Clumpy winds and the obscuration of Active Galactic Nuclei

The role of star-formation driven outflows in the obscuration of the central source in the Active Galactic Nuclei (AGN) is discussed. The outflow from a sub-parsec scale accretion disc is numerically modelled for parameters appropriate to the Galactic Cent

Mon.Not.R.Astron.Soc.000,000–000(0000)Printed5February2008

A(MNLTEXstyle lev2.2)

ClumpywindsandtheobscurationofActiveGalactic

Nuclei.

SergeiNayakshin1 andJorgeCuadra2

arXiv:astro-ph/0602366v1 16 Feb 2006

1Department

2Max-Planck-Institut

ofPhysics&Astronomy,UniversityofLeicester,Leicester,LE17RH,UK

f¨urAstrophysik,Karl-Schwarzschild-Straße1,85741GarchingbeiM¨unchen,Germany

5February2008

ABSTRACT

Theroleofstar-formationdrivenout owsintheobscurationofthecentralsource

intheActiveGalacticNuclei(AGN)isdiscussed.Theout owfromasub-parsecscaleaccretiondiscisnumericallymodelledforparametersappropriatetotheGalacticCentre.Theresultingobscurationpatternisverypatchy,withsomelinesofsightbecomingopticallythicktoThomsonscattering.A xedobserverwouldseecolumndepthchangingbyfactorsofmanyovertimescalesofordermonthstohundredsofyears,dependingonthephysicalsizeoftheout owregion.SuchwindsmayberelevantforobscurationofsomeAGNandespecially“changinglookAGN”.However,averagedovertheskyasseenfromthecentralsource,thesewindsarealwaysopticallythinunlesswindout owratesaresuper-Eddington.Asimplescalingargumentshowsthatthisistruenotonlyforstellar-drivenwindsbutforanyAGNwinds.WethereforeconcludethatAGNwindsareunabletoaccountforthevastmajorityofopticallythickobscuredAGN(asigni cantfractionofallAGN).WesuggestthatthemostlikelysourceofopticallythickobscurationinAGNisawarpedparsecscaleaccretiondisc.

Keywords:Galaxy:centre–accretion:accretiondiscs–galaxies:active–methods:numerical

1INTRODUCTION

AccordingtothesimplestversionoftheUni edmodelofActiveGalacticNuclei(AGN),thecentralsourcehasfunda-mentallythesamepropertiesinallclassesofAGN(e.g.,An-tonucci&Miller,1985;Antonucci,1993;Maiolino&Rieke,1995;Risalitietal.,1999;Sazonov&Revnivtsev,2004).ItistheorientationofourlineofsightwithrespecttothedustyobscuringenvironmentofanAGNthatmakesitappearasatypeI(relativelyun-obscured)ortypeIIobject(obscured).Despiteofalltheconvincingobservationalevidencefortheexistenceoftheabsorber,mostfrequentlythoughttobear-rangedinashapeofatorus,noconvincingtheoreticalmodelhaseverbeenproducedtoexplainitsproperties.Recently,Nenkovaetal.(2002)andRisalitietal.(2002)demonstratedthattocomplywiththesmallobservedtorussizes,theab-sorbermustnotbeuniformindensitybutshouldberatherclumpy,i.e.consistingofinpidualsmallerclouds.Theoret-icalmodelsofclumpytori(e.g.,Krolik&Begelman,1986,1988;Vollmeretal.,2004),however,haveallthesamefun-

E-mail:Sergei.Nayakshinatastro.le.ac.uk

damentaldi culty.Astheobservationsrequirethetorustobegeometricallythick,thevelocitydispersionofthecolddustycloudsmustbeoftheorderoftheirorbitalvelocityintheinnerparsecofagalacticcentre.Opticallythicktoriwouldthenhavecloudscollidingatsuper-sonicspeedsmanytimesduringonedynamicaltime(Krolik&Begelman,1986,1988).Therefore,thesemodelshavetopostulate(somewhatarbitrarily,inourpointofview),thatthecloudscansurvivethesecollisions,behavinginessenceasrubberballs.

Recently,anumberofauthorsarguedthattheobscu-rationmustinsteadcomefromclumpywinds(e.g.,K¨onigl&Kartje,1994;Kartjeetal.,1999;Elvis,2000;Elvisetal.,2004;Elitzur,2005)emanatingfromthesurfaceofanac-cretiondisk,bethesewindsdrivenbylinepressure(Proga,2003),continuumradiationpressure,heating,orbyhydro-magneticforces.Inthispaperwewishtocriticallyassessthissuggestion.Inparticular,weconsiderAGNwindsdrivenbystarformationactivityinanaccretiondisc.StarorplanetformationinAGNdiscshasbeenlongpredictedbytheo-rists(Paczy´nski,1978;Kolykhalov&Sunyaev,1980;Lin&Pringle,1987;Collin&Zahn,1999;Gammie,2001;Good-man,2003),andrecentobservationsoftheyoungmassivestarsinthecentreofourGalaxy(e.g.,Genzeletal.,2003;

The role of star-formation driven outflows in the obscuration of the central source in the Active Galactic Nuclei (AGN) is discussed. The outflow from a sub-parsec scale accretion disc is numerically modelled for parameters appropriate to the Galactic Cent

2S.NayakshinandJ.Cuadra

Ghezetal.,2003)lendaverystrongsupporttothesethe-ories(Levin&Beloborodov,2003;Nayakshin&Sunyaev,2005;Paumardetal.,2006).UsingtheSPHtreatmentofstellarwindsdevelopedbyCuadraetal.(2005,2006),wecalculatetheobscuringpropertiesofsuchwindsforaspe-ci cexample.We ndthatthesewindshaveveryirregular,patchyobscurationpatterns.A xedobservershouldthenseethecolumndepththroughthewindvaryingontimescalesfrommonthsoryearstotensandhundredsofyears,dependingonthephysicalsizeofthewindlaunchingregionandtheSMBHmass.

However,observations(see§3below)suggestthatthestructureresponsibleforAGNobscurationshouldbeopti-callythickoveralarge(e.g.,athird)fractionofthewholesky.Ourmodelscannotachievesuchahighaveragelineofsightcolumndepthunlessthemasslossrateinthewindissuper-Eddington.Infact,weshowviasimpleanalyticalesti-matesthatthisconclusionisvalidwhateverthenatureofthewinddrivingmechanismis.Wefeelitwouldbeveryhardtoproducehighlysuper-Eddingtonwindsinaplausiblequasisteady-stateAGNmodelforatypical(verysub-Eddingtonluminosity)AGN.Thuswesuggestthatwinds,whileveryimportantfortheAGNphenomenon,cannotreplacethe“torus”initsroleintheuni cationschemesofAGN.Ei-therthetorusdoesexistdespitethedi cultiesfacedbythemodels,oranotheropticallythickstructure,e.g.,awarpedaccretiondiscplaysitsroleinAGN.

2STELLARWINDSFROMAGNDISCS

AGNdiscsareexpectedtobemassiveandthusself-gravitatingatlargeenoughdistances(R～>

0.01 0.1pc)fromthesuper-massiveblackholes(SMBH).Ifthedisccoolingtimeisshort,gravitationalcollapseandformationofstarsorevenplanetsispredicted(e.g.,Paczy´nski,1978;Kolykhalov&Sunyaev,1980;Shlosman&Begelman,1989;Gammie,2001).YoungmassivestarsinthecentralparsecofourownGalaxyweremostlikelycreatedinthisway(Paumardetal.,2006),withanapparentsigni cantover-abundanceofhighmassstars(Nayakshin&Sunyaev,2005)overtheirfractionin“normalgalactic”starformation.Asstarsarebornfromtheaccretiondisc,theylaunchpowerfulradiation eldsandstellarwinds,someofwhichwillbreakthroughthedisc.Lowmassproto-starsmayproveequallye ectiveinlaunchingwindsinthesecircumstancesastheratesatwhichgasiscapturedfromthediskintotheHill(orcapture)zonesoftheprotostarsaresuper-Eddington(Nayakshin,2006).Inaddition,whenthegasinthediskisdepletedtoabouthalfitsinitialsurfacedensity,stellarvelocitydispersionstartstogrow,andsoonthestellardiscbecomesgeometricallythickerthanthegasdisc(Nayakshin,2006).Stellarwindsthenescapefromabovethediscdirectly.

Weattempttosimulatethiscomplexsituationinasim-pli edsetting,concentratingonlyonthewindpartofit.Forthisreasonwedonotincludethegasinthenegligiblythinand ataccretiondiscfromwhichthestarsareborn.Tocaptureadegreeoftheexpectedpersityofthestellarpop-ulationsandconditionsinthisproblem,thestarsarepidedintotwogroups.The rstproduceswindswithvelocitiesvw=300km/sec,whereasthesecondhasvw=700km/sec.Bothtypesofstarshavemasslossratesof2.5×10 4M⊙

year 1,andaresituatedina atcircularlyrotatingKep-leriandiskofgeometricalthicknessH(R)=0.1R(theunitoflengthusedhereis1arcsecondatthedistanceof8kpc,whichisabout1.2×1017cmor0.04pc).Intotal,wehave200massshreddingstars,thusamountingtothemasslossrateof0.1M⊙year 1,whichisafactoroffewsuper-EddingtonforaSMBHmassofMBH=3.5×106M⊙.ThediscinnerandouterradiiareRin=1.5andRout=8,respectively.ThestellarsurfacedensityfollowsthelawΣ (R)∝R 1,andthestarsareintheKepleriancircularrotationaroundtheSMBH.Theinitialphase(i.e.theφ-coordinate)ofthestarsinthediskisgeneratedrandomly,asistheverticalco-ordinatewithinthe Hto+Hlimits.Initially,thecompu-tationaldomainis lledwithhottenuousgaswithvelocitygreaterthantheescapevelocity.Thisgasquicklyout owsfromtheregion.Attheendofthesimulationstellarwindsexceedtheinitialgasmassbyafactoroffewtens.

Toperformsimulations,weusetheGadget-2code(Springel,2005)modi edtomodelstellarwindsasnewSPHparticlesejectedfromthestellarwindsources.Thesemeth-odsweredescribedandthoroughlytestedinCuadraetal.(2005,2006).

AsnapshotofthesimulationisshowninFigure1attimet≈2200yearsafterthebeginningofthesimulation,whichcorrespondsto～36dynamicaltimesatR=1.Whilethereisnotruesteadystateinthis nitenumberofmovingstarssystem,thesnapshotisfairlytypicalofthemorphologyofthestellarwind.Theface-onview(leftpanel)showsthatsomeoftheshockedwindmanagedtocooldownandformedasmall-scaledisc.Theinnerpartofthesimulationdomainisthemostlikelyplaceforthedisctoformsincestellarwinddensityishighestthere,leadingtoshocks,thermalizationandrapidcooling;thefrequentcollisionsofgaseousclumps“waste”momentumofthewinds,and nally,theescapevelocityfromtheregionisaround600km/sec,i.e.higherthanwindvelocitiesoftheslowerwinds.Withtimethediscgrowsradiallytobothsmallerandlargerradii.However,theformationofthedisc(cf.Cuadraetal.,2006)isphysicallysigni cantonlyinthecasewhenthegaseousaccretiondiscfromwhichthestarswerebornisentirelyabsent,asisthecasepresentlyintheGalacticCentre.Intheoppositecasethecooleddiscandthe“spiralarms”seenatintermediateradiiwouldsimplyblendinwiththemuchmoremassiveunderlyinggasaccretiondisc.

Contrarytothefrequentcollisionsandhighescapeve-locityintheinnerregionofthecomputationaldomain,theconditionsintheouterregionsallowdirectescapeofboththefasterdi useandtheslowercoolerclumpywinds.Duetothe nalextentofthestellardiscandtheprojectionef-fect,thewindmorphologyremindsand“X”-shape(Figure1,rightpanel).Theedge-onview(rightpanel)ofthestellar

discshowsthattheplaneofthegasdiscatRtiltedwithrespecttotheorbitalplaneof～<

2isveryslightlythestellardisc.Thisisanartefactoftheinitialconditions.

TheleftpanelofFigure2showstheobscuringcolumndepthofthewindsasseenfromtheSMBH.Sincetheorien-tationofthetiltedinnerdiscisin uencedbytheinitialcon-ditions,weeliminateitsobscuration,onlyplottinggaswith

radialdistancesR～>

1.6fromtheSMBH,whichexcludesallofthedisc.Besides,hadweincludedinthesimulationsthemoremassivegasaccretiondiskwhosemidplanewouldco-incidewiththatofthestars,theinnerdiscseeninFigure

The role of star-formation driven outflows in the obscuration of the central source in the Active Galactic Nuclei (AGN) is discussed. The outflow from a sub-parsec scale accretion disc is numerically modelled for parameters appropriate to the Galactic Cent

ClumpywindsandtheobscurationofActiveGalacticNuclei.

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-1.0-0.50.0-3.0-2.5

-2.0-1.5log Σ [g cm-2]

-1.0-0.50.0

Figure1.Face-on(left)andside(right)viewsofthesimulationdomainforthe atstellardisccon guration.Redasterisksshowthelocationofthewind-producingstarsusedinthesimulation(thered“dot”inthecentreoftheleftpanelisnotastar,butalowcolumndepthregioncentredontheSMBH).

1wouldhavecompletelyblendedinwiththeaccretiondiscandwouldnotpresentmuchobscurationatall.

Notetheveryirregularpatchystructureofthe(yel-low)opticallythickerregions.Thecontrastbetweenthoseandneighbouringlessdensepatchesofskyisfrequentlyafactorof10ormore.Sincethewindsarerotatingatafractionofthelocalangularfrequency, ,thisimpliesthatthecolumndepthsampledbytheobserverwillvaryontimescalesasshortas～10 3to10 2of1/ ,i.e.

1/2

.Thedottedpat-tvar～0.04 0.4year(R/0.1pc)3/2M8

ternatthecosθ=0planearethedenseregionsofstellarwindsimmediatelynexttothestars.

TherightpanelofFigure2showstheisotropicmasslossratealongthelineofsight,de nedas˙≡M

dΣ4πR2ρvR

2

2πRt

,(2)

whereRtisthetypicalradiusofthetorus,andweassumedthatitcoversroughly2πoftheskyasseenfromtheSMBH.Now,assumethatthetorusismadeupofcloudsthatformintheout owingdiskwind,asinthesimulationspresentedinthispaper.Thetotalmassout owrateofthecloudsisthen

The role of star-formation driven outflows in the obscuration of the central source in the Active Galactic Nuclei (AGN) is discussed. The outflow from a sub-parsec scale accretion disc is numerically modelled for parameters appropriate to the Galactic Cent

4S.NayakshinandJ.

Cuadra

1.0

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0.5

Mass loss rate

1.01.5

Figure2.Columndepththroughthewind(left)andisotropicmassout owrate(right;inunitsofM⊙year 1)forthesimulationshowninFig.1.Notethelargevariationsoftheobscuringcolumndepthoversmallangularscales.ThegaseousinnerdiscseenintheleftpanelofFigure1hasbeenexcludedfromtheseFigures.

˙wind～M2

2πRtNaΣc K

～15

M⊙

1pc

1/2

M8

1/2

,(3)

whereM8istheSMBHmassinunitsof108M⊙,andwe

assumedthatcloudsradialvelocitiesareoftheorderofthelocalKeplerianspeed.Now,theEddingtonaccretionrateis4πGMBHmp/ cσT≈2M⊙year 1M8.Thusthere-quiredwindmasslossrate(equation3)isanorderofmag-nitudehigher,typically,thantheEddingtonaccretionrateforaM8～1objectandaparsec-scaletorus.Theestimate

˙windcouldbereducedsomewhatbypostulatingevenofM

smallertorussizes,butitishardtomakeRtmuchsmallerthan～0.1pcasthenthedustwouldbesublimatedbytheAGNradiation eld(e.g.,eq.3.2inEmmeringetal.,1992).However,weratherthinkthatestimate3istoooptimistic,e.g.thatanevenhighermassout owratesareneeded,astheobservationsandmodellingofopticallythicksourcesre-quireaminimumabsorbingcolumndepth,andhenceinagoodfractionofsourcesNaΣcmayactuallybemuchhigherthan1gcm 2.

Consideringaspeci ccaseofthelocalobscuredAGNstudiedby(Guainazzietal.,2005),wenotethatthebolo-metricluminositiesoftheseobjectsintheinfrared,X-rayandopticalbandsareintherangeL～1043 few×1044erg/sec,whichimpliesSMBHaccretionratesof“only”～0.01M⊙year 1forthestandardradiativee ciency.Henceiftheobscurationoftheopticallythickobjectsinthatsam-plewereprovidedbythewinds,wewouldconcludethattheSMBHaccretionprocessmustbeverywasteful,with～100 10,000timesmoremass owingoutoftheinnerparsecthanaccretingontheSMBH.(Notethatthesemod-eratelybrightAGNarenotlikelytobeinthenon-radiative

accretion owregimewhenvigorousout owsareinfactex-pected,e.g.,Blandford&Begelman,1999)Itwouldalsore-quireaveryhighmassin uxintotheinnerparsectosustainsuchwinds.Giventhedi cultyofdeliveringenoughfueltotheSMBHsevenintheearliergas-richepochs(Thompsonetal.,2005),itishardtoseehowsuchhighmassin uxescouldbemaintained.

4DISCUSSIONANDCONCLUSIONS

Accretingblackholescandrivestrongout owsviaX-rayheating,lineorcontinuousradiationpressure,andhy-dromagneticforces(e.g.,Begelmanetal.,1983;K¨onigl&Kartje,1994;Kartjeetal.,1999;Proga,2003).Starforma-tionontheoutskirtsofcoolmassiveaccretiondiscswillalsoresultinwindsdrivenbyout owsfromtheyoungmassivestars(Cuadraetal.,2005).Herewestudiedtheobscura-tionpropertiesoftheAGNout owsofthelasttype.Wefoundthattheseout owsarequiteclumpy,withthelinesofsightpassingthroughtheclumpsbecomingmoderatelyopticallythicktoThomsonscatteringforhighenoughwindmasslossrates.Suchout owsareboundtoplayaroleintheobscurationofAGN.

Wehoweverfeelthatthisrolecannotbedominantastheskyaveragedcolumndepthissigni cantlysmallerthantheonerequiredbythecurrent“torus”models(e.g.,Nenkovaetal.,2002;Elitzur,2005)andobservations(Guainazzietal.,2005)forrealisticmassout owrates,parabletotheaccretionratesontotheSMBH,whicharelikelytobeonly0.01 0.1oftheEddingtonaccretionratesinthelocalAGN(andaremuchsmallerifwealso

The role of star-formation driven outflows in the obscuration of the central source in the Active Galactic Nuclei (AGN) is discussed. The outflow from a sub-parsec scale accretion disc is numerically modelled for parameters appropriate to the Galactic Cent

ClumpywindsandtheobscurationofActiveGalacticNuclei.

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Figure3.SameastheleftpanelofFigure2butforslightlydi erentinitialconditionsandwiththecoldtiltedandwarpedinnerdiscincludedintheFigure.Noticethatthisverylight(only～3M⊙)discisaverye cientabsorber.

considerLowLuminosityAGN;see,forexample,Fig.1.9inHo,2005).WethusbelievethatthesearchfortheculpritoftheopticallythickobscurationofAGNshouldstillgoon.

Oneratherobviouscandidateisawarpedaccretiondisc.Thediscmaybesigni cantlywarpedbyinstabilitiesduetobackreactiontomassout oworduetotheAGNradiationpressure,oritmaybewarpedduetoprecessioninanon-axisymmetricgravitationalpotential(e.g.Schandl&Meyer,1994;Pringle,1996;Nayakshin,2005).Finally,thereisnoreasonforaninitialdisccon gurationtobe atifthemassdepositionismuchshorterthanthediscviscoustime,whichwasprobablythecaseintheGalacticCentre(Nayakshin&Cuadra,2005;Paumardetal.,2006).Asimulationranbyusforthepresentpaperandlaterrejected(becausetheinitialmassofthegasturnedouttoohighsothatitin uencedtheoutcomeofthesimulation)iscuriouslyusefulinthisrespect.Figure3showstheobscurationpatternofthissimulationinthesamewayasintheleftpanelinFigure2,butincludingthetiltedandslightlywarpedgasdisc.Exceptfortheinitialconditions,thissimulationisexactlyidenticaltotheoneshowninFigures1&2.AsFigure3shows,theobscurationprovidedbythewindsliterallypalesincomparisonwiththatproducedbythewarpeddisc,whosemassisonly～3M⊙.Anaccretiondiscisalonglivedstructurethatcanaccumulatealotofgas(stillasmallfractionoftheSMBHmass,however)andcanbeThomson-thickouttoparsecdistances.Obscurationbyawarpedaccretiondiscrequiresnocontinuousenergyinput,andwouldworkforloworhighluminosityAGNaslongasthediscsaresu cientlymassiveandstronglywarped.

TheauthorsacknowledgediscussionsandcommentsfromKenPoundsandSergeiSazonov.

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