Astronomy&Astrophysicsmanuscriptno.25496_hk_2 (cid:13)cESO2015 January21,2015 LettertotheEditor Mass without radiation: Heavily obscured AGNs, the X–ray background, and the black hole mass density A.Comastri1,R.Gilli1,A.Marconi2,3,G.Risaliti3,andM.Salvati3 1 INAF–OsservatorioAstronomicodiBologna,ViaRanzani1,40127Bologna,Italye-mail:[email protected] 2 DipartimentodiFisicaeAstronomia,UniversitàdiFirenze,viaSansone1,50019SestoFiorentino,Firenze,Italy 3 INAF–OsservatorioAstrofisicodiArcetri,LargoE.Fermi5,50125Firenze,Italy Received???Accepted??? 5 1 ABSTRACT 0 2 Arecentrevisionofblackholescalingrelationsindicatethatthelocalmassdensityinblackholesshouldbefivetimeshigherthan previouslydeterminedvalues.Thelocalblackholemassdensityisconnectedtothemeanradiativeefficiencyofaccretionthrough n thetimeintegraloftheAGNvolumedensityandasignificantincreaseinthelocalblackholemassdensitywouldhaveinteresting a J consequencesonAGNaccretionpropertiesanddemography.Onepossibilitytoexplainalargeblackholemassdensityisthatmost oftheblackholegrowthisviaradiativelyinefficientchannelssuchassuperEddingtonaccretion;however,thissolutionisnotunique. 0 Hereweshowhowitispossibletoaccommodatealargerfractionofheavilyburied,Compton-thickAGNs,withoutviolatingthelimit 2 imposedbythespectralenergydensityofthehardX–rayandmid–infraredbackgrounds. ] Keywords. galaxies:active–galaxies:bulges–quasars:supermassiveblackholes–X–rays:diffusebackground A G .1. Introduction where(cid:15) istheradiativeefficiency,U isthebolometric,comov- h T ingenergydensityofAGNsdescribedbyabolometricluminos- p Thestrongcorrelationsbetweenthemassesofthecentralsuper ityfunctionΦ(L,z), - omassive black holes (SMBH) and the global properties of their rhost spheroids such as luminosities, dynamical masses and ve- (cid:90) (cid:90) t dt slocitydispersions,i.e.thescalingrelations,canbeusedtocon- U = dz LΦ(L,z)dL. (2) avert the mass function of local galaxies into a black hole mass T dz [ function and, by integration, into a local SMBH mass density ThecomovingenergydensityU canbewrittenastheprod- 2 ρ•,loc. uctoftheX–rayenergydensityU Ttimesanaveragebolometric X v A recent comprehensive analysis of black hole mass mea- correctionfactor(cid:104)k (cid:105)toconverttheX–rayemissivitytobolo- 0 BOL surements and scaling relations concluded that the canonical metricemissivity(e.g.Lussoetal. 2012). 2 6black-hole-to-bulge mass ratio, MSMBH/MBulge, shows a mass Equation1iswidelyusedinAGNdemographicstudiestoin- dependence and varies from 0.1–0.2% at M (cid:39) 109M to fertheefficiencyofaccretionprocessesbycomparingthevalues 3 bulge (cid:12) 0∼ 0.5%at Mbulge = 1011M(cid:12) (Graham&Scott 2013,Kormendy obtainedbytheintegrationofthebolometricluminosityfunction .& Ho 2013). The revised normalization is a factor of 2 to 5 withthelocalblackholemassdensityobtainedfromthescaling 1 0largerthanpreviousestimatesrangingfrom(cid:39)0.10%(Merrit& relations.Agoodmatchbetweenρ•,loc andρ•,acc isachievedfor Ferrarese 2001,McLure&Dunlop 2002,Sanietal. 2011)to∼ anaccretionefficiencyconsistentwiththatexpectedfromastan- 5 10.23% (Marconi & Hunt 2003), therefore resulting in an over- dard Shakura & Sunyaev (1973) accretion disk, broadly sup- :allincreaseinthenormalization,whichisdominatedbymassive portingthepossibilitythatmostoftheSMBHmassisefficiently vbulges. accreted over the Hubble time (e.g. Fabian & Iwasawa 1999). i X Themassfunctionexpectedfromblackholesgrownbyac- Using the X–ray background as an integral constraint and as- sumingthatthebulkoftheX–rayemissivityisat(cid:104)z(cid:105) (cid:39)2,Elvis rcretionduringactivephasesandthecorrespondingintegralover athecosmictimeρ ,arecloselyrelatedtotheAGNluminosity et al. (2002) infer a lower limit on the accretion efficiency of •,acc (cid:15) >0.15,arguingthatmostSMBHarerapidlyspinning. functionviaamass–to–luminosityconversionfactor.Theargu- Mostrecentestimatesareingoodagreementwith(cid:104)(cid:15)(cid:105)(cid:39)0.1. ment,originallyproposedbySoltan (1982),canbeformulated For instance, Marconi et al. (2004) used luminosity functions asacontinuityequationforthemassdensityofblackholes(e.g. intheopticalB-band(Boyleetal. 2000),softX–ray,0.5–2keV Marconi et al. 2004; Shankar et al. 2004; Merloni & Heinz 2008). Assuming a constant radiative efficiency and neglecting band(Miyajietal. 2000),andhardX–ray,2–10keVband(Ueda et al. 2003) transformed into a bolometric luminosity function mergers,thecontinuityequationcanbeintegratedtoobtain using a luminosity dependent X–ray to optical spectral index α (Vignali et al. 2003). The correction for the number of OX missedAGNsinhardX–raysurveys(i.e.Compton-thick)isob- 1−(cid:15) 1−(cid:15) tainedbyrequiringthattheobservedX–rayluminosityfunction ρ•,accc2 =UT (cid:15) =(cid:104)kBOL(cid:105)UX (cid:15) , (1) matchestheX–raybackgroundspectrum.Theresultingestimate Articlenumber,page1of4 A&Aproofs:manuscriptno.25496_hk_2 ofρ ,assuming(cid:15)=0.1,isfullyconsistentwiththelocalmass τ (cid:29)1.TheformerprovidethelargestcontributiontotheXRB •,acc T densityρ =4.6+1.9×105M Mpc−3.Morerecently,Uedaet (X–ray background) spectrum at 20–30 keV, while the contri- •,loc −1.4 (cid:12) al. (2014)publishedanupdatedversionofthehardX–raylumi- butionofthelatterdependsmainlyonthefractionoftherepro- nosityfunction,includinganestimateoftheCompton-thickpop- cessed radiation f reflected or scattered towards the observer s ulationneededtofitthehardX–raybackgroundspectrum.They which, in turn, depends on the geometry of the absorber. In reached similar conclusions on the average accretion efficiency GCH07 the f value in the 2–10 keV band is assumed to be s using the Hopkins et al. (2007) bolometric corrections and the 0.02, in broad agreement with the observations of Compton- Vika et al. (2009) best fit value for ρ = 4.9±0.7 × 105M thickAGNsinthelocalUniverse(seeFig.1inGCH07).More- •,loc (cid:12) Mpc−3. over, the ratio between Compton-thin and unobscured sources Theagreementofthetwoapproachesdescribedaboveisnot isluminositydependentandsmoothlydecreasesfromR = 4at surprising despite the different assumptions of the luminosity low luminosities (logL = 42) to R = 1 at high luminosities X functionsandbolometriccorrections.Infact,botharelinkedto (logL = 46). The luminosity dependent obscured fraction is X reproducethehardX–raybackground,whichcanbeconsidered assumed to be the same for both Compton-thin and Compton- anintegralconstraintonthetotalmassaccretedoverthecosmic thick objects. As a result, the total (thin plus thick) obscured- timeandlockedinSMBH.Thealmostidenticalvaluesoftheas- to-unobscured ratio decreases from 8:1 at low luminosities to sumedlocaldensity(ρ )thusleadtoverysimilarconclusions 2:1 at high luminosities. The luminosity averaged ratio is ap- •,loc ontheaverageaccretionefficiency. proximately3whichmeansthatforeachunobscuredAGNthere At face value, and keeping all the factors entering in Eq. 1 are three Compton-thin and three Compton-thick sources (1.5 fixed at the fiducial values described above, a higher value of MCTand1.5HCT)andthusΣR =6.Anexcellentmatchbe- OBS the integral local mass density would lead to a similarly lower tween ρ and ρ , satisfying the XRB constraints, is found •,acc •,loc valuefortheaverageradiativeefficiency.Thepossibilitythatra- for(cid:15) =0.1(Marconietal. 2006). diatively inefficient accretion processes may be more common Weconsiderthemostconservativecaseofalocalmassden- thanpreviouslythoughthasbeenputforwardbyNovak (2013). sitywhichisafactorof2higherthanpreviouslyconsidered.The His argument is based on a simple linear scaling of the com- revisedvalue(Graham&Scott 2013,Kormendy&Ho 2013)is monlyacceptedvaluefortheaccretionefficiency((cid:15) ∼0.1)bya almostexactlyafactorof2higherthanthatobtainedbyMarconi factorof5,themaximumdiscrepancybetweentherevisedesti- etal. (2004)andVikaetal. (2009).Wealsoassumethatthead- mate(Graham&Scott 2013,Kormendy&Ho 2013)andprevi- ditionalmassdensityisnotduetoalowerefficiency,buttoanew ousvalues.Radiativelyinefficientprocesses(i.e.slimaccretion populationdescribedbythesameglobalparameters(cosmolog- disks)areadvocatedtoexplainthefastgrowthofSMBHinthe icalevolution,bolometriccorrection,andradiativeefficiency)of earlyUniverse(e.g.Madauetal. 2014). theknownAGNsresponsiblefortheXRB.Ifthiswerethecase, TherevisedestimateofthelocalmassdensityofSMBHmay equation3becomes have important consequences for AGN demography and accre- tion physics. In the following we discuss the impact of an in- 1−(cid:15) (cid:88) creasedfractionofheavilyobscuredSMBHwithrespecttocur- 2ρ c2 =(cid:104)k (cid:105)U (1+ R +R ), (5) •,acc BOL XO (cid:15) OBS NEW rentobservationalconstraints. i where R is the contribution of the new population respon- NEW 2. AlargepopulationofdeeplyburiedSMBH? sible for the local mass density excess. It is straightforward to showthat ThecontributiontotheSMBHmassdensityofunobscuredand obscuredAGNsandthedegeneraciesintrinsictothecomparison (cid:88) betweenρ andρ ,aremanifestifequation1iswrittenas R =(1+ R )=7, (6) •,acc •,loc NEW OBS implying that the contribution to the mass density of the new 1−(cid:15) (cid:88) ρ c2 =(cid:104)k (cid:105) U (1+ R ), (3) populationisthesameasthatalreadycontributingtotheXRB. •,acc BOL (cid:15) XO OBS Itgoeswithoutsayingthatthenewpopulationwouldvastlyex- ceed the limits imposed by the XRB itself, unless the sources where U is the X–ray energy density of unobscured AGNs, XO are so extremely obscured that X–rays are almost completely as determined from the soft X–ray luminosity function (e.g. suppressed. In other words, the upward revised normalization Hasingeretal. 2005),while(cid:104)k (cid:105)istheaveragebolometriccor- bol can be matched if each SMBH contributing to the XRB, has an rectionwhichisassumedtobethesameforobscuredandunob- X–ray quiet counterpart that adds mass, but almost no X–ray scuredAGNs(e.g.Lussoetal. 2012).Thesumisovertheratios radiation. tounobscuredAGNsoftheobscuredpopulationcontributingto Thisfigurecouldbemitigatednotingthatthepresentuncer- theSMBHmassdensityandtheX–raybackground: tainties onthe XRBspectral intensity at (cid:39) 20–30 keVare such thatitwouldbepossibletoaccommodateadditionalsourceswith (cid:88) thesamespectrumof HCTinGCH07.Morespecifically,ifthe R =R +R =R +R +R ×(0.02/f ). OBS Thin Thick Thin MThick HThick s reflection-dominatedsourcesarefourtimesmorenumerousthan (4) inGCH07(Fig.1),thepredictedXRBfluxat20–30keVwould stillbeconsistentwiththedata,withinthemeasurementerrors. In the following we refer to the Marconi et al. (2006) cal- IfthiswerethecasethesizeoftheX–rayquietpopulationwould culations, which are based on the Gilli, Comastri & Hasinger be reduced to R(cid:48) = 2.5. This new value is derived by the NEW (2007, GCH07) AGN synthesis model where the population of equality Compton-thick(CT)AGNsissplitintomildlyCT(MCT)when the optical depth for Compton scattering τ is of the order of T a few, and heavily CT (HCT), or reflection-dominated, when R +R =4×R +R(cid:48) ; (7) HCT NEW HCT NEW Articlenumber,page2of4 A.Comastrietal.:Masswithoutradiation Fig.1.ThebroadbandcosmicXRBspectrum.Thethick,solid,darkbluecurveandthethin,solid,blackcurveshowthetotalAGNspectrumand thecontributionofCompton-thick(mildly+heavily)AGNsintheGCH07model,respectively.Thethick,dashed,darkbluecurveandthethin, dashed,blackcurveareasabove,butassumea4timeslargerabundanceofheavilyCompton-thickAGNs.TheXRBmeasurementsfromvarious mission/instrumentcombinationsaremarkedbydifferentsymbolsaslabeled.Labelsontheleft(right)generallyrefertomeasurementsatE<10 (>10)keV.Therelevantreferencesareasfollows.ASCAGIS:Kushinoetal.(2002);ASCASIS:Gendreauetal.(1995);ROSAT:Georgantopoulos etal.(1996);BeppoSAX/MECS:Vecchietal.(1999);XMM(2002):Lumbetal.(2002);XMM(2004):DeLucaetal.(2004);RXTE:Revnitsevet al.(2003);Swift/XRT:Morettietal.(2009);Nagoyaballoon:Fukadaetal.(1975);SMM:Watanabeetal.(1997);HEAO-1:Gruberetal.(1999; opencirclesandfilledtriangles),Kinzeretal.(1997;hatchedarea);BeppoSAX/PDS:Fronteraetal.(2007);INTEGRAL(2007):Churazovetal. (2007);INTEGRAL(2010):Turleretal.(2010);Swift/Bat:Ajelloetal.(2008). we note that R is 1.5. In this scenario, only about 18% of new population should be characterized by an extremely faint, HCT the SMBH population would be X–ray quiet, but having sig- almostvanishing,X–rayemission. nificantly increased the fraction of HCT sources in the GCH07 The suppression of the hard X–ray emission (up to a few model, the fraction of deeply buried AGN (HCT plus X–ray hundred keV) would require both extremely high column den- quiet)wouldbeabout60%ofthetotalSMBHpopulation. sities (τ (cid:29) 10) and large covering factors (approaching 4π). WecanrelaxtheGCH07assumptionofthereflectionyield The detection and the measure of column densities in the high (fs = 0.02), and assume that both the GCH07 model HCT τregime(NH ∼1026 cm−2 orevenmore)isextremelychalleng- sources and the new population are described by a heavily ob- ing.Largecoveringfactorsoftheobscuringgas,withreflection scured, reflection-dominated spectrum, with the same value of yields much lower than 1%, are invoked to explain the obser- the reflection yield in the 2–10 keV band. In this case the new vations of a few heavily obscured and Compton-thick AGNs valuesoftheyieldofreflection-dominatedsourcesareobtained observed with Suzaku (e.g. Ueda et al. 2007; Comastri et al. from 2010).Thereisalsotentativeevidenceforalargercoveringfac- torforincreasingabsorptionamongChandraDeepFieldSouth faint sources (Brightman et al. 2014). The upper limits on the RHCT ×0.02= fN(cid:48)EW ×[RHCT +RNEW], (8) 2–10keVX–rayemissionofasmallsampleofluminousAGNs, hosted by nearby ULIRGS (Nardini & Risaliti 2011), are so wthheesereasRsuHmCTp=ti1o.n5s,iwsetohbetafirnacftNioEWn=a0s.s0u0m35edanidnfNG(cid:48)ECWH=007.0.1W4fiothr t1i0g−h3t−th1a0t−th4e.IcfotrhreespproesnednincegoufptpheersleimAiGtsNosnisfswairdeeosfprtheaedoardmeornogf RNEW=7 or RNEW=2.5, depending on the assumed level of the infraredgalaxies,theycouldbeexcellentcandidatesforthenew XRB. population. At face value, a population of reflection-dominated Compton-thick AGNs, with a suitable combination of covering 3. Discussion factorandcolumndensity,wouldstillbeconsistentwiththeup- A large population of heavily obscured, reflection-dominated perenvelopeofpresentuncertaintiesofthehardXRBspectrum. AGNs is considered here to fill the mass gap between the most While the new population could have so far escaped direct recentestimateofρ andpreviousvalues.Inordertoaccom- detection in present deep, soft (< 10 keV) X–ray surveys, and •,loc modateasignificantlyhighernumberofSMBHwithoutviolat- would remain largely undetected in shallow NuSTAR hard X– ingthelimitimposedbytheX–raybackground,thehypothetical ray surveys (Alexander et al. 2013), the reprocessed AGN lu- Articlenumber,page3of4 A&Aproofs:manuscriptno.25496_hk_2 minositywillemergeintheinfrared.Afirstorder,integralesti- Thiswouldimplythatthecolumndensitydistributionofnu- matemaybeobtainedusingthehardXRBspectralintensityas clearobscuringgasinSMBHistopheavyandskewedtowards a limit of the total energy output combined with suitable bolo- valuesoftheorderof1025−26cm−2orevenhigherandthecover- metric corrections (e.g. Fabian & Iwasawa 1999, Risaliti et al. ingfractionofobscuringmaterialmayapproach4π.Findingob- 2002). The results indicate that the AGNs responsible for the servationalproofoftheexistenceoftheseultra-obscuredSMBH XRB make 10–15% of the IR background around its ∼100µm is challenging. Deep mid-far-infrared photometric and spectro- peak. Their contribution increases at shorter wavelengths, be- scopicsurveys,matchedwithsensitiveX–raysurveyswouldbe ing of the order of 20–40% at 60µm and possibly higher at neededtoestimatethefractionofinfraredemittingAGNswith- 15µm depending on the dust effective temperature (Risaliti et out an X–ray counterpart and thus presumably heavily buried. al. 2002)andtherelativefractionofCompton-thicksourcesin High density gas illuminated by a hidden nuclear source may the adopted XRB model. The impact of the new population of betracedbymoleculartransitionsobservableinthefar-IRwith heavily Compton-thick AGNs hypothesized here is even more ALMA(Imanishi&Nakanishi 2014).Afractionofthemcould uncertain.Inthemostnaiveapproximation,theaboveestimates befoundamongthesourcesresponsiblefortheunresolvedXRB shouldberescaledbyafactorof2makingtheAGNcontribution in the 6–8 keV band (Xue et al. 2012). We are hopeful that a dominant at short wavelengths (< 20 µm). However, according few examples will be uncovered by forthcoming deep hard X– toarecent,comprehensivepopulationsynthesismodeloftheX– raysurveyswithNuSTAR(Harrisonetal. 2013)andultra-deep rayandinfraredbackgrounds(Shietal. 2013),thecontribution ATHENAsurveys. oftheX–rayemittingAGNstothe100µmbackgroundisnegli- Acknowledgements. This work was partially supported by the ASI/INAF gibleandisoftheorderof15%at5–10µm.Ifthiswerethecase, I/037/12/0–011/13, the PRIN–INAF–2011 and the PRIN–INAF–2012 grants. therewouldbeplentyofroomtofitalargepopulationofheavily WethankMarcellaBrusa,MarcoMignoliandGianniZamoraniforusefuldis- obscuredSMBHwithoutexceedingthelimitsimposedbytheIR cussions,andtheanonymousrefereeforconstructivecomments.ACacknowl- background. edgesthesupportofaCaltechKingsleyvisitorfellowship. Observationalconstraintsontheabundanceofsourceslikely to harbor a deeply buried new AGN may be obtained by mid- infraredphotometricandspectroscopicsurveyscarriedoutwith References the Spitzer Space Telescope. Highly obscured and Compton- Ajello,M.,Greiner,J.,Sato,G.,etal.2008,ApJ,689,666 thick AGNs are likely to be affected by heavy dust absorption Alexander,D.M.;Stern,D.,DelMoro,A.,etal.2013,ApJ,773,125 and steep mid-infrared to optical slopes. A widely adopted se- Boyle,B.J.,Shanks,T.,Croom,S.M.,etal.2000,MNRAS,317,1014 Brightman,M.,Nandra,K.,Salvato,M.,etal.2014,MNRAS,443,1999 lectionmethodisbasedontheopticalto24µmcolor(e.g.Fiore Churazov,E.,Sunyaev,R.,Revnivtsev,M.,etal.2007,A&A,467,529 et al. 2009). The optical depth of the silicate absorption fea- Comastri,A.,Iwasawa,K.,Gilli,R.,etal.2010,ApJ,717,787 DeLuca,A.,Molendi,S.2004,A&A,419,837 ture at 9.7µm is also considered a reliable tracer of heavy dust ElvisM.,Risaliti,G.,Zamorani,G.2002,ApJ,565,L75 absorption.Goodquality,mid-infraredspectrawereobtainedfor FabianA.C.,Iwasawa,K.1999,MNRAS,303,L34 FioreF.,etal.2009,ApJ,693,447 localluminousinfraredgalaxiesintheGOALSsurvey(Stierwalt Frontera,F.,Orlandini,M.,Landi,R.,etal.2007,ApJ,666,86 etal. 2013)andz∼0.7COSMOSgalaxies(Fuetal. 2010).The Fu,H.,Lin,Y.,Scoville,N.Z.,etal.2010,ApJ,722,653 Fukada,Y.,Hayakawa,S.,Kasahara,I.,etal.1975,Nature,254,398 fractionofopticallythick(τ9.7 >1)galaxies,whicharelikelyto Gendreau,K.C.,Mushotzky,R.F.,Fabian,A.C.,etal.1995,PASJ,47,L5 host a buried AGN on the basis of SED decomposition, is es- Georgantopoulos,I.,Stewart,G.C.,Shanks,etal.1996,MNRAS,280,276 Georgantopoulos,I.,etal.2011,A&A,531,A116 timated to be of the order of 10% or even lower. However, it Gilli,R.,Comastri,A.,&Hasinger,G.2007,A&A,463,79GCH07 shouldbenotedthatcurrentmid-infraredphotometricandspec- Graham,A.W.,Scott,N.2013,ApJ,764,151 Gruber,D.E.,Matteson,J.L.,Peterson,L.E.,Jung,G.V.1999,ApJ,520,124 troscopicsurveysarerelativelyshallowand,moreover,thereare Harrison,F.A.,etal.2013,ApJ,770,103 many Compton-thick AGNs that do not display strong silicate Hasinger,G.,Miyaji,T.,Schmidt,M.2005,A&A,441,417 Hopkins,P.F.,Richards,G.T.,Hernquist,L.2007,ApJ,654,731 absorption features (e.g. Georgantopoulos et al. 2011). Given ImanishiM.,Nakanishi,K.2014,AJ,148,9 thepresentuncertaintiesandthedifficultyofdistinguishingthe Kinzer,R.L.,Jung,G.V.,Gruber,D.E.,etal.1997,ApJ,467,361 Kormendy,J.,Ho,L.C.2013,ARA&A,51,511 hypothetical population of deeply buried sources from normal Kushino,A.,Ishisaki,Y.,Morita,U.,etal.2002,PASJ,54,327 andstar-forminggalaxies,weconcludethatthenewpopulation, Lumb,D.H.,Warwick,R.S.,Page,M.,DeLuca,A.2002,A&A,389,93 Lusso,E.,Comastri,A.,Simmons,B.D.,etal.2012,MNRAS,425,623 isunlikelytoexceedtheIRbackgroundspectralintensity.Can- Madau,P.,Haardt,F.,Dotti,M.2014,ApJ,784,L38 didatesofthenewpopulationmaybefoundamongsourceswith Marconi,A.,HuntL.K.2003,ApJ,589,L21 a steeply rising SED from the near- to the far-infrared and/or Marconi,A.,Risaliti,G.,Gilli,R.,etal.2004,MNRAS,351,169 Marconi,A.,Comastri,A.,Gilli,R.,etal.2006,Mem.S.A.It.77,742 among those with the highest extinction in the silicate absorp- McLure,R.J.,Dunlop,J.S.2002,MNRAS,331,795 Merloni,A.,Heinz,S.2008,MNRAS,388,1011 tionfeatureat9.7µm. Merrit,D.,Ferrarese,L.2001,ApJ,547,140 Miyaji,T.Hasinger,G.,Schmidt,M.2000,A&A,353,25 Nardini,E.,Risaliti,G.2011,MNRAS,415,619 4. Conclusions Novak,G.S.2013,MNRAS,submitted,arXiv:1310.3833 Revnivtsev,M.,Gilfanov,M.,Jahoda,K.,Sunyaev,R.2005,A&A,444,381 Risaliti,G.,Elvis,M.,Gilli,R.2002,ApJ,566,L67 Weputforwardthepossibilitythatasizeablepopulationofhith- Sani,E.,Marconi,A.,Hunt,L.K.,Risaliti,G.2011,MNRAS,413,1479 erto uncovered ultra obscured AGNs would explain the revised Shakura,N.I.,Sunyaev,R.A.1973,A&A,24,337 Shankar,F.,Salucci,P.,Granato,G.L.,etal.2004,MNRAS,354,1020 upwardvalueoftheSMBHlocalmassdensityρ•,loc. Shi,Y.,Helou,G.,Armus,L.,etal.2013,ApJ,764,28 We are aware of the strong degeneracies between the var- Soltan,A.1982,MNRAS,200,115 Stierwalt,S.,Armus,L.,Surace,J.A.,etal.2013,ApJS,206,1 ious parameters used to build our argument and, in particular, Turler,M.,Chernyakova,M.,Courvoisier,T.J.L.,etal.2010,A&A,512,A49 theradiativeefficiency(cid:15).Withinthereasonableassumptionsthat Ueda,Y.Akiyama,M.,Ohta,K.,Miyaji,T.2003,ApJ,598,886 Ueda,Y.,Eguchi,S.,Terashima,Y.,etal.2007,ApJ,664,L79 the evolution of the X–ray luminosity function and bolometric Ueda,Y.,Akiyama,M.,Hasinger,G.,etal.2014,ApJ,786,104 corrections are robustly constrained by recent surveys, and the Vecchi,A.,Molendi,S.,Guainazzi,M.,etal.1999,A&A,349,L73 Vignali,C.,Brandt,W.N.,Schneider,D.P.2003,AJ,125,433 spectralintensityoftheX–raybackgroundandtheaccretionpro- Vika,M.,Driver,S.P.,Graham,A.W.,Liske,J.2009,MNRAS,400,1451 cessesareselfsimilarandnotevolving,alargenumberofdeeply Watanabe,K.,etal.1997,inTheFourthCGROSymposium,ed.C.D.Dermer, buriedSMBHmaybeaviablesolutionbeyond,radiativelyinef- M.S.Strickman,&J.D.Durfess,AIPConf.Proc.,410,1223 Xue,Y.Q.,Wang,S.X.,Brandt,W.N.,etal.2012,ApJ,758,129 ficientaccretion,forexample. Articlenumber,page4of4