Circumnuclear HI disks in radio galaxies The case of Cen A and B2 0258+35

New HI observations of the nearby radio-loud galaxies Centaurus A and B2 0258+35 show broad absorption (Delta_v=400km/s) against the unresolved nuclei. Both sources belong to the cases where blue- and redshifted absorption is observed at the same time. In

Mem.S.A.It.Vol.1,1

cSAIt2008 Memoriedella

C.Struve1,2,R.Morganti1,2andT.A.Oosterloo1,2

arXiv:0807.4476v1 [astro-ph] 28 Jul 200812NetherlandsFoundationforResearchinAstronomy,Postbus2,7990AA,Dwingeloo,TheNetherlandsKapteynInstitute,UniversityofGroningen,Landleven12,9747AD,Groningen,TheNetherlandse-mail:struve@astron.nlNewHiobservationsofthenearbyradio-loudgalaxiesCentaurusAandB20258+35showbroadabsorption( vabsorp～400kms 1)againsttheunresolvednu-clei.Bothsourcesbelongtothecaseswhereblue-andredshiftedabsorptionisobservedatthesametime.InpreviousCenAobservationsonlyarelativenarrowrangeofredshiftedabsorptionwasdetected.Weshowthatthedatasuggestinbothcasestheexistenceofacir-cumnucleardisk.ForCenAthenuclearabsorptionmightbetheatomiccounterpartofthemolecularcircumnucleardiskthatisseeninCOandH2.HigherresolutionobservationsarenowneededtolocatetheabsorptionandtofurtherinvestigatethestructureandkinematicsofthecentralregionoftheAGNandthewaytheAGNarefueled.Abstract.Keywords.galaxies:active–galaxies:ISM–galaxies:individual:CentaurusAandB20258+35–galaxies:kinematicsanddynamics1.IntroductionThecentralregionofAGNandthewayAGNarefueledareimportanttopicsincurrentextra-galacticastronomy.MergershavealwaysbeenconsideredimportantforthetriggeringofAGNbyprovidingthemechanismthatcouldbringgastothecentralregionsandfueltheblackhole(e.g.Hibbard&vanGorkom1996;Mihos&HernquistWhilethisseemstobelikelyinpowerful

radiogalaxies,recentobservationsshowthat

otherAGN-e.g.low-powerradiogalaxies-

existwherethereisnoevidenceforaccretion

throughmergers.Theactivityinthesegalaxies

appearstobeassociatedwiththeslowaccre-tionofgasfromtheISM/IGM(Best2005;Balmaverde2008).ApowerfultooltolearnmoreaboutthestructureofthecentralregionsofAGNandthefuelingmechanismisthestudyofthedistributionandkinematicsofthegas.InordertoperformdetailedstudiesofsingleobjectswehaveselectedtwoHi-richradio-loudgalaxies,themergerremnantCentaurusAandB20258+35(NGC1167).Thelaterisunlikelytobearecentmergerbuthasayoungradiosourcesuggestingthat“coldaccretion”fromtheIGMplaysthedominantrolebyprovidingthefuelfortheAGNactivity(seeSect.3).Neutralhydrogenisanidealtracerofac-cretion,interactionandmergingevents.

New HI observations of the nearby radio-loud galaxies Centaurus A and B2 0258+35 show broad absorption (Delta_v=400km/s) against the unresolved nuclei. Both sources belong to the cases where blue- and redshifted absorption is observed at the same time. In

2Struveetal.:CircumnuclearHidisksinradio

galaxies

observations(Morgantietal.2008).Thetotalabsorptionwidthrangesfrom400to800kms 1.Thearrowindicatesthepreviouslyknown(redshifted)absorption.Theverticallinegivesthesystemicvelocity.Rightpanel:Position-velocityplotoftheHi(grey-scaleandthincontours)andsuperimposedtheCOemission(thickcontours;fromLiszt(2001),takenalongpositionangle139 .NotethattheCOobservationsdonotextendbeyondaradiusofabout1arcmin.FiguretakenfromMorgantietal.(2008).Fig.1.CentaurusA:Leftpanel:HiabsorptionagainsttheunresolvednucleusasderivedfromtheATCAHiinabsorptionhasbeenoftendetected

inradiosources(seee.g.Conway1997;

Vermeulenetal.2003;Morgantietal.2005).

Theseabsorptionstructureshaveshownava-

rietyofcharacteristics.Hiabsorptionpro les

onlyredshifted(relativetothesystemicveloc-

ity),wereinitiallyfoundinanumberofradio

galaxiesandinterpretedasgascloudsthatare

fallingintothenucleus,possiblyindicating

accretionofgaswhichdeliverthefuelforthe

AGN(e.g.vanGorkometal.1989).However,

moresensitiveandbroaderbandobservations

haveshownthatthepictureismorecompli-

catedandthatblueshiftedabsorptionoccurs

evenmoreoftenthanredshiftedabsorption

(e.g.Vermeulenetal.2003;Morgantietal.

2005).

Inothercases,theHiabsorptioniscenteredon

thesystemicvelocityofthegalaxyandthese

caseshavebeenofteninterpretedasHiassoci-

atedwithacircumnucleardisk(ortorus),see

e.g.Conway(1997),Peck&Taylor(2001).

Supportforthepictureofacircumnuclear

diskcomesfromtheoreticalworkthatindeed

predicttheexistenceofsuchcircumnuclear

Hidisksaround(active)blackholes(seee.g.

Maloneyetal.1996;Loeb2008).Thus,the

pictureismorecomplicatedandthequestions

concerningthecentralstructureandfueling

remains.2.CentaurusATheprimegalaxytoinvestigatethesequestionsisCentaurusA,theclosestradio-loudgalaxyatadistanceofonly3.5Mpc.ObservationshaveshownredshiftedHiabsorptionagainsttheunresolvednucleus(e.g.vanderHulstetal.1983;Sarmaetal.2002).Thishasbeeninterpretedasgascloudsclosetothenucleuswhicharefallingin,feedingtheblackhole.However,ournewATCAobservations(Morgantietal.2008;Struveetal.inprep.)revealedforthe rsttimeblueshiftedabsorp-tion(w.r.t.thesystemicvelocity).Moreover,thedatashowthatthenuclearredshiftedab-sorptionissigni cantlybroaderthanreportedbefore,seeFig.1.Thetotalabsorptionwidthis400kms 1.Thelarge-scalegasanddustdiskhasacomplicatedwarpedstructurewhichisclosetoedge-on.ThankstoacarefulmodelingofthewarpeddiskofHiseeninemission(andpartlyinabsorption)wehaveshownthatthenuclearabsorptionmustarisemainlyinthevicinityofthenucleus(Struveetal.inprep.).Thisraisesthequestionofwhatthestructureinthevicinityoftheblackholeisandwhichmechanismfuelstheblackhole.Thefactthatred-andblueshiftedabsorptionisobservedatthesametimesuggeststhat

theHiabsorptionisnot,aswaspreviously

New HI observations of the nearby radio-loud galaxies Centaurus A and B2 0258+35 show broad absorption (Delta_v=400km/s) against the unresolved nuclei. Both sources belong to the cases where blue- and redshifted absorption is observed at the same time. In

Struveetal.:CircumnuclearHidisksinradiogalaxies

3

Fig.2.B20259+35:Leftpanel:HiabsorptionagainsttheunresolvednucleusasderivedfromthenewWSRTobservations(Struveetal.inprep.).Thetotalabsorptionwidthislargerthan400kms 1.Theverticallineindicatesthesystemicvelocity.Rightpanel:Position-velocityplotoftheHi(grey-scaleandcontours)takenalongthemajoraxis(PA=75 ).

claimed,evidenceofgasinfallintotheAGN,

butinsteadisduetoacold,circumnuclearrotatingdisk,similartothatseeninCO(Fig.1,Liszt2001)andatotherwavelengths(e.g.

Neumayeretal.2008).However,theabsorp-

tionislimitedbythebackgroundstructure.

VLBIobservations,whichhavemuchhigher

angularresolutionthantheATCAdata(beam

size～6′′),revealedsigni cantstructureon

milli-arcsecondscale:anunresolvednucleus,

ajetandacounterjet(e.g.Jonesetal.1996;

Tingay&Murphy2001).Thenucleusis

onlyvisibleathighfrequencies,whileatlow

frequencies(2.3GHzJonesetal.1996)the

VLBInucleusisobscured,suggestingthatthe

spectrumisinvertedandthenucleusisalso

invisibleat1.4GHz.Thus,theabsorption

observedwiththeATCAmustarisefroman

extendedstructure.

Thenucleardiskdetectedinmolecularand

ionized-gaslines(seeref.inMorgantietal.

2008)hasamajoraxisthatisroughlyper-

pendiculartothejetaxis.Therefore-ifthe

Hiislocatedinarotatingdiskandifthe

absorptionarisesagainstthejet/counter-jet

-onlyvelocitiesclosetothesystemicare

expected,giventhesmallopeningangleof

thejet.Thus,itwouldbehardtoexplainthe

largevelocitywidth.Theabsorptionislikely

tobeoccurringelsewhere.Interestingly,extra

ux(>50mJy)isclearlypresentontheshort

baselinesinthe1.4GHzVLBIexperiments(Tingay&Murphy2001),indicatingthepresenceofcontinuumstructuresthataretoolargetobeproperlyimaged.Therefore,theabsorptioncould(atleastpartly)alsoariseinfrontofthisdi usecomponent,explainingthefullabsorptionwidthobservedinthecaseofacircumnuclearHidisk.3.B20258+35B20258+35isayoungradiosource(～106yr,Girolettietal.2005)withatwo-lobesstructureextendedabout1kpc.Thestellarpopulationcontainsoldplusintermediateage(>4Gyr)stars,butdoesnothaveayoungcomponent(Emonts2006),suggestingthatthehostgalaxyNGC1167isnotarecentmerger.Therefore,theradiosourceisunlikelytobetriggeredbyamergingevent.Alarge(～160kpc),regularlyrotatingHidisk(con-tainingmorethan1010M⊙ofHi)hasbeenobservedwiththeWSRT(Emonts2006).NewWSRTobservationsshowthepresenceofhalogaswhichisbeingaccretedfromtheIGM(Struveetal.inprep.).Thelarge,regularrotatingHidisk(whichisnotwarped)furthercon rmsthatNGC1167isanoldmergerbutthepresenceofhalogassuggeststhat“coldaccretion”fromIGMmaybeoccurring,andcouldperhapsberelatedtothetriggeringof

theAGN.

New HI observations of the nearby radio-loud galaxies Centaurus A and B2 0258+35 show broad absorption (Delta_v=400km/s) against the unresolved nuclei. Both sources belong to the cases where blue- and redshifted absorption is observed at the same time. In

4Struveetal.:CircumnuclearHidisksinradiogalaxiesAgainsttheradiosource(unresolvedin

theWSRTobservations)fairlysymmetric

blue-andredshiftedabsorptionisfound, vabsorp>400kms 1(Fig.2,Struveetal.in

prep.).UnlikeforCenA,thecontinuumstruc-

tureofB20258+35isextendedaroundthe

nucleusonthearcsecondscaleinalldirections

andnotonlyalongthejetaxis(Girolettietal.

2005).Hence,thedatasuggeststhatthe

absorptioniscausedbyacold,circumnuclear

disk.

4.ConclusionsandOutlook

Usingnew,broadbandobservationsofCenA

andB20258+35wehaveshownthepresence

ofblue-andredshiftedHiabsorptionagainst

theunresolvednuclei.Bothgalaxiesbelong

tothegroupofradio-loudgalaxieswhere

blue-andredshiftedabsorptionisobserved

atthesametime.Previously,forCenAonly

anarrowrangeofredshiftedabsorptionwas

detected.

Wehaveshown(Morgantietal.2008)that

theHiabsorptioninCenAisnotevidence

ofgasinfallintotheAGN,butinsteadisdue

toacold,circumnuclearrotatingdisk.The

absorptionpro leoftheWSRTdatasuggests

thesameinterpretationforB20258+35.Both

observationsareinagreementwiththeoretical

workwhichpredictstheexistenceofsuch

circumnucleardisks.However,theseresults

leavethefuelingquestionopen.

Sensitive,higherresolutionobservations

(VLBIforCenAandVLAA-arrayfor

B20258+35)arenowneeded(andproposed)

todetectandlocatetheabsorptioninorder

tofurtherexplorethecharacteristicsofthe

nuclearHiandtotestthehypothesesof

circumnucleardisks.

Bothsourcesareuniqueobjects.CenAis

theclosestAGNandthereforethelinear

scaleismuchbetterthanforanyotherradio-

loudsource.Hence,theinnerstructureand

fuelingmechanismcanbestudiedingreat

detail.Becausethebackgroundstructureof

B20258+35isextendedaroundalldirections

oftheblackhole,higherspatialresolution

datawillallowtoinvestigatethecomplete2-DkinematicsinthevicinityoftheAGN.Forbothsourcesitwillbeinterestingtoinvestigatethepresenceofvelocitygradients(whichmightbeevidenceforcircumnucleardisks)aswellasradialmotions(infall/out ow?).Acknowledgements.ThisresearchwassupportedbytheEUFramework6MarieCurieEarlyStageTrainingprogrammeundercontractnumberMEST-CT-2005-19669”ESTRELA”.TheAustraliaTelescopeisfundedbytheCommonwealthofAustraliaforoperationasaNationalFacilityman-agedbyCSIRO.TheWSRTisoperatedbytheNetherlandsFoundationforResearchinAstronomywithsupportfromtheNetherlandsFoundationforScienti cResearch.ReferencesBalmaverde,B.2008,A&A,486,119Barnes,J.E.2002,MNRAS,333,481Best,P.N.2005,MNRAS,362,25Conway,J.1997,Proc.of“High-SensitivityRadioAstronomy,p.153Emonts,B.H.C.2006,PhDthesis,Univ.ofGroningenGiroletti,M.,etal.2005,A&A,441,89Hibbard,J.E.,vanGorkom,J.H.1996,AJ,111,655Jones,D.L.,etal.1996,ApJ,466,L63Liszt,H.,2001,A&A,371,865Loeb,A.,2008,JCAP,5,8Maloney,P.R.,etal.1996,ApJ,466,561Mihos,J.C.,Hernquist,L.1996,ApJ,464,641Morganti,R.,etal.2005,A&A,444,L9Morganti,R.,etal.2008,A&A,485,L5Neumayer,N.,etal.2008,ApJ,671,1329Peck,A.B.,Taylor,G.B.2001,ApJ,554,147Sarma,A.P.,etal.2002,ApJ,564,696Tingay,S.J.,Murphy,D.M.,2001,ApJ,549,L55vanGorkom,J.,etal.1989,AJ,97,708vanderHulst,J.M.,etal.1983,ApJ,264,L37Vermeulen,R.,etal.2003,A&A,404,861

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