Mass without radiation: heavily obscured AGN, the X-ray Background and the Black Hole Mass Density

0.2 MB·English

by  A. Comastri

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
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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-
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