A&A447,97–112(2006) Astronomy DOI:10.1051/0004-6361:20054031 & (cid:1)c ESO2006 Astrophysics The host galaxy/AGN connection (cid:1),(cid:1)(cid:1) in nearby early-type galaxies Is there a miniature radio-galaxy in every “core” galaxy? B.Balmaverde1 andA.Capetti2 1 UniversitádiTorino,ViaGiuria1,10125,Torino,Italy e-mail:[email protected] 2 INAF-OsservatorioAstronomicodiTorino,StradaOsservatorio20,10025PinoTorinese,Italy e-mail:[email protected] Received11August2005/Accepted20September2005 ABSTRACT ThisisthesecondofaseriesofthreepapersexploringtheconnectionbetweenthemultiwavelengthpropertiesofAGNinnearbyearly-type galaxiesandthecharacteristicsoftheirhosts.Weselectedtwosampleswith5GHzVLAradiofluxmeasurementsdownto1mJy,reaching levels of radio luminosity as low as 1036 ergs−1. In Paper I we presented a study of the surface brightness profiles for the 65 objects with availablearchivalHSTimagesoutofthe116radio-detectedgalaxies.Weclassifiedearly-typegalaxiesinto“core”and“power-law”galaxies, discriminatingonthebasisoftheslopeoftheirnuclearbrightnessprofiles,followingtheNukersscheme.Herewefocusonthe29coregalaxies (hereafterCoreG). We used HST and Chandra data to isolate their optical and X-ray nuclear emission. The CoreG invariably host radio-loud nuclei, with an averageradio-loudnessparameterofLogR=L /L ∼3.6.TheopticalandX-raynuclearluminositiescorrelatewiththeradio-corepower, 5GHz B smoothlyextendingtheanalogouscorrelationsalreadyfoundforlowluminosityradio-galaxies(LLRG)towardevenlowerpower,byafactor of∼1000,coveringacombinedrangeof6ordersofmagnitude.Thissupportstheinterpretationofacommonnon-thermaloriginofthenuclear emission also for CoreG. The luminosities of the nuclear sources, most likely dominated by jet emission, set firm upper limits, as low as L/L ∼10−9inboththeopticalandX-rayband,onanyemissionfromtheaccretionprocess. Edd The similarity of CoreG and LLRG when considering the distributions host galaxies luminosities and black hole masses, as well as of the surfacebrightnessprofiles,indicatesthattheyaredrawnfromthesamepopulationofearly-typegalaxies.LLRGrepresentonlythetipofthe icebergassociatedwith(relatively)highactivitylevels,withCoreGformingthebulkofthepopulation. We do not find any relationship between radio-power and black hole mass. A minimum black hole mass of MBH = 108 M(cid:3) is apparently associatedwiththeradio-loudnucleiinbothCoreGandLLRG,butthiseffectmustbetestedonasampleoflessluminousgalaxies,likelyto hostsmallerblackholes. IntheunifyingmodelforBLLacsandradio-galaxies,CoreGlikelyrepresentthecounterpartsofthelargepopulationoflowluminosityBLLac nowemergingfromthesurveysatlowradiofluxlimits.Thissuggeststhepresenceofrelativisticjetsalsointhesequasi-quiescentearly-type “core”galaxies. Keywords.galaxies:active–galaxies:bulges–galaxies:nuclei–galaxies:ellipticalandlenticular,cD–galaxies:jets– galaxies:BLLacertaeobjects:general 1. Introduction whichtoexploretheclassicalissueoftheconnectionbetween hostgalaxiesandAGN. The recent developments in our understanding of the nuclear All evidence now points to the idea that most galaxies regionsofnearbygalaxiesprovideuswithanewframeworkin host a supermassive black hole (SMBH) in their centers (e.g. Kormendy & Richstone 1995) and that its mass is closely (cid:1) Based on observations obtained at the Space Telescope Science linkedtothehostgalaxiesproperties,suchasthestellarveloc- Institute, which is operated by the Association of Universities itydispersion(Ferrarese&Merritt2000;Gebhardtetal.2000). for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. Thisisclearlyindicativeofacoevolutionofthegalaxy/SMBH (cid:1)(cid:1) AppendixAandBareonlyavailableinelectronicformat systemanditalsoprovidesuswithindirect,butrobust,SMBH http://www.edpsciences.org mass estimates for large sample of objects. Furthermore, Article published by EDP Sciences and available at http://www.edpsciences.org/aaor http://dx.doi.org/10.1051/0004-6361:20054031 98 B.BalmaverdeandA.Capetti:Aminiatureradio-galaxyinevery“core”galaxy? the innermoststructureofnearbygalaxieshavebeenrevealed to separate these early-type galaxies into core and power-law by HST imaging, showing the ubiquitous presence of singu- galaxiesfollowingthe Nukersscheme, rather than on the tra- lar starlight distributions with surface brightness divergingas ditionalmorphologicalclassification(i.e.intoEandS0galax- Σ(r) ∼r−γ withγ>0(e.g.Laueretal.1995).Thedistribution ies).Herewefocusonthesub-sampleformedbythe29“core” ofcuspslopes(Faberetal.1997)isbimodal,withapaucityof galaxies. objectswith0.3 < γ < 0.5.Galaxiescanthenbeseparatedon WeadoptaHubbleconstantH =75kms−1Mpc−1. 0 thebasisoftheirbrightnessprofilesinthetwoclassesof“core” (γ ≤ 0.3)and“power-law”(γ ≥ 0.5)galaxies,in closecorre- spondencetotherevisionoftheHubblesequenceproposedby 2. Acriticalanalysisoftheclassificationascore Kormendy&Bender(1996). galaxies Butdespite these fundamentalbreakthroughswe still lack In Paper I we adopted the classification into power-law and a clear picture of the precise relationship between AGN and core galaxies following the scheme proposed by Lauer et al. hostgalaxies.Forexample,whilespiralgalaxiespreferentially (1995).We then separated early-type galaxies on the basis of harbourradio-quietAGN,early-typegalaxieshostbothradio- theslopeoftheirnuclearbrightnessprofilesobtainedusingthe loudandradio-quietAGN.Similarly,radio-loudAGNaregen- Nukers law (i.e. a double power-law with innermost slope γ) erally associated with the most massive SMBH as there is a defining as core-galaxies all objects with γ ≤ 0.3. Since this median shift between the radio-quiet and radio-loud distribu- strategy has been subsequently challenged by Graham et al. tion,butbothdistributionsarebroadandoverlapconsiderably (2003),whointroducedadifferentdefinitionofcore-galaxies, (e.g.Dunlopetal.2003). it is clearly important to assess whether the identification of Inthisframework,intwosensesearly-typegalaxiesappear an object as a core galaxy is dependenton the fitting scheme tobethecriticalclassofobjects,wherethetransitionbetween adopted. the two profiles classes occurs(i.e. in which core and power- law galaxiescoexist)and in which theycan hosteither radio- Graham et al. argued that a Sérsic model (Sérsic 1968) loud and radio-quiet AGN. We thus started a comprehensive providesa better characterizationof the brightnessprofilesof study of a sample of early-type galaxies (see below for the early-typegalaxies.Inparticulartheypointedoutthat,among sampledefinition)toexploretheconnectionbetweenthemul- otherissues,i)thevaluesoftheNukerslawparametersdepend tiwavelengthpropertiesofAGNandthecharacteristicsoftheir ontheradialregionusedforthefit;ii)theNukersfitisunable hosts. Since the “Nuker” classification can only be obtained to reproduce the large scale behaviour of early-type galaxies when the nuclear region, potentially associated with a shal- and, most importantly for our purposes; iii) the identification low cusp, can be well resolved, such a study must be limited ofacoregalaxyfromaNukerfitmightnotberecoveredbya tonearbygalaxies.Themostcompactcoreswillbebarelyre- Sérsicfit.Conversely,theywereabletofitpower-lawgalaxies solvedatadistanceof40Mpc(where10pcsubtend0(cid:6).(cid:6)05)even (intheNukersscheme),aswellasdwarfellipticals(Graham& intheHSTimages.Furthermore,highqualityradio-imagesare Guzmán2003),witha singleSérsiclawoverthewholerange requiredforaninitialselectionofAGNcandidates. of radii. They also suggested a new definition of core-galaxy Wethenexaminedtwosamplesofnearbyobjectsforwhich astheclassofobjectsshowingalightdeficittowardthecenter radio observations combining relatively high resolution, high withrespecttotheSérsiclaw(Trujilloetal.2004). frequency and sensitivity are available, in order to minimize In this context, we discuss in detail here the behaviourof thecontributionfromradioemissionnotrelatedtothegalaxy’s the most critical objects, i.e. the two core galaxies for which nucleusandconfusionfrombackgroundsources.Morespecifi- theNukerlawreturnsthesmallestvaluesforthebreakradius, callywefocusonthesamplesofearly-typegalaxiesstudiedby namely UGC 7760 and UGC 7797 for which rb = 0(cid:6).(cid:6)49 and Wrobel & Heeschen (1991) and Sadler et al. (1989)both ob- rb = 0(cid:6).(cid:6)21 respectively.We fit both objectswith a Sérsic law. servedwiththeVLAat5GHzwithafluxlimitof∼1mJy.The Thefinalfits,showninFig.1,wereobtainediteratively,fitting twosampleswereselectedwithaverysimilarstrategy.Wrobel theexternalregionswhileflaggingtheinnermostpointsoutto (1991) extracted a northern sample of galaxies from the CfA a radius at which the residual from the Sérsic law exceeded redshift survey (Huchra et al. 1983) satisfying the following a threshold of 5%. The Sérsic law in general provides a re- criteria: (1) δ ≥ 0, (2) photometric magnitude B ≤ 14; markablygoodfittotheouterregions,withtypicalresidualsof 1950 (3) heliocentric velocity ≤3000 kms−1, and (4) morphologi- ∼1%,butasubstantialcentrallightdeficitisclearlypresentin cal Hubble type T ≤ −1, for a total number of 216 galaxies. bothobjects.Thisindicatesthatbothobjectscanbeclassified Sadleretal.(1989)selectedasimilarsouthernsampleof116E ascore-galaxiesintheGrahametal.scheme. andS0with−45 ≤ δ ≤ −32.Theonlydifferencebetweenthe Using the brightness profiles for the core-galaxies for twosamplesisthatSadleretal.didnotimposeadistancelimit. whichweobtainedNukerfitsinPaperI(14additionalobjects) Nonetheless,thethresholdinopticalmagnitudeeffectivelylim- we obtained similar results. Very satisfactory fits can be ob- itsthesampletoarecessionvelocityof∼6000kms−1. tainedwithaSérsiclawontheexternalregionsofthesegalax- InCapetti&Balmaverde(2005,hereafterPaperI),wefo- ies, but they all show an even clearer central light deficit, as cused on the 116 galaxies detected in these VLA surveys to expectedgiventhepresenceofwellresolvedshallowcores. boost the fraction of AGN with respect to a purely optically Weconcludethat,forthegalaxiesofoursample,theobjects selectedsample.WeusedarchivalHSTobservations,available classified as core-galaxiesin the Nuker scheme are recovered for 65 objects, to study their surface brightness profiles and assuchwiththeGrahametal.definition. B.BalmaverdeandA.Capetti:Aminiatureradio-galaxyinevery“core”galaxy? 99 Fig.1.Sérsicfitsforthetwocoregalaxiesofoursamplewiththesmallestvaluesforthecoreradius.Asubstantialcentrallightdeficitisclearly presentinbothobjects,conformingtothe“core”classificationintheGrahametal.scheme. 3. Basicdataandnuclearluminosities thisisexpectedinthepresenceofanuclearpointsource,since the convolution with the Point Spread Function produces a The basic data for the selected galaxies, namely the reces- smooth decrease of the slope toward the center when only a sion velocity (corrected for Local Group infall onto Virgo), diffuse galactic component is present. We therefore preferred the K band magnitude from the Two Micron All Sky Survey toadopta “local” approachto identifynuclearsources,based (2MASS), the galactic extinction and the total and core radio on the characteristic up-turn they cause in the nuclear bright- fluxesweregiveninPaperI. nessprofile. In the following three subsections, we derive and discuss Morespecifically,weevaluatedthederivativeofthebright- themeasurementsforthenuclearsourcesintheoptical,X-ray ness profile in a log-log representation for the sources of our andradiobands. sample.Inordertoincreasethestabilityoftheslopemeasure- mentthishasbeenestimatedbycombiningthebrightnessmea- suredovertwoadjacentpointsoneachsideoftheradiusofin- 3.1.Opticalnuclei terest, yieldinga second orderaccuracy.We then lookfor the Thedetectionandmeasurementofanopticalnuclearsourceat presence of a nuclearup-turnin the derivativerequiringfor a the center of a galaxy is a challenging task particularly when nucleardetectionadifferencelargerthan3σfromtheslopesat itrepresentsonlyasmallcontributionwithrespecttothehost the local minimum and maximum. This a rather conservative emission,asislikelyoftentobethecaseoftheweaklyactive definition since the region over which the up-turn is detected galaxiesmakingupoursample. extends over several pixels while we only consider the peak- Differentapproacheshavebeenemployedintheliterature. to-peakdifference. The most widely used method is to fit the overall brightness Toillustratethiswefocusonthreecases.IntheHSTimage profileofagalaxywithanempiricalfunctionalformandtode- aswellasinthebrightnessprofileofIC4296anucleusclearly fineagalaxyas“nucleated”whenitshowsalightexcessinits stands out against the underlying background and the central centralregionwithrespecttothemodel(e.g.Laueretal.2004; steepeningataboutr = 0(cid:6).(cid:6)15ishighlysignificant.NGC4373 Ravindranath et al. 2001). The drawback of this “global” ap- is the detection with the least significance of our sample, in proachisthatitassumesthatthemodelcanbeextrapolatedin- whichthe presenceof a nucleusis uncertainfromjust the vi- wardsfromtheradialdomainoverwhichthefitwasperformed. sual inspection of the optical image, but the derivative of the Furthermore,themeasurementandidentificationofthenuclear brightnessprofilerevealstheeffectofthepointsourcewithan component are coupled with the behaviour of the brightness increaseof0.013±0.004fromr = 0(cid:6).(cid:6)1andr = 0(cid:6).(cid:6)07.Instead profile at all radii and with the specific choice of an analytic inNGC1316wedonothaveevidenceforanycompactpoint form. Although this is not a significant issue for bright point source,bothinthe imageandin thebrightnessprofilederiva- sources,itisparticularlyworrisomeforthefaintnucleiweare tive,anditisconsideredasanon-detection. dealingwith.Nonetheless,Restetal.(2001)pointedoutthatin Adoptingthis strategy in 18 out of 29 objectswe identify generalnuclearlightexcessesareassociatedwithasteepening anopticalnucleus,withapercentageof∼60%ofthetotalsam- oftheprofileastheHSTresolutionlimitisapproached.Indeed ple.Insevenobjectswe didnotfindanyupturnandthese are 100 B.BalmaverdeandA.Capetti:Aminiatureradio-galaxyinevery“core”galaxy? Table1.(1)Opticalname(2)Chandraobservational identificationnumber,(3)exposuretime[ks],(4)referencefortheX-rayanalysis(see belowforthelist),(5)instrumentandfilteroftheHSTobservation,(6)opticalnuclearflux[ergcm−2s−1]. Name Chandradatasummary HSTdatasummary Obs.Id Exp.time Ref. Image F 0 UGC0968 – – – WFPC2/F814W <4.0E−14 UGC5902 1587 31.9 (1) WFPC2/F814W <6.3E−14 UGC6297 2073 39.0 (1) WFPC2/F814W <5.0E−14 UGC7203 3995 5.13 (1) WFPC2/F702W 4.0E−14 UGC7360 834 35.2 (2) NICMOS/F160W 1.9E−13 UGC7386 398 1.43 (3) NICMOS/F160W 1.6E−13 UGC7494 803 28.85 (2) NICMOS/F160W 3.3E−13 UGC7629 321 40.1 (4) WFPC2/F555W 1.4E−14 UGC7654 1808 14.17 (2) NICMOS/F160W 5.1E−12 UGC7760 2072 55.14 (5) WFPC2/F555W 6.8E−14 UGC7797 – – – WFPC2/F702W <1.0E−13 UGC7878 323 53.05 (4) WFPC2/F814W 4.3E−14 UGC7898 785 37.35 (6) WFPC2/F555W <1.8E−14 UGC8745 – – – WFPC2/F814W – UGC9655 – – – WFPC2/F702W – UGC9706 4009 30.79 (5) WFPC2/F702W 1.5E−14 UGC9723 2879 34.18 (7) WFPC2/F814W – NGC1316 2022 30.23 (8) NICMOS/F160W <9.6E−13 NGC1399 319 56.66 (4) WFPC2/F814W 1.4E−14 NGC3258 – – – ACS/F814W 4.9E−14 NGC3268 – – – ACS/F814W <1.5E−14 NGC3557 3217 37.99 (1) WFPC2/F555W – NGC4373 – – – WFPC2/F814W 2.4E−14 NGC4696 1560 85.84 (9) ACS/F814W 2.0E−14 NGC5128 463/1253 19.6+6.88 (10) NICMOS/F222W 4.5E−11 NGC5419 4999/5000 15+15 (1) WFPC2/F555W 6.7E−14 IC1459 2196 60.17 (11) WFPC2/F814W 4.4E−13 IC4296 3394 25.4 (12) NICMOS/F160W 9.5E−14 IC4931 – – – WFPC2/F814W 1.9E−14 (1)Thiswork,(2)Balmaverde&Capetti(2005),(3)Hoetal.(2001),(4)Loewensteinetal.(2001),(5)Filhoetal.(2004),(6)Randalletal. (2004),(7)Terashima&Wilson(2003),(8)Kim&Fabbiano(2003),(9)Satyapaletal.(2004),(10)Evansetal.(2004),(11)Fabbianoetal. (2003),(12)Pellegrinietal.(2003). consideredupperlimits. Note thatthisis againa conservative theup-turnandasthebackgroundregiona circumnuclearan- approach,sinceanuclearsourcecanstillbepresentbutitsin- nulus,0.1(cid:6)(cid:6)inwidth.Fortheundetectednucleiwesetasupper tensity might not be sufficient to compensate the downward limitsthelightexcesswithrespecttothestarlightbackground trendofthederivativesetsbythehostgalaxy. within a circular aperture 0.1(cid:6)(cid:6) in diameter. Then we use the Intheremaining4objectsthecentralregionshaveacom- PHOTFLAM and EXPTIME keywordin the image header to plex structureandno estimate of the opticalnucleusintensity convertthetotalcountstofluxes.Errorsonthemeasurements canbeobtained.Intwocases(UGC8745andUGC9723)the of the optical nuclei are dominated by the uncertainty in the central regionsare completely hiddenby a kpc scale edge-on behaviour of the host’s profile, while the statistical and abso- disk, while in NGC 3557 the study of its nuclear regions is lutecalibrationerrorsamounttolessthan10%.Theverypres- hampered by the presence of a circumnuclear dusty disk. In enceofthenucleuspreventsusfromdeterminingaccuratelythe UGC9655,theinnermostregion(r < 0(cid:6).(cid:6)1)hasalowerbright- hostcontributionwithinthecentralaperture.Ourstrategyisto ness than its surrounding; since only a single band image is remove the background measured as close as possible to the availablewecannotassessifthisisduetodustabsorptionorto nucleus, i.e. effectively we adopted a constantstarlight distri- agenuinecentralbrightnessminimumasinthecasesdiscussed butionintheinnermostregions.Analternativeapproachwould byLaueretal.(2002). betoextrapolatetheprofilewithaconstantslopeinstead.Our We measured the nuclear luminosity with the task definitionofnuclearsources(anincreaseintheprofile’sderiva- RADPROF in IRAF, choosing as the extraction region a cir- tive)implicitlyrequiresthattheobservedprofileliesabovethis cle centered on the nucleus with radius set at the location of extrapolation, but the resulting flux is reduced by at most a B.BalmaverdeandA.Capetti:Aminiatureradio-galaxyinevery“core”galaxy? 101 Table2.Coregalaxiesdata:(1)UGCname,(2)intrinsicnuclearX-rayluminosity(2–10keV)[ergs−1],(3)nuclearopticalluminosity(8140Å) [ergs−1]correctedforabsorptionusingthegalacticextinctionvaluesinPaperI,(4)nuclearradioluminosity(5GHz)[ergs−1]derivedfrom Paper I, (5) total radio luminosity (5GHz) [erg s−1] derived from Paper I, (6) Hα+[NII] line luminosity [erg s−1] from Ho et al. (1997) or a Phillipset al.(1986), (7) total K band galaxy’s absolute magnitude from2MASS,(8) logarithmof black hole massinsolar unity fromb Marconietal.(2003)orderivedusingthevelocitydispertion. Name LogLx LogνLo LogνLcore LogνLtot LogLHα+[NII] MK Log(MBH/M(cid:3)) UGC0968 – <39.77 36.94 36.94 38.98 –25.39 8.54 UGC5902 <38.40 <39.10 35.83 35.83 38.76 –24.25 8.00b UGC6297 <38.40 <39.06 36.46 36.46 39.09 –23.40 8.33 UGC7203 <38.93 39.85 37.44 37.44 38.74 –24.08 7.98 UGC7360 40.95 39.71 39.22 40.64 39.76 –25.11 8.72b UGC7386 39.72 38.76 38.38 38.38 39.60 –22.97 8.43 UGC7494 39.30 39.27 38.57 39.48 39.14 –24.41 9.00b UGC7629 38.23 38.79 37.73 37.95 37.98 –25.09 8.78 UGC7654 40.30 40.72 39.90 41.15 40.00 –25.48 9.53b UGC7760 38.40 38.73 37.30 37.3 37.90 –21.86 8.54 UGC7797 – <40.19 38.05 38.05 39.46 –24.61 8.33 UGC7878 <38.41 39.07 36.90 37.77 38.60 –24.43 8.16 UGC7898 <38.52 <39.13 37.46 37.59 – –25.34 9.30b UGC8745 – Dusty 37.81 37.97 39.21 –25.07 8.39 UGC9655 – Dusty 36.96 36.96 39.01 –24.74 8.44 UGC9706 38.26 39.25 37.31 37.31 39.11 –25.06 8.43 UGC9723 <38.18 Dusty 36.92 36.92 38.28 –23.82 7.73 NGC1316 39.62 <40.11 37.82 41.24 – –25.99 8.36 NGC1399 <38.79 38.65 37.20 38.73 – –24.75 9.07 NGC3258 – 39.91 37.42 38.57 – –24.39 8.67 NGC3268 – <39.42 38.21 38.21 39.67a –24.55 8.33 NGC3557 40.08 Dusty 37.94 39.41 – –25.70 8.67 NGC4373 – 39.79 38.15 38.15 – –25.51 8.49 NGC4696 40.04 39.59 38.65 40.05 39.30 –25.69 8.55 NGC5128 42.11 40.31 39.05 40.31 38.16 –24.64 8.38b NGC5419 40.69 40.79 38.42 39.83 – –26.14 9.02 IC1459 40.56 40.33 39.38 39.38 40.02a –24.70 9.18b IC4296 41.18 39.85 39.46 40.35 39.74a –25.91 9.04 IC4931 – 40.19 37.51 37.51 – –25.79 8.67 Fig.2. Brightnessprofileanditsderivativeforthreeobjectsofthesample,namelyIC4296,NGC4373andNGC1316.Thefirsttwogalaxies show, atdecreasing significancelevel,thecharacteristicup-turnintheprofileassociatedwithanuclear source. Thisisnot seeninthethird objectwhichisthenconsideredasanon-detection. 102 B.BalmaverdeandA.Capetti:Aminiatureradio-galaxyinevery“core”galaxy? factor of 2 (with respect to the case of constant background) forthenucleiwiththesmallestcontrastagainstthegalaxy.As willbecomeclearinthenextsections,errorsofthismagnitude onlyhaveamarginalimpactonourconclusions.Theresulting fluxesarereportedinTable1.Wefinallyderivedallthelumi- nosities referred to 8140 Å (see Table 2), after correcting for theGalacticextinctionastabulatedinPaperIandadoptingan opticalspectralindex1α =1. o 3.2.X-raynuclei ForthemeasurementsoftheX-raynucleiweconcentrateonly on the Chandra measurements, as this telescope provides the bestcombinationofsensitivityandresolutionnecessarytode- tect the faint nuclei expected in these weakly active galaxies. Data for 21 core galaxies are available in the Chandra public archive. When available, we used the results of the analysis of the X-ray data from the literature. We find estimates of the lumi- nositiesofthenuclearsources(usuallydefinedasthedetection ofahighenergypower-lawcomponent)basedonChandradata for16objectsofoursample(12ofwhicharedetectionsand4 Fig.3.RadiocorefluxdensityforCoreGobtainedat5GHzwithVLA (usedinthiswork)comparedtohigherresolutiondatafromSleeetal. areupperlimits)whichwerescaledtoouradopteddistanceand (1994); Nagar et al. (2002); Filho et al. (2002); Krajnovic´ & Jaffe convertedtothe2–10keVband,usingthepublishedpowerlaw (2002);Jones&Wehrle(1997).Thedottedlineisthebisectrixofthe index.InTable1wegiveasummaryoftheavailableChandra plane. data, while references and details on the X-ray observations andanalysisarepresentedinAppendixA. WealsoconsideredtheChandraarchivaldataforthe5un- Sadler et al. (1989), performed with the VLA at 5 GHz with publishedobjects,namelyUGC5902,UGC6297,UGC7203, a resolution of ∼5(cid:6)(cid:6). Although these represent the most uni- NGC3557andNGC5419.Weanalyzedtheseobservationsus- formandcomprehensivestudiesofradioemissioninearly-type ing theChandradata analysisCIAO v3.0.2,with the CALDB galaxies,theydonotalwayshavearesolutionsufficienttosepa- version 2.25, using the same strategy as in Balmaverde & ratethecoreemissionfromanyextendedstructure.Sadleretal. Capetti (2005). We reprocessed all the data from level 1 to (1989)arguedthatatdecreasingradioluminositythereisacor- level 2, subtracting the bad pixels, applying ACIS CTI cor- respondingincrease ofthe fractionalcontributionof the radio rection, coordinates and pha randomization. We searched for core. backgroundflaresandexcludedsomeperiodofbadaspect. ToverifywhethertheVLAdataoverestimatethecoreflux, We thenextractedthespectruminacircleregioncentered we searched the literature for radio core measurements ob- onthenucleuswitharadiusof2(cid:6)(cid:6)andwetakethebackground tained at higherresolution(and/orhigherfrequency)than our inanannulusof4(cid:6)(cid:6).Wegroupedthespectrumtohaveatleast data. This would improve the estimate of the core flux den- 10countsperbinandappliedPoissonstatistics. sity, avoiding the contribution of extended emission or spuri- For two objects (NGC 3557 and NGC 5419) we obtain a ous sources to the nuclear flux as well as revealing any radio detectionofanuclearpower-lawsourcebyfittingthespectrum structure.Bettermeasurements,fromVLBIdataorfromhigher using an absorbed power-law plus a thermal model, with the frequency/resolutionVLA data, are available for most CoreG hydrogencolumndensityfixedattheGalacticvalue.Detailsof (23outof29)andcompactcoresweredetectedinallbut2ob- theresultsaregiveninAppendixA.Fortheremaining3galax- jects.TheradiocorefluxesaretakenfromNagaretal.(2002) ies we set an upper limit to any nuclear emission, with the (15GHzVLAdataand5GHzVLBIdata),Filhoetal.(2002) conservative hypothesis that all flux that we measure is non- andKrajnovic´ &Jaffe(2002)(8.4GHzattheVLA),Jones& thermal. We then fit the spectrum with an absorbed (to the Wehrle(1997)(8.4GHzVLBIdata)andSleeetal.(1994)(PTI galacticvalue)powerlawmodelwithphotonindexΓ=2. 5GHzinterpolateddata). In Fig. 3 we compare the radio core flux density used TheX-rayluminositiesforallobjectsaregiveninTable2. in our analysis against observations made at higher resolu- tion.Overallthereisasubstantialagreementbetweenthetwo 3.3.Radionuclei datasets,withamediandifferenceofonly∼0.25dex(afactor ∼1.6), with only two objects substantially offset (by a factor The radio data available for all objects of our sample are of∼10).However,sincethesedataarehighlyinhomogeneous drawn from the surveys by Wrobel & Heeschen (1991) and andgiventhegeneralagreementwiththe5GHzVLAmeasure- 1 We define the spectral index α with the spectrum in the form ments,weprefertoretainthevaluesofWrobel&Heeschenand Fν ∝ν−α. Sadleretal. Nonetheless,we alwayscheckedthatusingthese B.BalmaverdeandA.Capetti:Aminiatureradio-galaxyinevery“core”galaxy? 103 Fig.4. Radiocoreluminosityfortheearly-typegalaxieswitha“core”profileversustheoptical(left)andX-ray(right)nuclearluminosities. higher resolution core fluxes our main results are not signifi- Table3.Correlationssummary. cantlyaffected(seeAppendixBforaspecificexample). Sample Var.A Var.B r Slope rms AB 4. Themultiwavelengthpropertiesofnuclei CoreG LO Lr 0.59 0.76±0.21 0.62 ofcoregalaxies LX Lr 0.78 1.36±0.20 0.59 LLRG L L 0.94 0.82±0.11 0.32 O r Having collectedthe multiwavelengthinformationforthe nu- L L 0.95 0.99±0.11 0.33 clei of our core galaxies we can compare the emission in the X r differentbands.Firstofall,wecanestimatetheratiobetween LLRG+CoreG LO Lr 0.90 0.89±0.07 0.56 L L 0.89 1.02±0.10 0.58 theradio,opticalandX-rayluminosities:themedianvaluesare X r Log(νL/νL ) ∼ −1.5(equivalenttoastandardradio-loudness r o parameterLogR ∼ 3.6)2 andLogR =Log(νL/L ) ∼ −1.3, X r X thesamebehaviourofthestrongerradiogalaxies,extendingit bothwithadispersionof∼0.5dex.Theseratiosareclearlyin- downwardby 3 ordersof magnitude in radio-coreluminosity dicativeofaradio-loudnatureforthesenucleiwhencompared astheyreachlevelsaslowasL ∼1036ergs−1. to both the traditional separation into radio-loud and radio- r We estimated the best linear fit for the combined quiet AGN (Log R = 1, e.g. Kellermannet al. 1994),as well CoreG/LLRGsampleinboththeL vs.L andL vs.L planes. as with the radio-loudnessthresholdintroducedby Terashima r o r X Thebestfitswerederivedasthebisectrixofthelinearfitsus- & Wilson(2003)basedontheX-raytoradioluminosityratio ing the two quantities as independentvariables following the (LogR = −4.5).Furthermore,thenuclearluminositiesinall X suggestion by Isobe et al. (1990) that this is preferable for three bands are clearly correlated (see Fig. 4 and Table 3 for problemsneedingsymmetricaltreatmentofthevariables.The asummaryoftheresultsofthestatisticalanalysis):thegener- presence of upper limits in the independentvariable suggests alized(includingthepresenceofupperlimits)Spearmanrank thatwe couldtakeadvantageofthemethodsofsurvivalanal- correlation coefficientρ is 0.63 and 0.89 for L vs. L and L r o r ysisproposedbye.g.Schmitt(1985).However,thedrawbacks vs. L respectively,withprobabilitiesthatthecorrelationsare X discussedbySadleretal.(1989)and,inourspecificcase,the notpresentofonly0.002and0.0001. non-randomdistributionofupperlimits,argueagainstthisap- Both results are reminiscent of what is observed for the proach.Wethereforepreferredtoexcludeupperlimitsfromthe radio-loud nuclei of low luminosity radio-galaxies (LLRG). linearregressionanalysis.Nonetheless,aposteriori,1)theob- Chiaberge et al. (1999)and Balmaverde & Capetti (2005)re- jects with an undetected nuclear component in the optical or ported on similar multiwavelength luminosity trends for the X-rayareconsistentwiththecorrelationdefinedbythedetec- sample of LLRG formed by the 3C sources with FR I mor- tionsonly;2)theapplicationoftheSchmidtmethodsprovides phology. The connection between the CoreG and LLRG be- correlation slopes that agree, within the errors, with our esti- comesmoreevidentif we addLLRG inthe diagnosticplanes mates. (see Fig. 5 and Table 4). The early-type core galaxies follow Weobtained(indicatingthePearsoncorrelationcoefficient 2 R=L5GHz/LB.AsinSect.3wetransformedtheopticalfluxesto withrandslopewithm)rro =0.90andmro =0.89±0.07,rrx = theBbandadoptinganopticalspectralindexα =1. 0.89 m = 1.02±0.10 for the radio/optical and radio/X-ray o rx 104 B.BalmaverdeandA.Capetti:Aminiatureradio-galaxyinevery“core”galaxy? Fig.5. Comparisonofradioandoptical(left)andX-ray(right)nuclearluminosityforthesampleofcore-galaxies(filledcircles)andforthe reference3C/FRIsampleoflowluminosityradio-galaxies(emptycircles).Thethreesourcesincommonaremarkedwithafilledsquare.The solidlinesreproducethebestlinearfits. correlations respectively. The slopes and normalizations their other properties, such as the structure of the host, black derived for CoreG, LLRG and the combined CoreG+LLRG holemass,radio-morphologyandopticalspectra. sample (see Table 3) are consistent within the errors and this Oursamplewasselectedtoincludeonlyearly-typegalax- indicates that there is no significant change in the behaviour ieswithacoreprofile,e.g.withanasymptoticslope(towardthe betweenthetwosamples.Onlythedispersionisslightlylarger nucleus)oftheirsurfacebrightnessprofilesγ < 0.3.Recently fortheCoreGnucleibeingafactorof∼4ratherthan∼2forthe de Ruiter et al. (2005) showed, from the analysis of a com- LLRGsamplealone. bined sample of B2 and 3C sources, that they are all hosted Chiaberge et al. (1999) first reported the presence of a byearly-typegalaxiesandthatthe presenceofa flat coreis a correlation between radio and optical emission in the LLRG characteristicofthehostgalaxiesofallnearbyradio-galaxies. and they concluded that this is most likely due to a com- A strong similarity between CoreG and LLRG emerges mon non-thermal jet origin for the radio and optical cores. when comparing the mass of their supermassive black holes. Recently Balmaverde & Capetti (2005) extended the analysis When no direct measurement(taken from the compilationby to the X-ray cores; the nuclear X-ray luminosity also corre- Marconi & Hunt 2003) was available, we estimated M us- BH lateswiththoseoftheradiocoresandwithamuchsmallerdis- ing the relationship with the stellar velocity dispersion (taken persion(∼0.3dex)whencomparedtosimilartrendsfoundfor fromtheLEDAdatabase)intheformgivenbyTremaineetal. otherclassesofAGN(seee.g.Falckeetal.1995),againpoint- (2002).Thedistributionsof M (seeFig. 6)of thetwo sam- BH ing to a common origin for the emission in the three bands. plesarealmostindistinguishable3,astheyhavemedianvalues Furthermore, the broad band spectral indices of the 3C/FR I of Log M = 8.54 and Log M = 8.70, for CoreG and BH BH cores are very similar to those measured in BL Lacs objects LLRG respectively,and they also cover the same range, with (for which a jet origin is well established) in accord with the mostobjectswithLogM =8−9.5. BH FR I/BL Lacs unified model (we will return to this issue in Furtherindicationsof the natureofCoreG coresandtheir Sect.7). connection with LLRG come from the emission lines in The core galaxies of our sample thus appear to smoothly their optical spectra. LLRG are characterized as a class by extendtheresultsobtainedforLLRGtomuchlowerradiolu- their LINER spectra (e.g. Lewis et al. 2003) and this is the minosity, expandingthe multiwavelenghtnuclear correlations case also for the CoreG of our sample. In the NED database, to a total of 6 orders of magnitude. This strongly argues in althoughabouthalf of the CoreG do nothave a spectral clas- favourofa jetoriginforthe nuclearemissionalso inthecore sification, 13 objects are classified as LINERs4. The only galaxies and that they simply represent the scaled down ver- sionsofthesealreadylowluminosityAGN. 3 The probability that the two samples are drawn from the same parentdistributionis0.32,accordingtotheKolmogorov-Smirnofftest. 4.1.Coregalaxiesvs.lowluminosityradio-galaxies 4 This result provides further support to the suggestion by Chiabergeetal.(2005)thatadualpopulationisassociatedwithgalax- The results presented above indicate that the nuclei of the ieswithaLINERspectrum,beingformedbybothradio-quietandby CoreGshowaverysimilarbehaviourtothoseofLLRG.Here radio-loudobjects.TheCoreGarepartofthislattersub-populationof we explorein more detail how CoreG and LLRG compare in radio-loudLINER. B.BalmaverdeandA.Capetti:Aminiatureradio-galaxyinevery“core”galaxy? 105 Table4.Radiogalaxiesofthe3C/FRIsampledata:(1)name,(2)intrinsicnuclearX-rayluminosity(2–10keV)[ergs−1],(3)nuclearoptical luminosity(8140Å)[ergs−1],(4)nuclearradioluminosity(5GHz)[ergs−1]and(5)totalradioluminosity(178MHz)[ergs−1]fromChiaberge etal.1999, (6)Hα+[NII]lineluminosityfromCapettietal.2005[ergs−1],(7)total K bandgalaxy’sabsolutemagnitudefrom2MASS,(8) logarithmofblackholemassinsolarunityderivedusingthevelocitydispertionorfromaMarconietal.(2003). Name LogLx LogνLo LogνLcore LogνLtot LogLHα+[NII] MK Log(MBH/M(cid:3)) 3C028 <41.60 <41.35 <38.86 42.32 – –26.06 – 3C029 – 41.32 40.25 41.01 40.40 –25.72 –8.11 3C031 40.60 40.89 39.40 40.21 39.89 –25.67 –8.70 3C066B 41.00 41.61 39.90 40.59 40.22 – – 3C075 <41.00 – 39.30 40.28 – – 8.84 3C076.1 – – – 40.64 – – 8.88 3C078 41.90 42.55 40.88 40.74 40.71 –26.24 8.61 3C083.1 40.95 40.2 39.10 40.75 39.26 – – 3C084 42.60 42.93 42.10 40.63 – –26.11 8.58 3C089 – <40.84 40.95 42.12 – –26.05 8.85 3C189 41.78 42.03 40.54 42.80 – – – 3C264 – 41.89 39.91 40.57 40.02 –25.09 8.67 3C270 40.95 39.71 39.22 41.27 39.54 –25.11 8.72a 3C272.1 39.30 39.26 38.57 40.10 38.56 –24.41 9.00a 3C274 40.30 40.71 39.90 41.78 39.15 -25.48 9.53a 3C277.3 – 41.30 39.93 41.35 40.51 –25.20 – 3C288 – 41.89 41.24 42.59 – – – 3C293 – – 40.29 40.94 – –25.44 8.14 3C296 41.30 40.53 39.62 40.39 – – 8.80 3C305 – – 39.68 40.96 – –25.45 8.07 3C310 – 41.26 40.35 41.74 – – 8.21 3C314.1 – <41.39 <39.14 41.71 – – – 3C315 – – 41.23 41.85 – – – 3C317 41.30 41.24 40.64 41.27 – –26.13 8.32 3C338 <40.48 41.22 39.96 41.16 40.54 – 8.92 3C346 43.30 43.03 41.74 41.99 41.41 –26.32 – 3C348 <42.30 41.53 40.36 43.45 – –26.40 – 3C424 – <41.64 40.44 41.88 – – – 3C433 – – 39.69 42.29 – – – 3C438 <42.60 <41.78 41.14 43.12 – –27.00 – 3C442 – 40.05 38.12 40.57 39.72 – – 3C449 <40.48 41.03 39.06 40.11 39.50 – 8.54 3C465 – 41.49 40.37 41.06 40.72 – 9.14 exceptionisUGC7203,withaSeyfertspectrum,butitsdiag- and 3C 274), while in the Southern sample we have the well nosticlineratiosareborderlinewiththoseofLINERs(Hoetal. studiedradio-galaxiesNGC1316(FornaxA), a FRII source, 1997).Concerningthe emission line luminosity,Capettietal. NGC5128(CenA)andIC4296.Aliteraturesearchshowsthat (2005) found a tight relationship between radio core and line at least another 11 sources have extended radio-structure in- luminositystudyinga groupofLLRG formedbythe 3C/FRI dicativeofacollimatedoutflow,althoughinseveralcasesthis complemented by the sample of 21 radio-bright (F > 150 canonlybeseen inhighresolutionVLBIimages,suchasthe r mJy) UGC galaxies defined by Noel-Storr et al. (2003). Line mas scale double-lobes in UGC 7760 or the one-sided jet of luminosityforourCoreGclearlyfollowthesametrenddefined UGC7386(Nagaretal.2002;Falckeetal.2000). by LLRG, althoughwith a substantiallylargerdispersion,not Conversely,hostsof3C/FRIradio-sourcesareonaverage unexpectedgiventheirlowlineluminosityandthenonunifor- more luminous than core-galaxies (see Fig. 6, left panel) al- mityofthedatausedforthisanalysis. thoughthere is a substantial overlapbetween the two groups: Considering the radio structure, several objects of our themedianvaluesareM =−24.8andM =−25.7forCoreG K K CoreG sample have a radio-morphologywith well developed and3C/FRIrespectively,withaKSprobabilityofonly0.003 jets and lobes: UGC 7360, UGC 7494 and UGC 7654 are of being drawn from the same population. This reflects the FR I radio-galaxiespart of the 3C sample (3C 270, 3C 272.1 well known trend, already noted by Auriemma et al. (1977), 106 B.BalmaverdeandA.Capetti:Aminiatureradio-galaxyinevery“core”galaxy? Fig.6. DistributionsforCoreG(shadedhistograms)andforLLRG(emptyhistogram)of(leftpanel)blackholemass M and(rightpanel) BH absolutemagnitudeM .TheLLRGhistogramshavebeenre-normalizedmultiplyingbyafactor29/19forM and29/17forM respectively, K BH K i.e.thenumberofobjectsinthetwosamplesforwhichestimatesoftheseparametersareavailable. for which a brighter galaxy has a higher probability of being astrongerradioemitter,andwhichispresentalsoinoursam- ple (Paper I). The selection of relatively bright radio sources, suchasthe3C/FRI,correspondsto abiastowardmorelumi- nous galaxies. Indeed, within our sample, imposing a thresh- oldintotalradio-luminosityofL > 1039 erg/s,5 thelowend tot forLLRG,decreasesthemedianmagnitudeto–25.1,incloser agreementwiththe3C/FRIvalue. We concludethatthepropertiesofourlowradioluminos- ity CoreG show a remarkable similarity to those of classical LLRG, in particular, they share the presence of a flat core in theirhost’sbrightnessprofiles,theyhavethesamedistribution inblackholemasses,aswellasanalogouspropertiesconcern- ing their optical emission lines and radio-morphology.These results indicate that core galaxies and LLRG can be consid- ered, from these differentpoint of view, as being drawn from the same population of early-typegalaxies. They can only be separatedon the basisof their differentlevelofnuclearactiv- ity,withtheLLRGformingthetipoftheicebergof(relatively) high luminosity objects. Furthermore, the emission processes associated to their activity scale almost linearly over 6 orders Fig.7. Emission line vs. radio core luminosity for CoreG galaxies (filledcircles)andfortheLLRG3C/FRIsample(emptycircles),from ofmagnitudeinallbandsforwhichdataareavailable. Capettietal.(2005). 5. Blackholemassandradioluminosity Theissueoftherelationshipbetweentheblackholemassand holeestimates,thattheradio-luminositytightlycorrelateswith theradio-luminosityhasbeendiscussedbyseveralauthors,tak- theblackholemass,withalogarithmicindexof∼2.5.Thisre- ing advantage of the recent possibility to measure (or at least sultwassubsequentlychallenged,bye.g.Ho(2002).We here estimate) M . Franceschiniet al. (1998)pioneeredthis field BH re-explore this issue limiting ourselves to the sample of core showing, from a compilation of objects with available black early-typegalaxies;whilethissubstantiallyrestrictstheacces- 5 The5GHzluminositywasconvertedto178MHzforconsistency sible range in MBH andit appliesonlyto radio-loudnuclei,it withthe3C/FRIvaluesadoptingaspectralindexof0.7. has the substantial advantage of performingthe analysis on a