarelikelytobehotteratagivenX-rayluminositythanradio-quietgroups.Theyattributethistotheeffectofheatinginducedbyradiogalaxies.Dunn&Fabian(2006)analysed

amoreheterogeneoussampleof30clustersthatshowclear

signaturesofradioactivityeitherintheformof’bubbles’orintheformofacentralradiocoreandstudysomeofthepropertiesofAGNheating.

Morerecently,Bestetal.(2007)investigatedtheradiopropertiesofgalaxieslocatedinthecentresof625nearbygroupsselectedfromtheSloanDigitalSkySurveyDataRelease4(DR4;Straussetal.2002)andarguethatAGNheatingfromtheclustercentralgalaxyprobablyovercom-pensatestheradiativecoolinglossesingroupsofgalaxies,thereforeaccountingfortheobservedentropyfloorinthesesystems.However,Bestetal.(2007)didnotstudythether-malpropertiesoftheICMusingX-raydata.Lastly,Jethaetal.(2006)examinedthecentralregionsof15galaxygroupswithCHANDRAandfindthatrepeatedoutburstshavealong-termcumulativeeffectontheentropyprofilesintheirsample.

ByfollowingCrostonetal.(2005)andBestetal.(2007),thepresentworkassessestheimportanceof(central)ra-dioemissiononthethermalpropertiesoftheintraclustermediumingroupsandclustersofgalaxiesfromastatisti-calpointofview.Ourapproachprovidesanimprovementwithrespecttopreviousworksasitaddressestheissueofsmallstatistics(presentinCrostonetal.2005),lackofin-formationonthethermalpropertiesoftheICM(presentinBestetal.2007)anditalsoextendstheinvestigationoftheAGN-clusterpropertiesuptothehighestclustermasses(∼1016M⊙),whileboththeBestetal.(2007)andCrostonetal.(2005)analysesonlygouptorichgroups/poorclus-tersscales.Finally,wedistinguishradiosourcesinclustercentresonthebasisoftheirradiomorphology.Asitwillbediscussedthroughoutthepaper,suchadistinctionhasanimportantimpactontheanalysisofthegeneralpropertiesoftheICM.

Here,wehaveconsideredastatisticallysignificantsampleof145X-rayselectedclustersbrighterthan3×10−12ergs−1cm−2,takenfromtheREFLEXandNORASsurveysandendowedwithcentralradioemission>abovethe3mJyfluxlimit.TheabovelimitsensurenessbothintheX-rayandradioselections.∼80%complete-Weinvestigatethethermalpropertiesofourclustersampleandcomparethemtopre-existingresultsfromtheliterature.WehavealsomadeuseofdetailedMonteCarlosimulationstostudythebehaviourofradiosourcessurroundedbyadensemediumsuchasthatofrichclusters.

Thelayoutofthepaperisasfollows:§2presentstheparentcataloguesfromwhichoursamplewasdrawn,while§3discussestheproceduretoassignaradiosourcetoclus-tercentres.§4describesthefinalcatalogue,withparticu-larattentiontotheissueofextendedvsunresolvedradiostructures.§5illustratesourresultsonthefractionofX-rayselectedclusterswhichhostacentralradiosource(§5.1),ontheX-rayandradioluminosityrelationswithclustertem-perature(§5.2),ontheX-ray,radioandmechanicalluminos-ityrelationswiththeclustersvelocitydispersion(§5.3)andonthemassdepositionratedM

Theinterplaybetweenradiogalaxiesandclusterenvironment

3

activityandthermalpropertiesoftheclusterswhichhostthem.

ThroughoutthisworkweadoptaflatcosmologywithamatterdensityΩ0=0.3andavacuumenergydensityΩΛ=0.7,apresent-dayvalueoftheHubbleparameterinunitsof100km/s/Mpc,h0=0.7.

2THEPARENTCATALOGUES2.1

NORASandREFLEX

TheRosat-EsoFluxLimitedX-raygalaxyclustersurvey(REFLEX;B¨ohringeretal.2004)providesinformationontheX-rayproperties,fluxes,redshiftsandsomeidentifica-tiondetailsforasampleof447,zTheNorthernROSATAll-Skygalaxysurvey(NORAS;B¨ohringeretal.2000)contains484clusters(includingthesupplementstotheoriginalsurveycatalogue)withmeasuredredshiftsuptoz∼0.45.Thisnumberdecreasesto245ifoneconsidersthefluxlimitforREFLEXcompleteness.ThesurveyprobesGalacticlatitudes|b|󰀂20◦anddeclinationsδ󰀂0◦,andisestimatedtobe∼82%completewithrespecttotheREFLEXsurveyatthesame3×10−12ergs−1cm−2fluxlevels.

X-rayluminositiesinthe[0.1-2.4]KeVbandarepro-videdforboththeREFLEXandNORASclusters.However,whileinthecaseoftheREFLEXsurveythesearegivenintheconcordanceΛCDMcosmology,theNORAScataloguestillreferstoanEdSmodelwithh0=0.5.Inordertocorrectforthisproblem,weconvertedtheNORASX-rayluminosi-tiesfromtheoriginalEdScosmologytotheoneadoptedin

thisworkbysimplywritingLΛCDMX=LEdSX·(xΛCDM/xEdS)2

,wherexisthecomovingcoordinateevaluatedatthespecificredshiftofeachobject.

FortheconversionfromX-rayluminositiesintherele-vant[0.1-2.4]KeVbandtoclustermasseswefollowedtheapproachpresentedbyBorganietal.(2001).Asafirststep,wecomputedbolometricandK-correctionstothe[0.1-2.4]KeV-observedbandbyusingtherelationprovidedbyB¨ohringeretal.(2004).Then,fortherelationbetweentem-peratureandbolometricluminosity,weconsideredthephe-nomenologicalexpression:Lbol=L6

󰀂

TX

1015h−1

0

(e.g.Ekeetal.1998),where󰀅2/3

[Ω0∆vir(z)]1/3(1+z)KeV,(2)

wetake76%ofgastobehy-drogen,Mviristheclustervirialmassexpressedinsolar

masses,βistheratiobetweenthekineticenergyofdark

matterandthegasthermalenergy,and∆vir(z)isthera-tiobetweentheaveragedensityρwithinthevirialradiusandthemeancosmicdensityatredshiftz.Inourcaseweassumeβ=1.15,asfoundbytheSantaBarbaraclus-tercomparisonproject(Frenketal.1999).Oncewehavecomputedthevirialmasses,weobtainthevirialradiiviarvir=(3Mvir/4πρ)1/3.Giventheuncertaintiesassociatedwiththeparametersusedinequations(1)and(2)andtheerrorsrelatedtothedeterminationofLXinboththeRE-FLEXandNORASsurveys,wenotethatthevaluesquotedthroughoutthisworkforbothMvirandrvirhavetobecon-sideredestimates.2.2

NVSS

TheNRAOVLASkySurvey(Condonetal.1998)isara-diosurveythathasobservedtheentireskynorthof-40degdeclinationat1.4GHz.TheNVSSwasobservedwiththearrayintheDconfiguration(DnCformostofthesouthernsky),whichprovidesanangularresolutionof45arcsec.Morethan1.8millionsourceswereobserveddowntoafluxlimitS1.4GHz=2.5mJy,andthesurveyisshowntobe∼80%completeatfluxesbrighterthan3mJy.Thermsuncertain-tiesinrightascensionanddeclinationvaryfrom<1arcsecforrelativelystrong(S1.4GHz>∼15mJy)pointsourcesto∼7arcsecforthefaintestdetectableobjects.

3MATCHINGPROCEDURE

Asexplainedintheintroduction,thescopeofthepresent

workistwo-fold:first,itaimsatinvestigatingthepropertiesofthoseradiosourcesthatresideintheproximityofclustercentres.Second,itstudiestheimpactandpossibleeffectsofradiosourcesonthesurroundingICM.NotethatforourpurposesclustercentresareassumedtocoincidewiththecentresofX-rayemission,sothatinmost(butnotall)casestheadoptedcentreiscoincidentwiththepositionofthecentralclustergalaxy(hereafterCG).

IntheregionwhereX-rayandradiosurveysoverlap(δ󰀂−40◦),wecompiledacatalogueofX-rayselectedclus-terswithenhancedcentralradioemissionbyperformingacross-correlationanalysisbetweenobjectsbrighterthan3×10−12ergs−1cm−2intheNORASandREFLEXsam-plesandradiosourcesfromtheNVSSdatasetwith1.4GHzfluxesabove3mJy.Thesefluxlimitswererequiredtoen-sureahighdegreeofcompletenessbothintheprimaryX-rayselectionandinthesearchforradiocounterparts(cfr.Sections2.1and2.2).Thereare425clustersfromtheRE-FLEXsurveyfulfillingtheaboveconditiononX-rayfluxes.Thisnumberfallsto305whenweconsideronlythosesourcesthatlieatδ󰀂−40◦,necessaryfortheoverlappingofthera-dioandX-raysurveys.Thetotalnumberofclustersusedforouranalysisisthen550,where245comefromtheNORASsurvey.

Asuitablematchingradiusshouldmaximizethenum-berofrealassociationsandminimizespuriousmatches.Herewehavetoconsiderthreefactors:i)thewiderangeofred-shiftsspannedbyclustersintheREFLEXandNORASsur-veys(fromz∼5·10−3toz∼0.3),ii)thefactthatX-rayandradioemissionintheproximityoftheclustercentrecan

4M.Magliocchetti&M.Br¨uggen

bedisplacedfromoneanotherandiii)thatNORASandREFLEXareflux-limitedsurveysprobingonaveragemoreluminousandmoremassiveclustersatincreasingredshifts.Allthesefactorsconvergeatindicatingthatwecannotuseafixedmatchingangularradius.Infact,anangularradiussuitableforsourcesaroundz∼0.2wouldbetoolargeforthemorelocalobjectsandasaresultwouldincrease(evenbyalargefactorfortheverylocalclusters)thechancesofrandomassociations.Ontheotherhand,anangularradiussuitableforlocalsourceswouldmostlikelymissrealradio-to-Xassociationsathigherredshifts.

Consequently,wehavechosentoadoptavaryingangu-larradiusinourcross-correlationanalysis.SinceonaverageclustersofhighermassarefoundtohostmoremassiveandthereforelargerCGs(e.gvonderLindenetal.2007)–sothatthepossibilityforagreaterdisplacementbetweenX-rayandradioemissionoriginatingfromwithintheeventualCGishigher–wehaveadoptedamatchingradiusoftheformθmatch=Qθvir,whereθviristheangularextensionoftheclustergivenbyθvir=rvir/x,withrvirvirialradiusandxcomovingdistance.

ParticularattentionwaspaidtothedeterminationofthevalueforthequantityQusedinθmatch.WefindthatQ=0.015(i.e.θmatch1.5%oftheprojectedvirialradius)producesalargeenoughsampleofX-rayclusterswithacentralradiocounterpartwhileminimizingcontaminationfrominterlopers.Theseinterloperscanresidebothinside(i.e.radioemittersotherthanthecentralsource)andout-side(i.e.spuriousmatchesoriginatingfromprojectionef-fects)theclustersinoursample.

ByapplyingtheabovematchingcriteriatotheRE-FLEXandNORASsamples,wefindthat81REFLEXclus-tersoutof305–correspondingto26.5%ofthesample–havearadiocounterpartthatisoffsetfromthecentreofX-rayemissionbylessthan1.5%ofthevirialradius,whileinthecaseofNORASwehave67radiomatchesoutof245objects(i.e.27.3%oftheoriginaldataset)withinthesameangulardistance.Itisreassuringthatthepercentageofmatchesisapproximatelythesameinthetwoclustersurveys.ThesefiguresarealsoinagreementwiththeresultsfromBestetal.(2007)fortheoccurrenceofcentralradiocounterpartsintheirSDSSclustersample(seetheirFigure2).Crostonetal.(2005)andDunn&Fabian(2006)findsomehowhighervalues(ontheorderof50%),buttheselectioncriteriaforthesetwosamplesaredifferentfromours,especiallysincebothgroupsconsidersystemsthatareonaverageatmuchlowerredshiftsthanoursample.

WenotethatthreesourcesinREFLEX(namelyRXCJ0338.4-3526,RXCJ1501.1+0141,RXCJ2347.7-2808)andtwoinNORAS(RXCJ1229.7+0759andRXCJ1242.8+0241)allowedformorethanoneradiocounterpartwithintheadoptedmatchingradius.Visualinspectionofradiomapshasshownthatinallfivecasesthecentralradiosourceshadextended/multiplestructures.Theradiofluxassociatedwitheachoftheseobjectswastakentobethesumoftheirsub-components.WewillgetbacktothispointinSection4.1.

Chancesofcontaminationfromspurioussourcesinthejointradio–X-raycatalogueasobtainedabovehavebeenes-timatedbysimplyverticallyshiftingallradiopositionsby1degreeandbyre-runningthematchingprocedure.Bydoingso,wefindthatinthecaseofbothREFLEXandNORAS

Figure1.DistributionofresidualsbetweenradioandX-rayposi-tionsforobjectsintheREFLEX(solidline)andNORAS(dashedlines)catalogues.

theprobabilitythataradiosourcefallsbychancewithinthesearchradiusfromthecentreofX-rayemissionisabout5.6%.TheadoptionofasmallermatchingradiussensiblyreducesthenumberofradiosourcesfoundintheproximityofthecentreofX-rayemission,whileleavingthechancesforspuriouscontaminationalmostunaltered.Forinstance,ifinsteadoftakingQ=0.015weconsiderQ=0.01,inthecaseofREFLEXweonlyobtain56matches,whilethees-timatedpercentageofrandomcoincidencesremainsalmostthesame(∼4.5%).Ontheotherhand,increasingtheal-lowedmatchingradiusbyevenasmallamountgreatlyin-creasesthenumberofpossiblespuriouscounterparts:forQ=0.02thisfigurealreadygetsashighas13%.

ThedistributionofresidualsbetweenradioandX-raypositionsasafunctionofthe(comoving)distancefromthecentreoftheclusterisshowninFigure1.ThesolidlineindicatesthecaseforREFLEXclusters,whilethedashedlinereferstotheNORASsample.AllradiopositionsdifferfromtheX-rayonesbylessthan∼50Kpc.Ifacentralclustergalaxyispresent,thisimplies–asexpected–thatradioemissionoriginatesfromit.

Thenumberofclustersbrighterthan3·10−12ergs−1cm−2endowedwithacentralradiocounterpartabovethe3mJylevelasderivedfromouranalysisis148.Threeclusters,however,havemultipleidentificationsintheNO-RASandREFLEXcatalogues(i.e.appearinbothsurveys).Byremovingthesesourcesfromtheradio-NORASsample,weendupwith145clusterswithauniquecentralradiocoun-terpart.Thisisthesamplethatwillbeusedthroughoutthiswork.

4DESCRIPTIONOFTHESAMPLE

Relevantpropertiesfortheradiosourcesfoundintheprox-imityofNORASandREFLEXclustercentresbyfollowing

Theinterplaybetweenradiogalaxiesandclusterenvironment

5

theproceduredescribedinSection3aresummarizedinTa-ble2whichreportsinthevariouscolumns:(1)Nameoftheparentcluster.

(2)-(3)RA(2000)andDec(2000)oftheradiosource.(4)RadiofluxexpressedinmJy(F1.4GHz).

(5)LogoftheradioluminosityexpressedinW/Hz/sr(log10LR).

(6)Redshiftofthecluster(z).

(7)Distancebetweenclustercentreandradiocounterpart(Kpc).

(8)Notesontheradioappearance,i.e.whetherpoint-like(blankspace)orextended/sub-structuredsource(denotedwithan’S’).

Radioluminositieshavebeencalculatedaccordingtotherelation:

L1.4GHz=F1.4GHzD2(1+z)3+α,

(3)

whereDistheangulardiameterdistanceandαisthespec-tralindexforradioemission[F(ν)∝ν−α].Sincewedonothavemeasurementsforα,weassumedα=0.75whichisthetypicalvaluefoundforearly-typegalaxiesandsteepspec-trumsourcessuchasjetsandlobes.Notethat,sincethemaximum0.3,estimatesfromequation(3)donotdependontheprecisevalueofthespectralindex.4.1

Radiosourceswithextendedemission

Theissueofextended/sub-structuredsources(point(8)inTable2)deservesadigression.AsmentionedinSection3,fiveclustersareassociatedwithradiosourcesthathavemul-tiplecomponentswithinthechosensearchradius.AvisualinvestigationoftheradioimagestakenfromtheNVSSsur-veyforallthe145radioobjectsassociatedwithNORASandREFLEXclustersshowsthatabout42percentofthemdif-ferfromapoint-likestructure.(NotethatsincetheNVSSresolutionis45′′,multiplestructurescloserthan∼9Kpcatz=10−2upto∼250KpcatthehighestredshiftsprobedbytheREFLEXandNORASsurveyswillbeunresolvedintheNVSSradiomaps.WewillgetbacktothispointinSections5.1and5.2).Moreprecisely,thishappensfor61sources,outofwhich33arefoundinREFLEXclustersand28inNO-RASclusters.Thepercentagequotedaboveismuchhigherthanwhatisfoundfor’field’radiogalaxieswithinthesameredshiftrangeandatsimilarradiofluxlevels(∼5%cfr.Magliocchettietal.2001).WewillgetbacktothispointinSections6and7.

Havinglabelledtheradiosourceswithan’S’,wedonotmakeanyfurtherdistinctiononwhethertheradioemis-sionpresentsextendedblobs,itappearsasacloseordistantdouble-lobedobjectorfeaturesatriple/morecomplexstruc-ture.Forthepurposesofthepresentwork,theonlydivisionweareinterestediniswhethertheradiosourceexhibitssignsofextendedemissionornot.

Generally,itwasrelativelyeasytorecognizedifferentsub-structuresthatbelongtothesameradiosource.Never-theless,thereareanumberofcasesthatremainuncertainandthathavebeenlabelledwithan’S?’inTable2.Toplaysafe,thesesourceshavenotbeenincludedinthe”sub-structure”sampleandinthefollowinganalysestheywillbetreatedaspoint-likeobjects.

RadiofluxesforextendedradiosourcesinTable2have

Figure2.Toppanel:numberofclustersperX-rayluminosityinterval.Thethick(green)linesillustratethecaseforNORAS,whilethethin(blue)onesarefortheREFLEXsample.SolidlinesshowtheentireFX󰀂3×10−12ergs−1cm−2population,whiledashedonesonlyrepresentthoseobjectswhichexhibitrelevant(F1.4GHz󰀂3mJy)centralradioemission.Lowerpanel:ratiobetweennumberofclusterswithacentralradiocounterpartandthewholeFX󰀂3×10−12ergs−1cm−2clusterpopulation.Open(green)circlesareforNORASobjects,filled(blue)dotsforRE-FLEX,while(red)squaresandassociatederrorbarsrepresentthecombinedREFLEX+NORASdataset.

beenobtainedbyaddingupallthefluxesoftheirdifferentcomponents.Radioluminositieshavethenbeencomputedbymeansofequation(3)fortheresultingtotalfluxes.

5

OBSERVATIONALPROPERTIESOFTHESAMPLE

Inthissection,wewillshowthemostinterestingproper-tiesandrelationsofoursample.InadditiontothedatadescribedinSection4,someextrainformationsuchastem-peraturesandvelocitydispersionshasbeengatheredfromtheBAXandVIZIERdatabases.Allthebest-fitvaluesfortherelationspresentedinthefollowingpagesaresumma-rizedinTable1,whilewedeferadetailedinterpretationoftheresultstoSections6and7.5.1

Fractionofradiodetections

Asaninterestingfirstexercise,wecanestimatethenumberofX-rayclustersbrighterthan3ergs−1cm−2thathostacentralradiosourceaboveF1.4GHz=3mJy.Figure2shows,asafunctionofX-rayluminosity,thetotalnumberofX-rayclustersusedinthiswork(solidlinesinthetoppanelofFig-ure2;thethicklinerepresentstheNORAScasewhilethethinoneisforREFLEXclusters),thenumberofX-rayclus-terswithacentralradiocounterpart(dashedlinesinthetoppanelofFigure2),andtheratiobetweenthesetwoquan-tities(bottompanelofFigure2,wheresquaresandasso-

6M.Magliocchetti&M.Br¨uggen

Table1.Best-fitvaluesforthetrendspresentedinSection5.Allrelationsareofthekindlog10y=blog10x+a.X-rayluminositiesLX

areexpressedin[1044erg/s],radioluminositiesLRin[W/Hz/sr],temperatureskTin[KeV],velocitydispersionsσin[Km/s]andmassdepositionratesdM

LRvskT(Fig.7b)

.2+0.1

b=5.6+0−0.2;a=20.40−0.09(33).4+0.1

b=7.2+0−0.3;a=19.5−0.2(31).3+0.1

b=6.3+0−0.2;a=20.1−0.1()

σvskT(Fig.8b)

.04+0.03

b=0.44+0−0.06;a=2.55−0.01(16).07+0.06

b=0.53+0−0.11;a=2.54−0.02(18).03+0.04

b=0.49+0−0.09;a=2.55−0.01(34)

dM

Figure3.Ratiobetweennumberofclusterswithacentralra-diocounterpartbrighterthan3mJyintheNVSSmapsandthe

wholeFX󰀂3×10−12ergs−1cm−2clusterpopulationasafunc-tionofvirialclustermass.Open(green)circlesareforNORASobjects,filled(blue)dotsforREFLEX,while(red)squaresandassociatederrorbarsrepresentthecombinedREFLEX+NORASdataset.Thedashedlineindicatesthebestfittothedata(seetextfordetails).

Figure4.Toppanel:fractionofX-rayselectedclusterswithfluxesFX󰀂3×10−12ergs−1cm−2whichpresentcentralra-dioemissionbrighterthanaluminosityLR=1021.9W/Hz/srasafunctionofvirialclustermass.Open(green)circlesareforNORASobjects,filled(blue)dotsforREFLEX,while(red)squaresandassociatederrorbarsrepresentthecombinedRE-FLEX+NORASdataset.Bottompanel:sameasabovebutforthevolume-limitedsampleLR󰀂1022W/Hz/sr,z󰀁0.14(cfr.Fig.12b).

ciatederrorbarsindicatethecombinedREFLEX+NORAScase).Notethatthereisaclearpreferenceforradiosourcestoappearinpoorclusters.Infact,despitetherelativelypoorstatisticsatlowluminosities,thepercentageofclus-tershostingacentralradio-AGNbrighterthan3mJydrops

43

fromabout[50%-100%]forLX<∼10erg/sto∼10%atthehighestluminositiesprobedbyNORASandREFLEX.

Thistrendbecomesevenmoreintriguingifoneconsid-ersthefractionofclustersthatpresentcentralradioemissionabove3mJyasafunctionoftheclustervirialmassevaluatedasinSection2.1(Figure3).Inthiscase,thereisatrendforcentralradiodetectionsinclusterstodecreasewithcluster

114.5

M⊙h−mass;wefindthatallbutoneofthe12Mvir<0∼10

poorclustersandgroupsidentifiedinNORASandRE-FLEXareassociatedwithacentralradioemittingsource

brighterthan3mJyintheNVSSmaps,whilethisfigurerapidlydecreasestoa∼20percentassoonasonemovesto

−115

massesMvir>∼10M⊙h0.Wecanquantifythisresultby

−α

writing:fR=NRADIO/NTOT,whereCLUSTERS=(Mvir/M∗)

−113

M∗=(3.5±0.9)·10M⊙h0andα=0.35±0.08(dashedlineinFigure3).

Obviously,selectioneffectsbiastheresultspresentedinFigures3and2.TheadoptedfluxlimitforNORASandREFLEXimpliesthatX-rayluminousandthereforemoremassiveclustersarepreferentiallyobservedathigherred-

Theinterplaybetweenradiogalaxiesandclusterenvironment

7

shifts.Ontheotherhand,theradio-selectionfunctionde-rivedfromhavinguseda3mJylimitforourradiosample(solidlineinFigure12b)impliesthatwhilelocallyoneiscapableofdetectingallradiosourcesbrighterthan∼1019.5W/Hz/sr,attheredshiftsprobedbymassiveclustersonlyradiosourcesbrighterthan∼1022W/Hz/srcanbeob-served.Furthermore,probingthemostluminousclustersathighredshiftsmayimplyselectingrelaxedclustersi.e.thosewhicharecool-coreones.Ifradio-loudAGNaretheoriginforanyfeedback,thenthismaybeanextraselectioneffect,inthatcool-coreclustersaremorelikelytohostpowerfulradiosources.

Nevertheless,theobservationalfactthatallbutoneofthelocalclustersandgroupshostacentralradio-luminoussourcesuggeststhat–bygoingdeepenoughinradiomaps–thismightalsobethecaseforthemajorityofX-rayselectedclusters.Thisconclusionbecomesmorestringentbythefactthatourcriterionforassociatingradiosourceswithclustercentresnecessarilymissesrelativelycommonobjectssuchastriplestructureswherethecentralcomponentistoofaintinradiotobeseen(double-lobedsourceswitha’radio-quiet’galaxysetintheirmiddle).

Toobtainanunbiasedtrendfortherecurrenceofradiosourcesinclustercentres,wecompileavolume-limitedsam-ple.Thisispresentedinthebottom-panelofFigure4forthevolume-limitedsamplez󰀁0.14,LR󰀂1022W/Hz/sr(cfr.Figure12b),chosensuchthatthenumberofsourcesavail-ablefortheanalysisismaximised.Inthiscase,thefractionofX-rayclusterswithcentralradioemissionisfoundtobearoundthe20percentlevel,withpossiblyaslightprefer-enceforradiosourcestoinhabitmoremassiveclusters.Wenotehoweverthatthisresultonlyholdsforrelativelylargeclustersastheradioluminositycuthasexcludedallclusters

lessmassivethanMvir∼1014.5M⊙h−1

0.

Bestetal.(2007)useasampleof625groupsandclus-tersofgalaxiesselectedfromtheSloanDigitalSkySurveyandcross-correlatedwiththeNVSSandFIRST(FaintIm-agesoftheRadioSkyatTwentycentimeters;Beckeretal.1995)radiosamplestostudythepropertiesofradio-loudAGNinthebrightestclustergalaxies(coincidinginthema-jorityofthecaseswithourdefinitionofCGs,seevonderLindenetal.2007).Intheirworktheyshowthatthefrac-tionofbrightestclustergalaxiesthatareradio-loudAGN

increaseswith(stellar)massas∝Mstellar1.0

uptoaplateaulevelofabout∼20−30percentreachedfortheirhighestmassrangewhichiscentredataroundMstellar∼1011.5M⊙.OursampleofX-rayselectedclustersallowstoextendtheBestetal.(2007)analysistothehigher-massregionprobedbyrichgroupsandclustersofgalaxies.Thetrendpre-sentedinthetoppanelofFigure4thenshowsthefractionofclusterswithacentralradiocounterpartbrighterthanLR󰀂1021.9W/Hz/sr(limitwhichiscomparabletothedefinitionofradioloudnessgiveninBestetal.2005)asafunctionofclustermass.Atallclustermasses,thepercent-ageofclustershostingacentralradio-loudsourceissubstan-tiallyconstantandequalto∼20percent.Ourresults,bothforwhatconcernsthepresenceofaplateauinthedistribu-tionofobjectsthatareradio-loudAGNandforthevalueofsuchfraction,aretheninagreementwiththoseofBestetal.(2007).Itisquiteintriguingthatthesamedependence(orratherindependence)onmassoftheoccurrenceofcentralradio-loudsourcesinmassiveextragalacticobjectsisfound

Figure5.Fractionofradio-detectedclustersthatpresentextended/sub-structuredradioemissionasafunctionofvirialclustermass.Open(green)circlesareforNORASobjects,filled(blue)dotsforREFLEX,while(red)squaresandassociateder-rorbarsrepresentthecombinedREFLEX+NORASdataset.Thedashedlineindicatesthebestfittothedata(seetextfordetails).

independentlyofwhethersuchamassisthatofa’field’galaxy(seeBestetal.2007),ofaclustercentralgalaxyoroftheclusterthatsurroundsit.

Anotherinterestingpointconcernstheintermittencyoftheradio-loudAGNphenomenon.Bestetal.(2005)arguethattheobservationalfindingthat∼20-30percentofallthemostmassivegalaxiesintheirsamplepresentsigna-turesofradioactivityimpliesthatthisactivityhastobere-triggeredsooftenthatthegalaxyspendsaboutaquarterofitslife-timeinanactivestate.Thisfuel-supplyrequire-mentbecomesevenmoredemandingifoneconsidersthattheBestetal.(2005)selectionincludesradiosourcesthatarebrighterthanthoseconsideredinthiswork.Asalreadydiscussed,Figure3suggeststhatifonegoestolow-enoughradioluminosities,itislikelythatthemajorityofclustershostacentralradiosource,justasitisfoundlocallyforoursample.Thisresultwouldthenimplyanalmostconstantre-triggeringoftheradiosource,sotoallowthehostinggalaxytospendalmostallitslifetimeinanactivestate.

Asafinalpointinthissection,wehaveconsideredthoseradiosourceswithintheclustercentresthatpresentextendedand/orsub-structuredradioemissionandinvesti-gatedwhethertherewasadependenceoftheirrecurrencerateonclustermass.TheresultsarepresentedinFigure5.TheverymarginaltrendfortheirfractiontodecreaseMvir(fS=NSub/NRADIO≃󰀆Mvir/1.6·1012h−1

mostlikelyduetothecombinedeffectofthe0M⊙

resolution󰀇−0.with

1)isca-pabilitiesoftheNVSSsurveywiththealreadymentionedselectionbiasesinflux-limitedsurveyswhichpreferentiallyidentifymassiveclustersathighredshifts.Thisimpliesthat,atvariancewithwhathappensmorelocally,closesubstruc-tureswillnotbeobservedassuchatthoseredshifts,result-

8M.Magliocchetti&M.Br¨uggen

Figure6.Fractionofradio-detectedclustersthatpresentextended/sub-structuredradioemissionasafunctionofredshift.Open(green)circlesareforNORASobjects,filled(blue)dotsforREFLEX,while(red)squaresandassociatederrorbarsrepresentthecombinedREFLEX+NORASdataset.

inginanetlossofsub-structured/extendedsourcesstart-ingfromz∼0.05andparticularlybeyondz∼0.1(cfr.Figure6).Nonetheless,inthelocaluniverseand/oratlowclustermassesthereisatendencyforthemajority(>50%)ofcentralradiosourcestopossessextendedradioemission.CombiningthisresultwiththatofFigure3wecanthencon-cludethat,atleastlocally,themajorityofclusters(whichinthiscasearerathergroups)hostanextendedradiosourceintheircentre.Wewillinvestigatetheconsequenceofthisfindinginthenextsub-sections.5.2

LuminosityvsTemperature

X-rayscalingrelationsofgalaxyclusters,suchastheX-rayluminosity-temperaturerelation,playanimportantrolewhenusingclustersofgalaxiestoconstraincosmologicalpa-rameters.Whengravitationalheatingistheonlysourceofheating,clustersshouldbehaveself-similarlyandtheX-rayluminosity(duetothermalbremsstrahlung)shouldscaleasLX∝T2(Kaiser1986).

Mostauthors(e.g.White,Jones&Forman1997;Ar-naud&Evrard1999andmorerecentlyPopessoetal.2005)reportLX−kTcorrelationswithlogarithmicslopescloseto3intheclusterregime,thoughattemptstoremovetheeffectsofcentralcoolingflows(Allen&Fabian1998;Marke-vitch1998)resultsinflatterrelations.However,theaboveresultisfoundtobreakatthemass/luminosityscalesofgroupsofgalaxies.Helsdon&Ponman(2000a,b)reportasteeperslopeof4.9±0.8forasampleofX-raybrightloosegroups,and4.3±0.5foralargersampleincludingbothlooseandcompactgroups,whileXue&Wu(2000)foundaslope5.6±1.8fromdatafor38groupstakenfromtheliterature.Morerecently,Osmond&Ponman(2004)–byanalysingasampleof60galaxygroupswithintheGEMSproject–have

measuredanLX−kTslopethatiscomparablewiththat

foundinclusters(butseetheirdiscussionaboutpossiblesystematicswhichcouldhaveaffectedtheirresult).

Theobserveddeparturefromself-similarityhasbeeninterpretedasexcessofentropyinlowmassclustersduetosomeformofpre-heatinggeneratedbysourcessuchassu-pernovaeand/orAGN(seee.g.Tozzi&Norman2001).Ev-idenceforheatingexcessin19groupscontainingradio-loudAGNhasrecentlybeenreportedbyCrostonetal.(2005).Fromthetheoreticalpointofviewanumberofworkshaveshownthatradio-loudAGNcanprovideasufficientlyener-geticandwidelydistributedheatingmechanismtosolvetheentropyfloorproblem(e.g.Br¨uggen&Kaiser2001;Br¨uggen&Kaiser2002;B¨ohringeretal.2002;Fabianetal.2003;Heinzetal.2006).

Inordertoinvestigatetheeffectsofthepresenceofacentralradiocomponentingroupsandclustersofgalax-iesfromastatisticalpointofview,wehavegatheredtem-peraturesforoftheradio-detectedclustersbelongingtooursamplefromtheBAXdatabase.Thissamplewassub-dividedaccordingtowhetherthecentralradiosourcepresentedanextended(31objects)oranunresolved(33objects)structure.TheLXvskTtrendforthetwosub-populationsispresentedintheleft-handpanelofFigure7,whereopencirclesrepresentclustersinhabitedbypoint-likeradiosources,whilefilledsquaresareforclustersassociatedwithcentralsourceswithextendedradioemission.Thedif-ferenceinbehaviourbetweenthesetwopopulationsisstrik-ing:atagiventemperature,theoverwhelmingmajorityofclustersassociatedwithextendedradiosourceshavealowerX-rayluminositythanthosethathostpoint-likeradioob-jects.Or,fromadifferentpointofview,atagivenlumi-nosityclustersassociatedwithextendedradiosourcesaresystematicallyhotterthanthosethatarenot.

Whenwemakeaχ2fittoourentiresampleofX-rayselectedclustersthathostacentralradiosource,wefindaslopeoftheLX−kTrelationofb=2.8±0.1.Thisslope

becomesappreciablyshallower(b=2.35+0.08

−0.07)ifoneonlyincludesunresolvedradiosources,andremarkablysteeper(b=4.0±0.2)forthesub-populationofextendedradioob-jects.Intriguingly,X-rayclusterswithapoint-likecentralradiosourcetraceadistributionthatisverysimilartotheLX∝T2expectedforself-similarsystems.ThepresenceofextendedradiosourcesbreaksthisrelationcausingamuchsteeperdependenceofX-rayluminosityonclustertemper-ature.ThismaypointtowardsamoreefficientmodewithwhichextendedsourcescanpermeatethesurroundingICMandtransferheatwithinthecluster.AswillbediscussedingreaterdetailinSections6and7,theabsoluteeffectofextendedradio-loudAGNontheICMdecreaseswithclus-termass.ThisisclearlyseeninFigure7a.Withincreasingmass(i.e.increasingT),thedifferencebetweenpoint-likeandextendedradiosourcesdecreases.

Theissueofpoint-likevsextendedradiosourceshasbeeninvestigatedfurtherbyconsideringvolume-limitedsamplesofclusters.Infact,giventhe45′′resolutionofNVSS(cfrSection4.1andFigure6),theaboveresultonthedif-ferentbehaviouroftheLX−kTrelationshipforclustersthathostcompactorextendedradiosourcescouldbebi-asedbythepresenceofradiostructuresthatareextendedbutappearaspoint-likeintheNVSSmaps.Wehavethenrepeatedtheaboveanalysisbysplittingthesamplesinto

Theinterplaybetweenradiogalaxiesandclusterenvironment9

Figure7.Left-handpanel(a):X-rayluminosityversusclustertemperatureforthoseX-rayselectedclustersbrighterthan3ergs−1cm−2whichexhibitcentralradioemissionabove3mJy.Open(red)circlesrepresentclustersassociatedtopoint-likeradiosources,whilesolid(green)squaresareforthoseclustersinhabitedbyradioobjectspresentingextendedstructures.Thedashed(green)lineindicatestheLX−kTbestfitforthesub-populationofextendedradiosources,whilethedotted(red)oneisforpoint-likeradioobjects(seeTable1).Thesolidlineisthebestfittothewholeradioclustersample.Right-handpanel(b):radioluminosityofthesourcesassociatedwithclustercentresversusclustertemperature.Symbolsareasbefore.

Figure8.Left-handpanel(a):X-rayluminosityLXversusvelocitydispersionσforasubsampleof39clusterswithenhancedcentralradioemission.Open(red)circlesidentifypoint-likeradiosourceswhilefilled(green)squaresrepresentextendedones.The(green)dashedand(red)dottedlinesrespectivelyindicatetheLX−σbestfitstothesub-populationsofextendedandpoint-likeradiosources(seeTable1).Thesolidlineisthebestfittothewholeradioclustersample.Right-handpanel(b):velocitydispersionversusclustertemperature.Symbolsareasbefore.

sourceswithz󰀁0.05and0.0510M.Magliocchetti&M.Br¨uggen

b=2.9+0.8+0.4

−0.6;a=−1.5−0.6andb=2.7±0.3;a=−1.3±0.2.Onecanthenseethattheresultsobtainedforthemorereli-ablelocalsubsetsofclustersareinverygoodagreementwiththosederivedpreviouslyinthisSection(cfrTable1)regard-lessoftheclusterredshiftdistribution.Thisisreassuring.Harderistheintepretationofthehigher-redshiftresults.Inthiscaseclustersinhabitedbypoint-likeandsub-structuredradiosourcespresentaverysimilardependenceoftheX-rayluminosityonthetemperature.Whetherthiseffectisduetothefactthatthe’point-like’sampleofsourcesinthisredshiftrangealsoincludesafractionofsub-structured(butunresolved)objects,thereforeproducinganetmixofthetwopopulationsorwhetheritsexplanationresidesinthemorephysicaleffectthatathighredshiftwearepref-erentiallyselectinghigher-masssystemswheretheabsoluteeffectofextendedradio-loudAGNontheICMissmallerisdifficulttotell.Higher-resolutionradioobservationsoftheclustercentresareneededtodisentanglethesetwoeffects.

TheaverageLX−kTbehaviourobtainedbyconsideringbothpopulationsofextendedandpoint-likeradiosourcesisinremarkableagreementwithwhatisfoundinthelitera-tureforthewholeclusterpopulation(e.g.White,Jones&Forman1997;Arnaud&Evrard1999;Popessoetal.2005).Thisresult,togetherwiththefactthatweexpectmostX-rayselectedclusterstohostacentralradioobjectbrighterthan∼1020W/Hz/sr(see§5.1),suggeststhattheobservedLX∝T3scalingisduetothesuperpositionoftwoseparatepopulations:clustersinhabitedbyunresolvedradioobjectsandclusterswithextendedradioemission.

Theaboveanalysisunderlinesthefundamentalimpor-tanceofextendedradio-loudAGNinbreakingtheself-similarityofclusters,atleastonscaleskT<∼3KeV.Atthispointonecanwonderhowthelevelofradioactivitydependsontheclustertemperature.AplotofLRvskTispresentedintheright-handpanelofFigure7.Independentoftheradiomorphology,thereisamuchgreaterspreadthanintheLXvskTplot.However,forkT<∼3keV,i.e.intheregimewhereradio-loudAGNheatingisveryefficient,theLR−kTrelationisrelativelytight,providingsomeevidence–atleastinlow-masssystems–forarelationbetweenradioluminosityandclustertemperature,inthathotterclustershostmorepowerfulradiosources.Notethatthisresultisfoundtoholdforbothunresolvedandsub-structuredradiosources.

5.3VelocityDispersions

Velocitydispersionmeasurementsforthispartofouranaly-sishavebeentakenfromtheVIZIERdatabaseandinmostcasesrelyontheworksofMahdavietal.(2000)andMahdavi&Geller(2001).X-rayluminositieshavebeencomparedtovelocitydispersionsforthe39clustersforwhichmeasure-mentswereavailableintheleft-handpanelofFigure8.Best-fitvaluesforthefitsareprovidedinTable1.Thetrendseenforthewholepopulationofclustersinhabitedbyacentralradiosource(LX∝σb,withb=3.8±0.4)isingeneralagreementwithwhatisfoundintheliteraturefordifferentsetsofclustersandgroupsofgalaxies(e.g.b∼3.6,Popessoetal.2005;b∼4.4,Madhavi&Gheller2001;b∼3.9fortheclustersconsideredinOsmond&Ponman2004).Thisbe-haviour,togetherwithwhatwasfoundinSection5.2forthe

Figure9.RatiobetweenmechanicalandX-rayluminosityversusclustervelocitydispersion.Open(red)circlesidentifypoint-likeradiosourceswhilefilled(green)squaresrepresentextendedones.The(green)dashedand(red)dottedlinesrespectivelyindicatetheLmech/LX−σbestfitstothesub-populationsofextendedandpoint-likeradiosources(seeTable1);thesolidlineisthebestfittothewholeradioclustersample.

LX−kTrelation,showthatradio-identifiedclustersprovideafairsampleoftheentireclusterpopulation.

However,thesimilaritybreaksifoneconsidersclustersinhabitedbyunresolvedandextendedradiosourcessepa-rately.Fortheformercase,wefindthatb=6.2+0.4

−0.3,whilein

thecaseofextendedradioemissionwefindb=5.5+1.0

−0.8.Thedifferenceintheobservedbehaviourofthesetwosub-groupswiththegeneralcase(cfr.Figure8a)maybeattributedtothefactthatinoursampleextendedradiosourcesseemtobesystematicallyassociatedtohighervelocitydispersionsystems.Eventhoughtheavailabledataisnotgoodenoughtoprovidemorethana1σevidenceforthiseffect,thenetresultofcombiningextendedandpoint-likeradiosourcesisaflatteningoftheLX−σrelationtothe’standard’valueb∼4.AsalreadyobservedinSection5.2,itisintriguingthatthesumoftwopopulationswithdissimilarbehavioursgivesrisetoatrendthatmatchespreviousmeasurementsobtainedfordifferentclustersets.Thissuggestsagainthatclusterscanbedividedintotworatherdifferentgroups:clus-tersthathostpoint-likeradiosourcesandthoseassociatedwithextendedradioemission.

ItwouldbeinterestingtoinvestigatetheLX−σre-lationinthe42.5

erg/sregime,thisiscurrentlynotpossible.∼10

sothatForthesamereason,wecannotdetectanydifferenceintherelationbetweentem-peratureandvelocitydispersion(right-handpanelofFigure8),apartfromamarginalpreferenceforextendedradiosourcestoresideinhighervelocitydispersionsystems.The

Theinterplaybetweenradiogalaxiesandclusterenvironment

11

slopeoftheσvskTrelationisb∼0.5inallcases,againinagreementwithmostoftheresultsfoundintheliterature.

Byanalysingthecavitiesandbubblesthatarepro-ducedinclustersandgroupsbytheinteractionbetweenra-diosourcesandthesurroundinghotgas,Birzanetal.(2004)provideausefulempiricalrelationtoestimatethemechan-icalluminosityreleasedbythecentralradiosourceintotheICM:Lmech

1025WHz−1

Eventhoughthe1.4GHzsynchrotron󰀃0.40±0.13

(4)

luminosityisnottooreliableatpredictingthemechanicalluminosityascanbeseenfromthelargeerrorsintheexponentofEq.4,wewilluseittocalculatetheenergeticbalancebetweenAGNheat-ingandradiativelosses(asinBestetal.2007andBirzanetal.2004).ThisispresentedinFigure9asafunctionoftheclustervelocitydispersion,whereonceagainsolidsquaresareforextendedradiosources,whileopencirclesrepresentpoint-likestructures.InagreementwithBestetal.(2007)wefindthat,whileonthesmallestmassscales(σThereisnosignificantdifferencebetweenthebehaviourofextendedandpoint-likesources(Lmech/LX∝σ−7),eventhoughthedatadoesnotallowustoconcludeanythingaboutthegroupregime.

Thus,forthesmallermasssystemswehavefoundthefollowing:(i)mechanicalheatandradiativelossesbalanceand(ii)thereisacorrelationbetweenX-rayluminositiesandclustertemperaturesandbetweenradioluminositiesandclustertemperaturesinthelowtemperatureregime(Sec-tion5.2).Thissuggeststheconclusionthatinobjectssuchasgroupsorsmallclustersofgalaxiesthereisastrongin-terplaybetweencentralradiosourceandthesurroundinggas.Inthiscase,morepowerfulradiosourcesleadtohot-tersystems.Theeventualpresenceofextendedradiostruc-turessuchasjetsandlobesisoffundamentalimportancetothethermalstateoftheICM,astheycancarryenergythroughoutthewholeclusterandappeartobemoreefficientatheatingthesurroundingmedium.Theheatingprovidedbythecentralradiosourceisalsofoundtobesufficienttobalanceradiativelosses.

Thesituationisdifferentathighermasses.Inthiscase,wefindthatthereisnocorrelationbetweentheradiolu-minosityofthecentralsourceandtheclustertemperature.Also,theeffectoftheradio-loudAGNonthethermalstateoftheclusterisindependentofthecapabilitiesofsuchanobjecttopermeatethesurroundingmedium(nodifferenceintheLX−kTrelationbetweenextendedandpoint-likesources).AGNheatingisfoundtobeinsufficienttocoun-terbalancetheradiativelossesoftheICM,witharelativeenergeticimportancewhichsteeplydecreases(as∝σ−7)withclustermass.

Adifferentsourceofprominentheatingmustbeinvokedinthislattercase.Onesuchmechanismmaybethermalcon-duction(seee.g.Br¨uggenetal.2003;Hoeft&Br¨uggen2004;Roychowduryetal.2005;Ruszkowski&Begelman2002;Fu-jita&Suzuki2005;Narayan&Medvev2001;Voigt&Fabian2004).Hoeft&Br¨uggen(2004)haveprovidedevidencefortherelativeimportanceofheatingduetothermalconduc-

Figure10.X-rayluminositywithinthecoolingvolumeversusradioluminosityLRofthecentralsource.Symbolsareasbefore.

tiontoincreasewithclustermass,inagreementwithourresults.5.4

Coolingcoreclusters

Clustersofgalaxiescanbeseparatedintotwoclasses:clus-terswithdensegaseouscoresinwhichthecoolingtimeislessthantheHubbletime,(coolcoreclusters),andclusterwithlessdensegaseouscores(non-coolcoreclusters).

Chenetal.(2007)haveinvestigatedtheinfluenceofcoolcoresonclusterscalingrelations.TheiranalysishasshownthatincoolcoreclusterstheX-rayluminosityisenhancedoverthatofnon-coolcoreclusters,whileotherparameterssuchastemperature,massandgasmassarelessaffectedbytheoccurrenceofacoolingcore.Thishasbeenexplainedbythefactthatatleastsomeofthenon-coolcoreclus-tersareindynamicallyyoungstatescomparedwithcoolingcoreclusters.Alternatively,thenon-coolcoreclustersmighthavehadtheircoolcoresheatedbyaradio-loudAGN.ThisissuggestedbyFig.7awhichshowstheLX−Trelationforclusterswithanextendedradiosourceandforthosewithoutit.Ourresultindicatesthatthescalingrelationsforclus-tersaresignificantlyaffectedbythepresenceofaradio-loudAGNattheircentres.

IntheworkbyBirzanetal.(2004),aclosecorrelationbetweenthemechanicalenergyoutputofthejetsandtheenergylossofthecentralX-rayplasmabycooling(relatedtothetraditionallydeterminedcoolingflowmassdepositionrate)hasbeenestablished.ThislendssupporttotheideathatthefeedingoftheAGNisconnectedtothecoolingrateandthattheenergyoutputoftheAGNregulatesthecoolingcorestructurebyfeedbackprocesses.Inanymodeloffeed-back,thepowerofthecentralAGNshouldberegulatedbythesupplyoffuelfromtheICM.Thus,asinathermostat,largeradiativelossesshouldgiverisetoastrong,centralradiosource,whichinturnwillheatuptheICM(seeChu-

12M.Magliocchetti&M.Br¨uggen

Figure11.Thermalenergyrequiredtoheattheclusterswhichhostintheircentresextendedradiosourcesfromthepredictedtothemeasuredtemperature(cfr.Figure7a)asafunctionofradiopower(seetextfordetails).

razovetal.2003).Themorestronglyaclustercoolsatitscentre,themorefuelcanbesuppliedtothecentralAGNwhich,inturn,candrivemoreenergeticradiolobesthroughtheICMwhichgetsheateduntilthecentralcoolingtimegoesup,themassdepositionrategoesdownandthesupplyoffuelisshortened.

Coolingflowmeasurementsfor15clustersbelongingtooursamplehavebeentakenfromWhiteetal.(1997).Fig-ure10showsthedependenceoftheradioluminosityofthecentralsource,LR,onthecoolingluminositydM

dt

vsLRrelationwhichdoesnotaffect

ouranalysisandconclusions).Giventhepaucityofsourcesandthefactthattheydidnotshowanydifferentbehaviour,inthiscasethebest-fitvaluefordM

dt

∝LbRwithb=0.25±0.02cfr.Table1)

whichisinagreementwithfeedbackmodels(e.g.Churazovetal.2003)forthefuelingofclusterradiosources.

6

CLUSTERSWITHCENTRALRADIOEMISSION:UNDER-LUMINOUSOROVERHEATED?

InSection5.2wehaveseenhowclustersthathostaradiosourceendowedwithanextendedstructureshowadepar-turefromthetypicalLX−kTrelationfoundfortheclusterpopulationasawhole(cfr.Figure7a),adeparturethat

isparticularlyremarkableinthecaseoflow-masssystems.

Whatisnotyetclearfromouranalysisisifextendedradiosourcescauseanunder-luminosityoranoverheatingoftheassociatedcluster.Ifitisindeedatemperatureincrementcausedbythecentralradiogalaxy,onemayexpectthecor-respondingheatexcesstobecorrelatedwiththeradiopowerofthecentralsource.

Toinvestigatethispossibility,wehaveconsideredthequantityEreq=3Nk∆T/2=3Mgask∆T/2µmHwhichistheenergyrequiredtoheattheICMfromthepredictedtothemeasuredtemperature.Intheaboveexpression,Nisthetotalnumberofparticles,Mgasthegasmassoftheclusterand∆TthetemperatureincrementofclusterswhichhostanextendedradiosourcewithrespecttotheLX−kTrelationobeyedbyclusterswithunresolvedcentralsources.Thegasmasswascomputedfromthetotalmassoftheclus-terbyusingaconstantgasmassfractionof0.2.WehaveplottedtherequiredenergyasafunctionofradiopowerinFig.11.Theradioluminosityandtheheatinputneededtoproducetheobservedtemperatureincrementinclustershostingextendedradiosourcesappeartobecorrelated,al-thoughwithalargescatter.Thiscorrelationfavoursamodelwherethetemperatureincreaseiscausedbyradiogalaxy-inducedheating.AsnotedbyCrostonetal.(2005)whoper-formedasimilaranalysisforgroupsofgalaxies,thelargescatterinthisplotisnotsurprisinggiventhattherearemanyunknownfactorssuchastheageofthesource,itshistoryandsize.

TheissueoftemperatureincrementcanbeinvestigatedfurtherbyperformingMonteCarlosimulationstobecom-paredwiththeX-rayandradioluminositydistributionsofthepresentsample.PossiblebiasesduetothejointeffectsoftheX-rayandradioselectionfunctions(highlightedbythesolidcurvesinthetwopanelsofFigure12),havebeenremovedbylimitingouranalysistofourcontiguousredshiftregionswhicharecompletebothinX-rayluminosityandin1.4GHzradiopower.Inordertomaximizethenumberofsourcesavailableforstatisticalanalyses,thesefourregionsinthez−LX−LRspacehavebeenidentifiedasfollows:1)z󰀁0.01;LX󰀂0.0015·1044;LR󰀂1019.5;2)0.01(5)

–whereX-rayluminositiesareinerg/sandradioluminosi-tiesinW/Hz/sr–andarethoseenclosedwithinthedashedlinesinFigure12.91clustersinourdatasetfulfilltheaboverequirements(crossesinFigure12),outofwhich45hostsub-structured/extendedradiosources(filledgreensquareswithcrossesontopinFigure12).

Simulatedsamplesinthez−LX−LRspacehavebeenobtainedbyassumingthattheclusterX-rayluminosityandradioluminosityofthecentralsourcearenotcorrelatedwitheachother.Eachsimulatedobjectinthefourregionsconsid-eredinouranalysiswasthengivenatripletofvaluesz,LXandLR,wheretheX-rayluminositywasassignedaccordingtotheB¨ohringeratal.(2002)clusterluminosityfunctionasderivedfromtheREFLEXclustersurveyoncetheirsamplewascorrectedbothformissingfluxandfluxerror:Φ(LX)dLX=Φ0

󰀂LX

L∗

󰀃dLX

Theinterplaybetweenradiogalaxiesandclusterenvironment13

Figure12.DistributionofX-ray(left-handpanel)andradio(right-handpanel)luminositiesasafunctionofredshiftforthe145NORAS+REFLEXclustersendowedwithacentralradio-activecomponent.ThesolidlinesshowtheminimumluminositiesprobedbytheX-rayand1.4GHzsurveyscorrespondingrespectivelytothelimitingfluxesof3·1012ergs−1cm−2and3mJy.Open(red)circlescorrespondtopoint-likeradiosources,whilefilled(green)squaresidentifysub-structuredobjects.Crosseshighlightthosesystems(91,outofwhich45showsignaturesofofextendedemission)whichfulfilltheluminosityrequirements(5)adoptedfortheMonteCarlosimulations(dashedhorizontallinesbothintheleft-handandright-handpanels)andthatthereforehavebeenincludedintheanalysispresentedinthissection(seetextfordetails).

Figure13.FractionalluminositydistributionsofX-rayselectedclustersinhabitedbyacentralradio-activeAGN.Thesolidpointswithassociatederrorbarscorrespondtothe91sourcesextractedfromthesamplepresentedinthisworkbyrequiringtheconditions(5)tobefulfilled.SolidlinesrepresenttheresultsoftheMonteCarlosimulations.

−3

withα=1.69,Φ0=1.07·10−7h3and0,50Mpc

44−2

L∗=8.36·10h0,50erg/s,whereh0,50=h0/0.5.

provedtoprovideaverygoodfittothelocal,AGN-fuelled,radiopopulation(seee.g.Magliocchettietal.2002):

RadioluminositieswereinsteaddrawnfromtheDun-lop&Peacock(1990)radioluminosityfunctionforsteep-spectrumsources(model7forpureluminosityevolution),

Ψ(LR,z)=Ψ0

󰀈󰀄

LR

Lc(z)

󰀅β󰀁−1

,(7)

14M.Magliocchetti&M.Br¨uggen

Figure14.ThesameasinFigure13butforthesub-populationofextendedradiosources(45objects).

whereα=0.69,β=2.17,Ψ0=10−6.97Mpc−3

(∆log10LR)−1andwheretheevolvingbreakluminosityLc(z)canbeexpressedaslog10Lc(z)=26.22+1.26z−0.26z2(inW−1Hz−1units).SincetheaboveexpressionwasderivedforanΩ0=1,h0=0.5(EdS)universe,followingDunlop&Peacock(1990)wehaverewrittenequation(7)inthecon-cordanceΛCDMcosmologybyusingtherelation:Ψ1(LR,1,z)

dV1

dz,

(8)

observedinFigure7a.ThecontributionofAGNheatingfromacentralradiosourceislessimportanti)inthecaseofpoint-likesourceswhicharenotobservedtopermeatetheICMandii)–independentofthetypeofradioobject–inthecaseofhighmasssystemswhichrequireadditionalformsofenergytobalanceradiativeloss.

ThisisinagreementwiththeresultsdisplayedinFig-ure11.Physically,itmeansthattheradiosourcesdisplaceonlyasmallfractionoftheX-rayluminousICMsothatthereisnonoticeableluminositydeficitinclusterswithra-diostructure.However,simulationsshowthatifasignifi-cantfractionofthejetenergyistransferredintopotentialenergythusleadingtoanexpansionoftheclusteratmo-sphere,alsotheluminosityoftheICMdecreases(seeHeinzetal.2006).SuchsimulationsalsosuggestthatasignificantfractionofthejetenergyisthermalisedintheICMonrel-ativelyshorttimescales.Onlongtimescales,however,theenergyinjectedintotheICMbyradiogalaxiesmustendupmostlyaspotentialenergybythevirialtheorem,thusim-plyinglong-termtemperatureincrementstobesmall.Thisindicatesthatouranalysisisunveilingtheshort-termeffectsofradio-loudAGNheating.TheextendedradiostructuresinfactoriginatefromrecentAGNactivityandarefoundincorrespondencewithongoingheatingintheICM.Ontheotherhand,thoseclusterswithnoextendedstructureshavenothadarecent(i.e.longerthanthelifetimeofaradiolobe)outbreakofradio-loudAGNactivityandthereforeshownosignsofrecenttemperatureincrements.

WhiletheX-rayluminosityofatypicalclusterremainsunaffectedbythepresenceofacentralradio-emittingcom-ponent,oursimulationsclearlyshowthatthisisnotthefateforthe1.4GHzluminosityofaradiogalaxysetatthebottomofthepotentialwellofacluster.Infact,theright-handpanelsofFigures13and14highlightthatthereisaremarkabledeficitoflow-luminosity(1021W/Hz/sr<∼22

10W/Hz/sr)radiosourceswhencomparedtotheLR<∼

wholeFR󰀂3mJypopulation.Atthesametime,radio-loud

where1and2refertotheoldandnewcosmologyanddV/dzisthevolumeelement.

Weperformed1000MonteCarloruns,whereeachrunwassetsotogivethesamenumberofsourcesperredshiftinter-valwithluminositiesabovethelimitsexpressedin(5)asintherealdataset.

TheluminositydistributionsofX-ray-selectedclustersinhabitedbyaradio-loudAGNareshowninFigures13and14whichprovidethefractionalnumberofclustersperX-ray(left-handpanels)and1.4GHzluminosity(right-handpanels)forbothrealdata(pointswithassociatederrorbars)andsimulatedsample(solidlines.Notethat,duetothehighnumberofrealizations,thesevaluesarevirtuallyerror-free).

Twoimportantconclusionscanbedrawnfromthedis-tributionsshowninFigures13and14.First,theX-raylu-minositydistributionofclusterswithacentralradio-activecounterpartisvirtuallyidenticaltothatfeaturedbythewholepopulationofX-rayselectedclustersbrighterthanthesamefluxlimit.Inotherwords,thepresenceofacen-tralradio-loudcomponent,howeverpowerful,doesnotaffecttheclusterX-rayluminosity.ThisresultisfoundregardlesstowhethertheradioAGNshowsanextendedorpoint-likestructure.IfwethencombinetheabovefindingwithwhatemergedfromSections5.2and5.3,wecanconcludethat–mostlyinthecaseofsmallmasssystems–theinteractionofextendedradiosourceswiththesurroundingenvironmentcausesaremarkableoverheatingoftheavailablegas,heat-ingwhichproducesthesteepeningoftheLX−kTrelation

Theinterplaybetweenradiogalaxiesandclusterenvironment

15

AGNwithintheclustercentresseemtofavourluminosities∼1024W/Hz/sr(∼3σdiscrepancyfromtheresultsforthewhole1.4GHzradiopopulation).Thisdifferentbehaviourbetweentheradio-luminositydistributionofrealsourcesandofsimulateddatasetsisfoundregardlessofwhetherthecen-tralradiosourcepresentsanextendedstructureornot.Thisresultimpliesadifferencebetweentheluminosityfunctionofradiosourcessittingintheclustercentresandthatofthewholeradiopopulation,inthesensethattheformer

oneismuchflatteratallluminositiesLRW/Hz/sr.Indeed,Figure4ofBestetal.(2007)showsthedif-ferentbehaviouroftheluminosityfunctionsasderivedfortheirsampleofBCGsandinthemoregeneralcaseofradiosourcesassociatedtoSDSSgalaxiesofdifferentmass,eventhoughtheauthorsdismisstheevidenceas’tentative’.

Thefactthatlow-luminosityradiosourcesareunder-representedinclustersismostlikelyduetothe’strangling’effectcausedbytheoverdensecentralregionsofmassivesystemsonradioobjectsthatarenotpowerfulenoughtoexpandthroughoutthesurroundingmedium.Ontheotherhand,ifwecombinetheresultspresentedinthissectionwiththeanalysisperformedinSection5.1,wecancon-cludethatextendedstructuresandhighradioluminositiesaremorelikelytooccurinsourcesthatarelocatedinrichenvironmentssuchasgroupsorclustersofgalaxies.Thisislikelycausedbyatightinterplaybetweentheintraclustermediumandtheenergyreleasedbyacentralradiosource,whichbooststheradioluminosityofthelatterone.Indeed,inagreementwithourresults,Barthel&Arnaud(1996)findthatforafixedjetkineticpower,radioluminositiescanbehigherinclustersduetotheconfiningeffectofthedenseICMwhichreduceslossesduetoadiabaticexpansionintheradiolobes,thereforeenhancingtheradiosynchrotronemis-sion.

7CONCLUSIONS

WehavecombinedtheREFLEXandNORASclustersur-veyswiththeNVSSdatasettoprovidealistofX-rayse-lectedclustersbrighterthan3·10−12ergs−1cm−2whichhostintheircentres(dist󰀁1.5%rvir)aradiosourcebrighterthan3mJy.Outof550clusterssetinthewholeskynorthofδ=−40◦,148systems(correspondingto∼27percentofthetotalsample)showsignaturesofcentralradioemis-sion.WefindthatthreesystemswereimagedbothinRE-FLEXandNORASintheoverlappingregion0◦<∼δ<◦Bythenremovingdouble∼2.5

betweenthetwosurveys.identi-ficationsweendupwithasampleof145clustersthathostacentralradiosourcewithF1.4GHz󰀂3mJy.Visualinves-tigationsofradiomapsshowthat61oftheradioobjects(i.e.∼42percentofthesample)associatedwithclustercentrespossessextendedradioemissionintheformofelon-gatedblobs,doubleortriple/multiplestructures.Archivaldata(mainlytakenfromtheBAXandVIZIERdatabases)haveprovidedextrainformationsuchastemperaturesandvelocitydispersionsforsomeofthesystemsinoursample.

Themainconclusionsofourworkcanbesummarizedasfollows:

(1)Althoughthefractionofclustersthathostacen-tralradiosourcebrighterthanLR=1022W/Hz/srisap-proximatelyconstantwithclustermassandequalto20%

(inagreementwiththeresultsofBestetal.2007),wefindthat11outof12(i.e.92percent)local/low-masssystemspresentcentralradioemissionwithluminosities

LR>20

W/Hz/sr.Thissuggeststhat,either,radiosources∼10

preferentiallyinhabitlow-massclusters,or–morelikely–thatbygoingdeepenoughinradioflux,mostofX-rayselectedclusterswillbefoundtohostacentralradio-loudAGN.

(2)Theluminosity-temperaturerelationofclustersthathostacentralradiosourcefollowsthatfoundforthewholeclusterpopulation(LX∝T2.8–e.g.White,Jones&Forman1997;Arnaud&Evrard1999;Popessoetal.2005).However,thereisasignificantdiscrepancyintheLXvskTrelationbetweenunresolvedandextendedradiosources.ThelatterpopulationshowsthemuchsteepertrendLX∝T4,whileforunresolvedstructuresonefindsLX∝T2.3,whichisclosetotheself-similarresult.ThedifferencebetweentheLXvskTtrendforpoint-likeandextendedsourcesbecomespar-ticularlynoticeablefortemperatureskT<∼3KeV,wherealmostallsystemswithextendedradioemissionliebelowthoseassociatedwithunresolvedradiostructures.

(3)Theradioluminositiesofthecentralsourcesshowasteep(LR∝T6)dependenceontheclustertemperatureforkT<∼3KeV.Suchacorrelationislostinmoremassivesystems.

(4)TheX-rayluminosityvsvelocitydispersionandthetemperaturevsσrelationsofoursampleareinagreementwiththosefoundintheliterature(LX∝σ3.8;kT∝σ2;e.g.Popessoetal.2005;Madhavi&Gheller2001;Osmond&Ponman2004).Nosignificantdifferencesarefoundbetweenpoint-likeandextendedsources,exceptforamildpreferenceforthelatterpopulationtoappearinhighervelocitysys-tems.Weremarkthoughthattheavailableσmeasurementsdonotallowustoprobetheverylow-massregimewherethedifferentbehaviourbetweenpoint-likeandextendedsourcescouldbemoreprominent.

(5)Themechanicalluminosityprovidedbythecentralradiosourceisfoundtobalanceradiativelossesonlyinsmall(i.e.σ<∼400km/s)systems.

(6)Itseemsplausiblethatheatingbylow-power(FRI)radiogalaxiescanexplaintheabsenceofcoolingflowsinclustersofgalaxies.Thisisinagreementwithconclusionsbyvariousotherauthors(e.g.Fabianetal.2003;Crostonetal.2005),whoconcentratedtheiranalysisongroupsofgalaxies.However,forthelargestclusters,theenergyre-leasedbytheradiosourcesmaynotbesufficienttobalanceradiativelosses.

(7)MonteCarlosimulationsshowthattheX-raylumi-nosityofaclusterisnotaffectedbythepresenceofacentralradiosource,howeverpowerful.Theluminositydistributionofthepresentsampleofradio-identifiedsystemsisthesameasthatofthewholeclusterpopulation.Combiningthisre-sultwithwhatwefoundinpoint(2),wecanconcludethatthepresenceofradiosourceswithanextendedstructureisresponsiblefortheover-heatingoftheintraclustergas.Theimportanceofsuchaheatingdramaticallyincreasesinlow-masssystems.

(8)Theluminositydistributionofradiogalaxiessittinginclustercentresisverydifferentfromthatofthetotalradiopopulation.Infact,intheformercasewefindthatlowluminosities(LR∼1021−1022W/Hz/sr)aredepressed,whilehigherluminositiesarestronglyboosted.Theneteffect

16M.Magliocchetti&M.Br¨uggen

ontheradioluminosityfunctionisaflatteninginthewhole

luminosityrangeLR<∼1024

W/Hz/sr.OurresultssupportastronginteractionbetweenAGNradioemissionandtheICM.Radiosourcesarepresentinthecentresofalmostalllocal/smallmasssystemsandweexpectthemajorityofclusterstoexhibitcentralradioemis-sionwithpowersLR>∼1020

W/Hz/sr.TheICMstronglyaffectstheluminosityoftheradiosourcethatsitsintheclustercentre:low-luminosityobjectsare’strangled’bytheoverdensecentralmedium,whilebrightoneshavetheirlu-minositiesboostedbytheinteractionwiththesurroundinggas(seee.g.Barthel&Arnaud1996).Moreover,theexis-tenceofacentralradiosource–especiallyifitexhibitsanextendedstructureandresidesinsmall-masssystems–leadstoasignificantheatingoftheICM.

Furthermore,radio-loudAGNareobservedtoprovideenoughmechanicalenergytobalanceradiativelosses.TheeffectofAGNheatingfromacentralradiosourcebecomeslessimportanti)inthecaseofpoint-likesourceswhicharenotobservedtopermeatetheICMandii)–independentofthetypeofradioobject–forhighmasssystems.ClustershotterthankT<∼3KeV,donotobeythetightLRvskTre-lationobservedforsmallsystems,theeffectonthethermalstateoftheclusterasprovokedbythepresenceofaradio-activeAGNisindependentofthecapabilitiesofsuchanobjecttopermeatethesurroundingmedium(nodifferenceintheLX−kTrelationbetweenextendedandpoint-likesources)andAGNheatingisfoundtobeinsufficienttobal-anceradiativelossesinthecluster.Inthiscase,adifferentsourceofheatingsuchasthermalconduction(e.g.Narayan&Medvev2001;Voigt&Fabian2004)hastobeinvoked.

Thedatasuggestasmoothtransitionbetweentheradio-AGNheatingmodeandthethermalconductionmode.Indeed,doubleheatingmodels–firstdevelopedbyRuszkowski&Begelman(2002)–havebeenprovedtopro-videagoodagreementwiththeobservedclusterproperties(seee.g.Br¨uggenetal.2003;Hoeft&Br¨uggen2004;Roy-chowduryetal.2005;Fujita&Suzuki2005).Withinthisframework,Hoeft&Br¨uggen(2004)haveshowntherela-tiveimportanceofheatingduetothermalconductiontoincreasewithclustermass,inexcellentagreementwithourfindings.

Obviously,itwouldbedesirabletopushthiskindofanalysistofainterfluxes(bothradioandX-ray)andhigherredshiftstoinvestigatewhethersomeofourfindingsonlyholdinthelocaluniverseoraremoregeneralpropertiesoftheclusterpopulation.ForthcomingsurveyslikeCOSMOS(seee.g.Finoguenovetal.2007)areexpectedtoprovidesuchanswers.

Acknowledgments

MMwishestothankS.Borgani,A.Merloni,P.Rosati&G.DeZottifordiscussionsandclarificationswhichgreatlyhelpedshapingupthiswork.MBwishestoacknowledgethesupportbytheDFGgrantBR2026/4withinthePrior-ityProgram“WitnessesofCosmicHistory”andthesuper-computinggrantsNIC1927and1658attheJohn-NeumannInstitutattheForschungszentrumJ¨ulich.Wethanktheref-ereeforhelpfulcomments.

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