Compton thick AGN in the XMM-COSMOS survey

23 Pages·2015·3.07 MB·English

A&A573,A137(2015) Astronomy
DOI:10.1051/0004-6361/201424924 &
(cid:2)c ESO2015 Astrophysics
(cid:2)
Compton thick AGN in the XMM-COSMOS survey
G.Lanzuisi1,2,P.Ranalli1,I.Georgantopoulos1,A.Georgakakis1,3,I.Delvecchio2,4,T.Akylas1,S.Berta3,
A.Bongiorno5,M.Brusa2,4,N.Cappelluti2,F.Civano6,A.Comastri2,R.Gilli2,C.Gruppioni2,G.Hasinger7,
K.Iwasawa8,9,A.Koekemoer10,E.Lusso11,S.Marchesi4,6,V.Mainieri12,A.Merloni3,M.Mignoli2,E.Piconcelli5,
F.Pozzi2,4,D.J.Rosario3,M.Salvato3,J.Silverman13,B.Trakhtenbrot14,(cid:2)(cid:2),C.Vignali2,4,andG.Zamorani2
1 InstituteofAstronomy&Astrophysics,NationalObservatoryofAthens,PalaiaPenteli,15236Athens,Greece
e-mail:[email protected]
2 INAF–OsservatorioAstronomicodiBologna,viaRanzani1,40127Bologna,Italy
3 Max-Planck-InstitutfürextraterrestrischePhysik,Giessenbachstrasse,85748Garching,Germany
4 DipartimentodiFisicaediAstronomia,UniversitádiBologna,viaRanzani1,40127Bologna,Italy
5 INAF–OsservatorioAstronomicodiRoma,viaFrascati33,00040MontePorzioCatone(RM),Italy
6 YaleCenterforAstronomyandAstrophysics,260WhitneyAvenue,CT06520NewHaven,USA
7 InstituteforAstronomy,2680WoodlawnDrive,HI96822-1839Honolulu,USA
8 ICREAandInstitutdeCiènciesdelCosmos,UniversitatdeBarcelona,IEEC-UB,MartíiFranquès,1,08028Barcelona,Spain
9 InstitutdeCiénciesdelCosmos(ICCUB),UniversitatdeBarcelona,MartíiFranqués,108028Barcelona,Spain
10 SpaceTelescopeScienceInstitute,3700SanMartinDrive,MD21218Baltimore,USA
11 MaxPlanckInstitutfurAstronomie,Königstuhl17,69117Heidelberg,Germany
12 EuropeanSouthernObservatory,Karl-Schwarzschild-Strasse2,85748Garching,Germany
13 KavliInstituteforthePhysicsandMathematicsoftheUniverse(IPMU)5-1-5KashiwanohaKashiwa,277-8583Chiba,Japan
14 DepartmentofPhysics,InstituteforAstronomy,ETHZurich,Wolfgang-Pauli-Strasse27,8093Zurich,Switzerland
Received5September2014/Accepted25November2014
ABSTRACT
Heavily obscured, Compton thick (CT, N > 1024 cm−2) active galactic nuclei (AGN) may represent an important phase in
H
AGN/galaxyco-evolutionandareexpectedtoprovideasignificantcontributiontothecosmicX-raybackgroundatitspeak.However,
unambiguously identifyingCTAGNbeyondthelocalUniverseisachallengingtaskeveninthedeepestX-raysurveys, andgiven
the expected low spatial density of these sources in the 2−10 keV band, large area surveys are needed to collect sizable sam-
ples. Through direct X-ray spectra analysis, we selected 39 heavily obscured AGN (N > 3×1023 cm−2) at bright X-ray fluxes
(F2−10 >∼ 10−14 ergs−1 cm−2)inthe2deg2 XMM-COSMOSsurvey.AfterselectingCTAHGNbasedonthefitofasimpleabsorbed
twopowerlawmodeltotheshallowXMM-Newtondata,thepresenceofbonafideCTAGNwasconfirmedin80%ofthesources
usingdeeper Chandradataandmorecomplexmodels.ThefinalsamplecomprisestenCTAGN(sixofthemalsohaveadetected
FeKαlinewithEW ∼1keV),spanningawiderangeofredshifts(z∼0.1−2.5)andluminosity(L2−10 ∼1043.5−1045 ergs−1)andis
complementedby29heavilyobscuredAGNspanningthesameredshiftandluminosityrange.Wecollectedtherichmulti-wavelength
informationavailableforallthesesources,inordertostudythedistributionofsupermassiveblackholeandhostproperties,suchas
blackholemass(MBH),Eddingtonratio(λEdd),stellarmass(M∗),specificstarformationrate(sSFR)incomparison withasample
of unobscured AGN.Wefindthathighly obscured sources tend tohave significantlysmaller M and higher λ withrespect to
BH Edd
unobscured sources, whileaweaker evolutionin M∗ isobserved. ThesSFRof highly obscured sources isconsistent withtheone
observedinthemainsequenceofstarforminggalaxies,atallredshifts.Wealsopresentandbrieflydiscussopticalspectra,broadband
spectral energy distribution (SED)and morphology for the sample of ten CT AGN. Both the optical spectra and SED agree with
the classification as highly obscured sources: all the available optical spectra are dominated by the stellar component of the host
galaxy,andtoreproducethebroadbandSED,ahighlyobscuredtoruscomponentisneededforalltheCTsources.Exploitingthehigh
resolutionHubble-ACSimagesavailable,weareabletoshowthatthesehighlyobscuredsourceshaveasignificantlylargermerger
fractionwithrespecttootherX-rayselectedsamplesofAGN.Finallywediscusstheimplicationsofourfindingsinthecontextof
AGN/galaxyco-evolutionarymodels,andcompareourresultswiththepredictionsofX-raybackgroundsynthesismodels.
Keywords.galaxies:nuclei–galaxies:Seyfert–quasars:general–X-rays:galaxies
1. Introduction Universeandtheinterplaybetweenactivegalacticnuclei(AGN)
andtheirhostgalaxies.ObscuredAGNforexample,areessen-
Observational and theoretical arguments suggest that the ob- tialforreconcilingthemassfunctionofblackholesinthelocal
scured phase of super massive black hole (SMBH) growth Universe with that expected from AGN relics, i.e. inferred by
holdsimportantinformationonboththeaccretionhistoryofthe integrating the luminosity function of AGN via the continuity
equation (e.g. Soltan 1982; Marconi et al. 2004). The spectral
(cid:2) Appendicesareavailableinelectronicformat shapeofthediffuseX-raybackgroundalsorequiresalargenum-
http://www.aanda.org berofmildlyobscuredAGN(Gillietal. 2007)andevenpossi-
(cid:2)(cid:2) Zwickypostdoctoralfellow. bly deeply buried ones with column densities in excess of the
ArticlepublishedbyEDPSciences A137,page1of23
A&A573,A137(2015)
Comptonthicklimit(CTAGN, N >∼ 1024 cm−2;Treisteretal. allowrobustX-rayspectralparameterestimationandhence,the
H
2009;Akylasetal.2012). identificationofsecureheavilyobscuredAGNsamples.
X-ray surveys provide a relatively unbiased census of the InthispaperwesearchforthemostheavilyobscuredAGN
accretion history in the Universe, as they can penetrate large in the COSMOS field (Scoville et al. 2007) starting from the
amounts of dust and gas, especially in the hard, 2−10 keV XMM-COSMOScatalog(Hasingeretal.2007;Cappellutietal.
band.X-raysurveysindeedfindthattheaccretiondensityofthe 2009)andusingdeeperChandradata(Elvisetal.2009)totest
Universe is dominatedby black holes that grow their mass be- the efficiencyof ourselection methodfor CT AGN. Moreover,
hind large columns of dust and gas clouds (e.g. Mainieri et al. using the wealth of multi-wavelength data available in the
2002; Ueda et al. 2003; Tozzi et al. 2006; Akylas et al. 2006; COSMOSfield,weexploretheaccretionandhostgalaxyprop-
Buchneretal.2014;Uedaetal.2014). erties of the obscured AGN sample, in order to place them in
However, the exact intrinsic fraction of CT AGN remains the context of AGN-Galaxy co-evolution scenarios. This stage
highlyuncertain,rangingfromabout10%ofthetotalAGNpop- shouldbecharacterizedbysmallBHmasses,highaccretionand
ulation up to 35%. Owing to ultra-hard X-ray surveys above star formation (SF) rates (e.g. Fabian 1999; Page et al. 2004;
10 keV performed with Swift and INTEGRAL, CT AGN are Draper&Ballantyne2010),andcouldbepossiblymerger-driven
commonly observed in the local Universe, representing up (Hopkins et al. 2008). Our multi-wavelength analysis attempts
to 20% of local active galaxies at energies 15−200 keV down to provide constraints on such models. Finally we present an
toafluxlimitof10−11ergcm−2s−1 (seeBurlonetal.2011,and atlas of the multi-wavelength properties (i.e. broadband SED,
referencestherein).Still, the identification of CT AGN beyond optical spectra, morphology) of the sample of 10 bona fide
thelocaluniverseis challenging.Recentresultsonpreliminary CT AGN. Throughoutthe paper, a standard Λ−CDM cosmol-
NuSTAR(Harrisonetal.2013)datawereonlyabletoputanup- ogy with H0 = 70 kms−1 Mpc−1, ΩΛ = 0.7 and ΩM = 0.3 is
perlimit(<33%)tothefractionofCTAGNbetweenz=0.5−1 used.Errorsaregivenat90%confidencelevel.
(Alexanderetal.2013).
Within the AGN/galaxy co-evolution perspective, dust and
gasenshroudedAGNrepresentanevolutionarypointthatiscrit- 2. Thedataset
ical for understanding how the growth of black holes relates
2.1.Multi-wavelengthcoverageintheCOSMOSfield
to the build-up of the stellar populations of their hosts. The
genericpictureproposedinvolvesgasinflowstriggeredbyinter- One of the main goals of the Cosmic Evolution Survey
nal(Hopkins& Hernquist2006; Ciotti & Ostriker 1997, 2007; (COSMOS; Scoville et al. 2007) is to trace star formation
Bournaud et al. 2011; Gabor & Bournaud 2013) or external and nuclear activity along with the mass assembly history of
(Sanders et al. 1988; Di Matteo et al. 2005; Hopkins et al. galaxies as a function of redshift. The 2 deg2 area of the
2006) processes. These result in a period of rapid black hole HST COSMOS Treasury program is bounded by 9h57.5m <
growth that takes place within dense dust and gas cocoons. It RA < 10h03.5m;1◦27.50 < Dec < 2◦57.50. The field
is then followed by a blow-out stage during which some form has a unique deep and wide multi-wavelength coverage, from
of AGN feedback depletes the gas reservoirs thereby regulat- the optical band (Hubble, Subaru, VLT, and other ground-
ingboththestar-formationandblackholegrowth.Thestudyof based telescopes), to the infrared (Spitzer, Herschel), X-ray
obscuredAGN hasthe potentialto provideimportantpiecesof (XMM-Newton and Chandra) and radio (Very Large Array
the puzzle,such asthe nature ofthe triggeringmechanism,the (VLA) and future Jansky-VLA, P.I. V. Smolcic) bands. Large
physicsofAGNoutflowsandtheirimpactonthehostgalaxy. dedicated ground-based spectroscopy programs in the optical
Theevidenceabovemotivatednumerousstudiestoidentify withMagellan/IMACS(Trumpetal.2009),VLT/VIMOS(Lilly
the most heavily obscuredAGN, determine their space density et al. 2009), Subaru-FMOS (P.I. J. Silverman), and DEIMOS-
relativetounobscuredsourcesandstudytheirhostgalaxyprop- Keck (P.I. N. Scoville) have been completed or are well under
erties(e.g.Hickoxetal.2009;Brusaetal.2009;Mainierietal. way. Very accuratephotometricredshiftsare available for both
2011;Donleyetal.2012;Rovilosetal.2014;Delvecchioetal. thegalaxypopulation(Δz/(1+z)<1%;Ilbertetal.2009)andthe
2014). At X-rays in particular, there has been an explosion re- AGNpopulation(Δz/(1+z)∼1.5%Salvatoetal.2009,2011).
cently in the quality and quantity of data available, a develop- The COSMOS field has been observed with XMM-Newton
ment that led to direct constraints on the fraction of even the foratotalof∼1.5Msataratherhomogeneousdepthof∼60ks
mostdeeplyshroudedCTsourcesoverawiderangeofredshifts (Hasingeretal.2007;Cappellutietal.2009).TheXMM-Newton
(e.g.Tozzietal.2006;Burlonetal.2011;Comastrietal.2011; catalogusedinthisworkincludes∼1800point-likesources,de-
Brightman&Ueda2012;Georgantopoulosetal.2013;Buchner tectedinoneormoreofthe3adoptedbands(0.5−2,2−10and
etal.2014;Brightmanetal.2014).Despitesignificantprogress, 5−10keV),andhavingahard(2−10keV)bandlimitingfluxof
therearestilldifferencesintheheavilyobscuredAGNsamples 2.5×10−15 erg cm−2s−1 (and5.6(9.3)×10−15 ergcm−2s−1 on
compiled by different groups using the same data. This is pri- 50%(90%) of the area). The adopted likelihoodthresholdcor-
marilyrelatedtovariationsinthespectralanalysismethods,e.g. responds to a probability of ∼4.5×10−5 that a catalog source
thecomplexityoftheX-rayspectralmodelsadoptedorhowun- isa spuriousbackgroundfluctuation.197sourcesareclassified
certaintiesduetophotometricredshiftsorthePoissonnatureof asstarsorunclassified(Brusaetal.2010),sotheyareexcluded
X-rayspectraarepropagatedintheanalysis.Ithasbeenshown fromthefollowingX-rayspectralanalysis.
for example, that a common feature of obscured AGN spectra At the hard band flux limit of the XMM-COSMOS survey,
is emission at soft energies in excess to the primary contin- current CXB models predict a fraction of CT-AGN between 1
uum, possibly due to scattered radiation into the line of sight and3%(Gillietal.2007;Treisteretal.2009;Uedaetal.2014),
(Brightman & Nandra 2012; Buchner et al. 2014). Taking this whileatafluxlimitofF2−10 ∼10−14ergcm−2s−1,abovewhich
componentintoaccountis clearlyimportantforidentifyingthe a basic spectral analysis is viable, this fraction is even smaller
<
mostobscuredAGNinX-raysurveys.Recently,Buchneretal. (∼1%,seeSect.8).Therefore,theXMM-COSMOScatalogmay
(2014)alsodemonstratedtheimportanceofadvancedstatistical seem not the best place where to look for, in order to select a
methodsthatproperlyaccountforvarioussourcesofuncertainty, largesampleofCTAGN,eventakingintoaccountthelargearea
A137,page2of23
G.Lanzuisietal.:ComptonthickAGNintheXMM-COSMOSsurvey
covered.However,the greatadvantageofthe XMM-COSMOS We adopted as source model a simple double power-law:
dataset is that there are deeper X-ray observations available in the primary power-law, modified by intrinsic absorption at the
the same area: the Chandra-COSMOS and its new extension, source redshift,representingthe transmittedcomponent,plusa
COSMOS legacy (Elvis et al. 2009; Civano et al. 2014), that second power-law to account for the soft emission commonly
together cover the same 2 deg2 area of the XMM-Newton ob- observed in local highly obscured sources (e.g. Done et al.
servation,with a homogeneousexposuretime of ∼160ks(flux 2003). This emission can be due to unobscured flux leaking
limitF2−10 ∼ 7×10−16 ergcm−2s−1).Thiswillallowustouse out in the soft band (through scatter or partial covering), ther-
thedeeperX-raydatatoevaluate,a posteriori,theefficiencyof mal emission related to star-formation, or the blend of emis-
ourXMM-basedCTAGNselectionmethod(seeSect.4.1),with sion lines from photo-ionizedcircumnuclear gas (Guainazzi &
thefinalgoaltoextend,inthefuture,thesestudiestothedeeper Bianchi2007)oracombinationofthesecomponents.Giventhe
fullChandracataloginCOSMOS.Giventhedifficultyofunam- relativelypoorphotonstatisticsandthelackofhighspectralres-
biguouslyidentifyingCT AGN beyondthe localUniverse,this olution,distinguishingbetweenthesedifferentoriginsisnotpos-
isavalidalternativeapproachtosimulations,toevaluatetheef- sible here, and the second power-law is only used as a simple
ficiencyoftheselectionmethod. description of the observed spectra in the soft band. However,
Furthermore, a great wealth of information has been made the presence of a second, soft component is essential in order
available in the latest years, regarding multi-wavelength, host torecoveracorrectestimateoftheintrinsicabsorptionaffecting
andSMBHpropertiesofthesourcesintheXMM-COSMOScat- the primary power-law, especially for highly obscured sources
alog:Trumpetal.(2009),Merlonietal.(2010)andRosarioetal. (Brightmanetal.2014;Buchneretal.2014).
(2013)presentSMBHmassesforlargesamplesoftype-1AGN
The normalization of the soft component is constrained to
(182,89and289,respectively,thelattercomputedfromacom-
be less than 10% with respect to the hard component.In well-
pilation of all the spectra available); Mainieri et al. (2011)
studied nearby AGN, a small percentage is the typical flux
present multi-wavelength properties of 142 obscured QSOs;
contribution of the soft component with respect to the unob-
Bongiorno et al. (2012, B12 hereafter) present host properties
scured primary one (e.g. Turner et al. 1997). The photon in-
(starformationrate,stellarmassetc.)ofasampleof1700AGN;
dex of the soft component is linked to that of the primary
Lusso et al. (2012, L12 hereafter) present bolometric lumi-
power-law,inordertominimizethenumberoffreeparameters.
nosities and Eddington ratios of a sample of 929 AGN, both
Bothpower-lawsare absorbedbya Galacticcolumndensityof
type-1 and type-2, and SMBH masses estimated trough scal- 1.7× 1020 cm−2 (Kalberla et al. 2005), observed in the direc-
ingrelationsfor488type-2AGN.Delvecchioetal.(2014,D14
tionoftheCOSMOSfield1.Theenergyrangeinwhichthefitis
hereafter)performedbroadbandSED decompositionforall the
performedis0.3−10keV.
160 μm Herschel detected COSMOS sources. The availability
of all this information is crucial in order to study the role of We notethattheminimumnumberofcountsforwhichthis
CTAGNinthecontextofAGN/galaxyco-evolution. kindofanalysiscanbeappliedisconstrainedonlybythemaxi-
mum relative error that one wants to allow for the free param-
eters in the fit. Because here we are mainly interested in re-
2.2.Sampleselection
coveringtheintrinsicabsorptionandluminosityofoursources,
The XMM-Newton spectra have been extracted as described in we fixed the photonindexΓ to 1.9for sourceswith fewerthan
Mainierietal.(2007).Theoriginalspectralextractionwasper- 100netcounts(70%ofthesourceshavefewerthan100counts).
formed only for the EPIC pn-CCD (pn) camera (Struder et al. ThisvalueistheonetypicallyfoundinAGNatanyluminosity
2001). All the spectral fits are performed with Xspec v. 12.7.1 level(Nandra&Pounds1994;Piconcellietal.2005;Tozzietal.
(Arnaud et al. 1996). We analyzed the 1625 X-ray pn spectra 2006). In this way the number of free parameters is three (NH
oftheidentifiedextragalacticsources,usingthesameautomated andthetwopower-lawnormalizations)andweareabletocon-
fitprocedurepresentedinLanzuisietal.(2013a).Thisprocedure strain,withatypicalrelativeerrorsmallerthan60%,theNH for
makesuseoftheC-statistic(Cash1979),especiallydevelopedto sources with more than 30 net counts in the full 0.3−10 keV
modelspectrawithasmallnumberofcounts.Itrequiresthesi- band. We discuss in Appendix A the detection limits of the
multaneousfitofthebackground(BKG)andverylimitedcounts XMM-COSMOS survey in the z−L2−10 plane, for highly ob-
binning(minimumof1countsperbin,inordertoavoidempty scuredsourceswithatleast30netcountsinfullband,compared
channels). withtheoneforthefullXMM-COSMOScatalog.
The global XMM-Newton pn background between 0.3 and
Figure1showsthedistributionoftheintrinsiccolumnden-
10 keV is complex, and comprises an external, cosmic BKG, sity N for the 1184 extragalactic sources with >30 counts.
H
passing throughthe telescope mirrors,and thereforeconvolved The first bin at N = 1020 cm−2 includes the 390 sources for
H
with the instrumental Auxiliary Response File (ARF), and the which the N is an upper limit (for clarity the y-axis of the
H
internal, particle induced BKG, which is not convolved by the plot is rescaled in the range 0−100). With this simple spectral
ARF. TheexternalBKG,whichdominatesatsoftenergies(be- fit, we areabletoselectten CT AGNcandidates(bestfit N ≥
low ∼1 keV) is modeled with two thermalcomponents(Xspec 1×1024 cm−2, initial Compton thick sample, CTK hereaftHer),
i
modelAPEC),withsolarabundances,oneforthelocalhotbub- plusalargersampleof29highlyobscured(N >3×1023cm−2),
ble(kT ∼0.04keV)andthesecondfortheGalacticcomponent butnominallynotCT(N <1×1024cm−2,inHitialComptonthin
(kT ∼ 0.12 keV), produced by the ISM of the Galactic disk H
sample, CTN hereafter)AGN. The globalpropertiesof all the
i
(Kuntz & Snowden 2000), plus a power-law reproducing the
39 sources comprised in both sample are described in the next
CosmicX-rayBackground(Γ ∼ 1.4),mostlyduetounresolved
section,beforeperformingmoredetailedspectralanalysis.
discretesources(Comastrietal.1995;Brandtetal.2002).The
particle induced BKG is well reproduced by a flat power-law
(Γ ∼ 0.5),plusseveralstrongemissionlinesatenergiesof1.5,
7.4, 8.0 and 8.9 keV, due to Al, Ni, Cu and Zi+Cu Kα lines, 1 In XSPEC notation the above model is expressed as
respectively(Freybergetal.2004). wa*(po+zwa*po).
A137,page3of23
A&A573,A137(2015)
and CTN samples can be considerednot variablein the 0.5−2
i
and 2−10 keV bands at 95% confidence level (variability es-
timators for the full XMM-COSMOS catalog are available in
Lanzuisi et al. 2013b) through the 3.5 years of observation of
theXMM-COSMOSsurvey.ThereforethemergedX-rayspec-
trausedintheanalysisshouldbeconsideredasrepresentativeof
thetypicalspectrumforeachsource.
3. CTspectralmodeling
Studies of the X-ray spectra of local Seyferts have identified
differentcomponentsassociatedwithdifferentphysicalprocess,
suchasreflection,scatteringandabsorptionofthedirectX-ray
emission.Properlymodelingthesecomponentsisimportantfor
meaningfulparameterderivation,especiallyinthecaseofheav-
ily obscuredand CT sources. Indeed,buildinga generalmodel
for the X-ray spectrum of CT AGN is difficult, because of the
complex interplay between photoelectric absorption, Compton
scattering,reflectionandfluorescenceemissionlinesalongdif-
ferentlinesofsight(seee.g.Murphy&Yaqoob2009,forade-
taileddiscussion).
Fig.1. Distribution of N (in units of 1022 cm−2) in the XMM-
H In addition to the primary power-law, obscured up to very
COSMOS survey. Only extragalactic sources with >30 pn net counts highenergies(typicallyupto7−10keVrestframe),aratherflat
areshown.Thegreen(red)shadedhistogramshowthesampleofCTN
i reflection componentis usually present, together with a strong
(isCaTnKui)ppsoeurrlcimesi.tT(fhoerficrlastribtyinthinecylu-adxeisstihser3es9c0alseoduricnetshfeorrawngheic0h−th1e00N).H (EW >∼ 1 keV) Fe Kα line, which is considered to be the un-
ambiguous sign of the presence of a CT absorber (Matt et al.
2000), and even used as one of the most reliable CT selection
techniquesforsourceswithmedium-goodqualityX-rayspectra
2.3.Theobscuredsample
(Corral et al. 2014). Several recent studies showed that a sec-
Figure2(left)showsthedistributionof0.3−10keVnetcounts ond component in the soft band, and a reflection component
for the CTKi (red) and CTNi (green) sources. As expected, all in the hard, are generally both needed by the data, even in the
these highly obscured sources are faint, and 40−50% of them case of low quality X-ray spectra (Brightman & Nandra 2012;
havebetween30and50counts,justabovethethresholdchosen Brightman&Ueda2012;Buchneretal.2014).
for the X-ray spectral analysis, with the CTKi sources having Because of the complex spectral shape and typical low
typicallyfewercounts.Asshownbythisplot,theabilitytopush flux levels, the definition of a source as CT, beyond the lo-
the analysisto very low countsis crucial, in orderto recovera cal Universe, is in general model dependent and therefore not
sizablesampleofsuchhighlyobscuredsourcesinrathershallow univocal: see for example the differences in CT samples in
X-raycatalogs. ChandraDeepFieldSouth(CDFS)datafromTozzietal.(2006),
Thetotalnumberofavailablespectroscopicredshiftsin the Georgantopoulos et al. (2008), Brightman & Ueda (2012),
obscuredsampleis19.Themajorityofthemhaveopticalspec- Georgantopoulos et al. (2013), summarized in Castelló-Mor
tra from the zCOSMOS survey (11 out of 19). Seven sources etal.(2013,thedisagreementistypically∼50%),orthecaseof
havebeenobservedwiththeIMACSspectrographonMagellan, IRAS09104+4109,aratherbrightandlowredshift(z = 0.442)
andonehasaSDSSspectrum.Theyareallclassifiedasnarrow hyper-luminousIRgalaxyforwhichtheclassificationasCThas
lineAGN(seeSect.6.1fortheopticalclassificationofthefinal beendebatedfor a decade(Franceschinietal. 2000;Piconcelli
CTK sample).Sourceswithoutspectroscopicclassification are etal.2007;Vignalietal.2011;Chiangetal.2013).
i
classifiedonthebasisoftheirspectralenergydistribution(SED) GiventhelowstatisticsavailablefortheX-rayspectrainour
bestfittemplatefromSalvatoetal.(2009,2011):15areclassi- sample,andtheneedtominimizethenumberoffreeparameters,
fied as type-2andfive as type-1(the photometricclassification wewillusetworathersimplemodels,inwhichthegeometryis
is however affected by large uncertainties). The redshift distri- fixed,basedonwhatweknowfromlocalCTsources.Thiswill
butionforCTK (in red)andCTN (ingreen)sourcesis shown allow us to constrain the amount of obscuration, the intrinsic
i i
inFig. 2(right)Thegrayshadedhistogramshowsthe distribu- luminosity,thestrengthofthescatteredemission,andtheEWof
tionofphotometricredshifts,wherewenotethatthefractionof theFeKαline.
sources with photometric redshiftincreases at higher redshifts, The first model (Tor hereafter) uses the TORUS tem-
being∼30%forz < 1 and∼60%for z > 1. We also underline plate presented in Brightman & Nandra (2012), which self-
that CTN sources tend to have higher redshift with respect to consistently reproduces photoelectric absorption, Compton
i
CTK ones. scatteringandlinefluorescence,asafunctionofN andsystem
i H
The XMM-Newton pn spectra analyzed here are obtained geometry(Fig.3left).Thesecond(Plhereafter)isbuiltupwith
as the sum of X-ray counts collected in different observations, anobscuredpower-law(PLCABS,Yaqoobetal.1997,thatprop-
performed during a period of ∼3.5 years. X-ray spectral vari- erly takes into account Compton scattering up to 15−20 keV),
ability, due to occultation by broad line clouds, and producing a PEXRAV component (Magdziarz & Zdziarski 1995) con-
large variation in observed column densities, has been found tributing with only the reflected component, and a redshifted
to be common in local AGN (e.g. Risaliti et al. 2009; see Gaussianline,withlineenergyfixedat6.4keV,toreproducethe
Torricelli-Ciamponietal.2014;andMarkowitzetal.2014,for Fe Kαline(Fig.3 right).Inbothmodels,a secondunobscured
systematicstudies).WecheckedthatallthesourcesintheCTK power-lawisincluded.
i
A137,page4of23
G.Lanzuisietal.:ComptonthickAGNintheXMM-COSMOSsurvey
Fig.2.Leftpanel:0.3−10keVnetcountsdistributionforCTK (red)andCTN (green)sources.Rightpanel:redshiftdistributionforCTK (red)
i i i
andCTN (green)sources.Thegrayshadedhistogramshowsthedistributionofphotometricredshifts.
i
Fig.3.Asanexample,thetwomodelsusedinthefitofsource2608atz=0.125areshown:Tor(left)andPl(right).IntheTormodeltheblueline
includestheprimarypower-law,thereflectioncomponentandfluorescencelines,allself-consistentlycomputedasafunctionofN andgeometry.
H
InthePlmodelthebluelinerepresentstheprimarypower-law,modifiedbyphotoelectricabsorptionandtakingintoaccountComptonscattering
(PLCABScomponentinXspec),ThemagentalinerepresentsthePEXRAVcomponent(i.e.reflectionproducedbyaslabofinfiniteopticaldepth
material),andthegreenlinerepresentsaGaussianemissionlinereproducingthefluorescentFeKαline.Inbothmodelstheredlinerepresentsthe
secondarypower-law,reproducingscatteredlightand/orthermalemissioninthesoftband.
Sinceforhighlyobscuredsourcestheprimarypower-lawis For the Tor model we have to make an assumption on the
observedonlyatthehighestenergies,ifobservedatall,inboth geometryof the system, using fixed torushalf-openingand in-
modelswe fixeditsphotonindexatthetypicalvalueof1.9for clination angles (θ = 60◦ and θ = 85◦ respectively). These
tor i
allthesources(includingthefourwith>100counts).Thepho- correspondtoasituationinwhichthelineofsightinterceptsthe
tonindexofthesecondarycomponentisalsofixedat1.9,while torusitself,andthetorushasanintermediateopeningangle.The
the ratio of its normalization with respect to the primary com- useofasmallerhalf-openingangleforthetorus(i.e.θ =30◦),
tor
ponentisa freeparameter,andisusedtoestimatetheintensity increasestheamountofreflection,whichcontributestotheflux
ofthescatteredcomponent2,expressedas%ofthefluxemitted belowtheabsorptioncut-off(seeFig.3left),andrequireshigher
by the soft component with respect to the flux emitted by the N valuestoreproducethesamedatapoints.Therefore,theN
H H
unobscuredprimarycomponent.AsmentionedinSect.2.2,the values reported in the following analysis, could be underesti-
scattered componentis constrained to be <10% of the primary mated, if the absorbingmaterial geometryis closer to a sphere
component. (seeBrightmanetal.2014).
For the Pl model, we fixed the ratio between the reflection
2 The secondary component must be a redshifted power-law andtheGaussianlinenormalizations,sothattheEWoftheemis-
(ZPOWERLWinXspec)inordertohavethenormalizationrelativeto sionlineisEW =1keVwithrespecttothereflectedcontinuum.
1keVrest-frame,asfortheTORUStemplateandthePLCABSmodel. This choice is made in order to minimize the number of free
A137,page5of23
A&A573,A137(2015)
parameters, and is justified by observational evidence, both at Given the relatively poor photon statistics available for all
low and high redshift (Matt et al. 1997; Guainazzi et al. 2000; our sources, and the availability of deeper X-ray data in the
Feruglioetal.2011),andbymodelpredictions(Ghisellinietal. COSMOS field, we decided to look more carefully into the
1994;Ikedaetal.2009;Murphy&Yaqoob2009,seediscussion X-ray properties of the small sample of CTK sources, plus
i
inSect.4.1). the 5 border-line sources. The idea is to collect all the avail-
Therefore the number of free parameters of the Tor model able X-ray data for this subsample, with the aim of improving
is three: the obscuring N and normalization of the primary the available statistics, and possibly confirm, with better data,
H
component(theTORUStemplate)andthenormalizationofthe theCTnatureofthesesources.
secondary component(ZPOWERLW). The number of free pa-
rameters for the Pl model is four: the obscuring N and nor-
H 4.1.XMM-Newtonpn,MOSandChandraspectra
malization of the primary component (PLCABS), the normal-
ization of the the secondary component (ZPOWERLW) and For the majority of our sources, XMM-Newton MOS1 and
the normalization of the reflection plus emission line complex MOS2 data are available, each with roughly the same net ex-
(PEXRAV+ZGAUSS). posureofpn.ThesumofMOS1andMOS2spectratypicallyin-
We stress that the Tor model is expected to give more ac- creasestheavailablenumberofcountsbyafactorof1.5.Weex-
curate NH and intrinsic luminosities for these highly obscured tractedthespectraforthecandidateCTsourcesfromtheMOS1
sources. This is because its reflection component is computed andMOS2camerasineachobservation,followingtheprocedure
for a more physically motivated geometry with respect to the developedinRanallietal.(2013)fortheXMM-CDFS.Wethen
PEXRAV component, in which the reflection is computed as mergedthespectratogether(MOS1+MOS2)producingaverage
producedbyaninfiniteslabofneutralmaterialwithagivenin- matricesusingstandardHeaSOFTtools3.
clination. The PEXRAV component was indeed introduced to Furthermore, merging the original C-COSMOS data (Elvis
reproducereflectedemissionfromaface-onaccretiondisk,and et al. 2009) with the new Legacydata (Civano et al. 2014, ob-
notreflectionfromanobscuringtorus.ThePlmodelismeantto servationsconcludedin April2014),wehavea Chandracoun-
mimic,asmuchaspossible,theglobalshapeoftheTormodel, terpart for all our sources. The Chandra observation of the
andtocomputeindependentlytheEWoftheFeKαline,which COSMOS field is muchdeeper(∼160ks average)andaffected
is not an output in the Tor model. Then, the Fe Kα line EW by much lower background levels. Therefore, even if the net
canbeusedasfurtherevidenceofthepresenceofCTabsorbing number of counts available from the Chandra spectra is typ-
material,anditsevolution,e.g.with NH,canbecomparedwith ically on the same order as the XMM-Newton pn one (in the
modelpredictions(seeSect.4.1). range 30−100, thus doubling the number of counts available),
ThePEXRAVcomponentisalsoknowntoreproduceadif- thesignal-to-noiseratioisalwaysmuchhigher.Theresultsofthe
ferentglobalcontinuumshapeandComptonhump,withrespect joint fit (pn+MOS+Chandra)are indeed more reliable and the
toCTdedicatedmodeling,i.e.MonteCarlosimulationsthatas- parametersbetter constrained (average error on N of 35% in-
H
sume a toroidal geometry and self-consistently include reflec- steadof56%),andtheycanbeusedasana-posterioritestofthe
tionandscatteringintheiransatz(e.g.theTorustemplatefrom reliabilityoftheXMM-pnonlyfits.Thesystematicdifferences,
Brightman et al. 2014, or MYTORUS model from Murphy & influxandphotonindex,betweenChandraandXMM-Newton,
Yaqoob 2009). The difference is significant enough to poten- foundfor faint sources (Lanzuisi et al. 2013a) are too small to
tially affect fitting resultsfor high signal-to-noisedata. For our haveanymeasurableeffect,giventhedataqualityofthesample
low signal-to-noise data, we checked that the overall contin- discussedhere.
uum propertiesobtained from the two models used here are in We performed a joint fit of the three spectra for each
good agreement. We show in Appendix B the results of this source, using the same models described in Sect. 3. For 8 out
comparison. of10sources,classifiedasCTfromtheXMM-Newtonpnspec-
trum alone, we can confirm, through the simultaneous fit of
Chandra and XMM-Newton pn and MOS spectra, that the N
4. Results H
is indeed >1 × 1024 cm−2, and therefore in the CT regime.
ThefitresultsaresummarizedinTable1fortheentireobscured ThereforetheywillbeincludedinthefinalCTKsample(CTK
f
sample. All the parameters, except for the Fe Kα line EW, are hereafter).TwosourcesfromtheoriginalCTK sample,namely
i
derived from the Tor model fit. The ten sources belonging to XID217andXID 60314,haveinsteada bestfitvalueof N ∼
H
the CTKi sample (thefirst groupofsourcesin Table1) showa 3−4 × 1023 cm−2: whenobservedwith deeperX-raydata they
CT absorber from both fits, and five of them have a detection fall into the Compton thin regime and are excluded from the
of an emission Fe line with EW consistent with the CT nature CTK sample, and will be included in the final CTN sample
> f
(i.e. EW ∼ 1 keV rest frame). For three sources, the NH is un- (CTNf hereafter).
constrainedattheupperend(theNH upperlimitoftheTORUS Furthermore, two of the border-line sources, namely
template is 1026 cm−2). Source XID 2608, the lowest redshift, XID 54490 and XID 70145 show, in the joint fit, a column
brightestCTcandidate,wasidentifiedasaCTcandidatealready density which is actually higher than the one measured by the
inHasingeretal.(2007,thesocalledpinksource,foritspeculiar XMM-Newton pn fit alone, and in the CT regime,and a strong
X-raycolors),andinMainierietal.(2007). Fe Kα line, again consistent with these sources being CT. We
Five sources (the second group of sources in Table 1) are therefore include them in the CTK sample, see Table 2. The
f
borderline:theyare eitherseen as CT fromonemodelbutnot otherthreeborder-linesourcesshowhighlevelsofobscuration,
bytheother(seeAppendixB),ortheyareinbothmodelsvery butinthe Comptonthin regime,andarethereforenotincluded
closetothedividingline(NH =1024cm−2)andwithlargeupper intheCTKf sample.
errorbars, that make them fully consistent with being CT. The Weconcludethattheselectionmethod,basedonthesimple
remaining 24 sources are highly obscured but in the Compton two power law model, applied to XMM-Newton spectra alone
thinregime.AllofthelatterhaveonlyanupperlimitfortheEW
oftheFeline. 3 http://heasarc.nasa.gov/lheasoft/
A137,page6of23
G.Lanzuisietal.:ComptonthickAGNintheXMM-COSMOSsurvey
Table1.ResultsoftheXMM-Newtonpnspectralfitforthehighlyobscuredsources,sortedfordecreasingN .
H
XMMID z Cl. C NH F2−10 Log(LI) Log(LO) Sc.% EW Cstat/d.o.f.
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
60361∗ 1.73 Ph 49 6.35+... 0.92+0.05 45.15+... 43.46+1.50 0.36±0.14 2.29+2.29 175.6/201
−4.80 −0.13 −0.61 −0.03 −1.79
70082∗ 2.429 Ph 34 3.19+5.53 1.47+0.19 45.34+0.43 43.65+0.15 <2.10 <3.07 238.7/255
−1.48 −0.16 −0.26 −0.03
60314 1.107 Ph 49 3.15+4.05 2.53+0.29 45.34+0.35 43.38+0.08 <5.20 <0.39 156.9/157
−2.28 −0.28 −0.69 −0.08
217 0.66 Sp 56 2.82+... 1.92+0.23 44.47+... 42.97+0.04 <5.60 0.61+0.77 144.7/157
−2.53 −0.19 −1.25 −0.06 −0.43
60152∗ 0.579 Sp 74 1.97+... 3.36+0.27 44.48+... 43.04+0.03 <0.80 0.57+0.63 164.0/162
−1.21 −0.42 −0.43 −0.06 −0.43
60342∗ 0.941 Ph 33 1.48+2.21 1.34+0.25 44.67+0.39 43.04+0.05 <1.22 <0.32 145.4/143
−1.40 −0.35 −1.32 −0.05
5511∗ 1.023 Ph 101 1.26+2.24 2.03+0.21 44.70+0.44 43.42+0.03 4.10±3.50 <0.38 215.5/246
−1.19 −0.31 −1.29 −0.04
54514∗ 0.707 Sp 121 1.23+0.55 4.45+0.45 44.51+0.17 43.57+0.07 1.50±0.70 0.35+0.43 214.0/220
−0.88 −0.76 −0.56 −0.06 −0.25
60211∗ 0.511 Sp 38 1.11+2.87 0.88+0.21 43.94+0.55 42.64+0.06 1.42±0.45 <0.23 122.6/144
−0.21 −0.51 −0.26 −0.23
2608∗† 0.125 Sp 142 1.10+0.59 4.72+0.79 43.30+0.19 42.17+0.05 0.24±0.08 0.50+0.33 256.8/299
−0.26 −0.96 −0.16 −0.10 −0.25
202 1.32 Sp 115 1.06+0.37 1.84+0.21 44.65+0.13 43.73+0.05 1.87±1.32 <0.92 233.0/270
−0.28 −0.19 −0.14 −0.04
60043 1.73 Ph 35 0.86+2.10 0.94+0.10 44.51+0.54 43.33+0.11 <3.80 <1.31 187.0/200
−0.59 −0.12 −0.53 −0.07
54490∗ 0.908 Sp 63 0.82+1.08 3.60+0.46 44.69+0.37 43.69+0.05 <2.60 <0.93 196.5/206
−0.40 −0.63 −0.31 −0.07
258 1.748 Ph 66 0.80+0.93 2.30+0.38 45.09+0.34 44.10+0.07 <2.62 <1.51 198.8/203
−0.36 −0.46 −0.27 −0.05
70145∗ 2.548 Ph 50 0.79+1.24 0.87+0.11 44.69+0.41 43.96+0.10 1.82±0.87 <3.67 135.5/111
−0.39 −0.13 −0.32 −0.08
60024 1.147 Sp 55 0.70+0.50 1.44+0.21 44.10+0.24 43.48+0.06 <3.59 <0.88 175.2/165
−0.38 −0.35 −0.37 −0.08
5006 2.417 Sp 80 0.67+0.40 2.25+0.22 45.21+0.21 44.24+0.08 <2.82 <1.55 293.4/293
−0.29 −0.32 −0.26 −0.06
5357 2.189 Ph 56 0.63+0.33 1.96+0.37 45.03+0.20 44.13+0.09 <3.60 <0.72 129.5/123
−0.19 −0.44 −0.17 −0.06
5185 2.19 Ph 32 0.57+0.90 1.94+0.45 45.05+0.42 44.12+0.12 <5.34 <2.46 105.7/111
−0.38 −0.40 −0.52 −0.10
5130 1.553 Sp 79 0.55+0.39 1.97+0.23 44.71+0.23 43.89+0.05 3.05±1.83 <1.61 224.5/251
−0.22 −0.27 −0.23 −0.05
348 0.779 Ph 36 0.52+0.84 0.47+0.23 44.39+0.42 43.64+0.07 <2.38 <0.21 179.8/190
−0.24 −0.33 −0.30 −0.09
54541 2.841 Sp 35 0.51+0.35 2.06+0.40 45.23+0.25 44.49+0.14 <2.63 <1.53 114.6/82
−0.28 −0.64 −0.39 −0.11
5534 2.12 Ph 33 0.50+0.83 1.51+0.26 44.89+0.43 44.08+0.07 <10.00 <1.71 134.2/129
−0.33 −0.27 −0.48 −0.05
60492 1.914 Ph 41 0.48+3.39 1.16+0.24 44.67+0.91 43.84+0.08 <2.91 <0.75 193.5/208
−0.30 −0.22 −0.45 −0.07
70084 1.27 Ph 54 0.46+0.58 2.43+0.36 44.51+0.35 43.93+0.05 <4.66 <1.36 132.8/119
−0.23 −0.38 −0.31 −0.06
5033 0.67 Ph 87 0.46+0.27 8.98+1.70 44.66+0.21 43.87+0.08 <0.93 <0.40 132.8/150
−0.18 −1.62 −0.24 −0.08
473 2.148 Ph 50 0.45+0.97 1.00+0.15 44.68+0.50 43.99+0.06 5.80±4.20 <2.31 203./188
−0.28 −0.18 −0.44 −0.07
70135 2.092 Ph 54 0.44+0.72 1.68+0.29 44.88+0.42 44.18+0.07 <9.93 <1.08 211.0/197
−0.28 −0.32 −0.46 −0.07
5496 0.694 Sp 81 0.44+0.35 6.86+1.35 44.48+0.26 43.84+0.08 <1.76 <0.53 166.5/152
−0.25 −1.47 −0.41 −0.12
5007 2.390 Ph 31 0.41+0.47 1.24+0.21 44.89+0.33 44.07+0.08 <3.63 <1.65 216.2/198
−0.23 −0.21 −0.37 −0.06
5222 0.332 Ph 120 0.39+0.16 10.05+0.96 44.03+0.15 43.37+0.05 1.78±0.53 <0.37 243.3/235
−0.13 −1.73 −0.19 −0.07
70007 1.848 Sp 38 0.39+0.50 0.77+0.11 44.41+0.36 43.79+0.05 7.30±5.15 <2.15 125.0/109
−0.24 −0.10 −0.43 −0.06
302 0.186 Sp 72 0.39+0.37 1.87+0.33 42.70+0.29 42.16+0.07 4.20±1.70 <1.43 157.1/166
−0.20 −0.47 −0.35 −0.10
5042 2.612 Sp 90 0.39+0.21 1.31+0.19 45.02+0.19 44.29+0.06 <7.66 <0.62 227.3/231
−0.16 −0.23 −0.25 −0.07
60436 2.313 Ph 48 0.37+0.37 1.07+0.16 44.76+0.30 44.06+0.07 <5.92 <0.49 182.9/174
−0.18 −0.15 −0.30 −0.05
5562 2.626 Sp 64 0.32+0.28 0.90+0.13 44.78+0.27 44.21+0.06 <10.00 <0.90 145.9/160
−0.19 −0.16 −0.40 −0.06
5427 1.177 Sp 55 0.32+0.20 3.42+0.52 44.63+0.21 43.99+0.06 <1.32 <0.95 148.4/183
−0.13 −0.64 −0.25 −0.07
70216 1.577 Ph 36 0.31+0.36 0.94+0.17 44.33+0.34 43.70+0.07 <4.52 <0.84 147.5/135
−0.20 −0.19 −0.47 −0.07
5014 0.213 Sp 155 0.31+0.11 20.70+2.80 43.92+0.14 43.31+0.05 1.71±0.35 <0.54 209.4/218
−0.09 −3.50 −0.17 −0.07
Notes.ThefirstgroupincludesthetensourcesbelongingtotheCTK sample,thesecondandthirdgroupsincludethe29sourcesbelongingtothe
i
CTN sample,withthefivesourcesinthesecondgroupbeingborderline.Column:(1)XMM-NewtonIDnumber;(2)redshift;(3)classification
i
(photometricorspectroscopicredshift);(4)0.3−10keVnetcounts;(5)columndensityinunitsof1024cm−2;(6)2−10keVobservedfluxinunits
of10−14 ergcm−2 s−1;(7)Logofthe2−10keV restframeintrinsicluminosity,inunitsof ergs−1.Theerrorsareobtainedtakingintoaccount
theerrorsontheobserved luminosityandonthecolumn density;(8) Logof the2−10keV restframeobserved luminosity, inunitsofergs−1;
(9)intensityofthescattercomponent,relativetotheprimarypower-law;(10)rest-frameequivalentwidthoftheFeKαline,inunitsofkeV,from
thePlfit;(11)bestfitCstat/d.o.f.∗SourceswhoseCTnatureisconfirmedbythecombinedanalysisofXMM-Newton(pnandMOS)andChandra
data(seeTable2).†Source2608wasalreadyidentifiedasaCTcandidateinHasingeretal.(2007)andMainierietal.(2007).
hasaselection efficiencyof ∼80%:giventheaboveresults,we decreasesstronglywiththenumberofcounts.Indeedourlowest
indeedmeasurea20%contaminationbynonCTsourcesinthe redshiftsource (XID 2608,z = 0.125)is also the one with the
sample, and 20% of missed CT outof the sample. This can be largest number of counts, and would be probably classified as
comparedwithresultsfromBrightman&Ueda(2012):through a soft source (see the spectrum in Fig. 4), in case of shallower
simulations, they estimated a similar, rather constant selection X-raydata.
efficiency,forsourceswithz > 1(inthisN regimetheshiftof Table 2 summarizes the final results for the CTK sam-
H f
theintrinsicspectrumthroughtheobservedbandhelpsinidenti- ple (8 from the CTK sample and 2 from the border-line sam-
i
fyingCTsources),whiletheselectionefficiencyatlowerredshift ple),orderedbyincreasingredshift.Column2showsthe IDof
A137,page7of23
A&A573,A137(2015)
Table2.ResultsofthejointfitofXMM-Newton-pn+MOS+ChandraspectrafortheCTK sample.
f
XMMID Ch.ID z TotC NH F2−10 Log(LI) Log(LO) Sc.% EW C/d.o.f.
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
2608 482 0.1251 271 1.92+0.50 4.72+0.46 43.68+0.10 42.10+0.03 0.34±0.15 0.85+0.19 542.6/579
−0.46 −0.24 −0.12 −0.04 −0.45
60211 368 0.5112 85 1.16+0.43 1.17+0.15 43.45+0.14 42.72+0.04 1.62±0.90 <0.61 158.2/217
−0.21 −0.22 −0.12 −0.07
60152 298 0.5792 121 2.10+0.54 1.87+0.35 44.15+0.12 43.07+0.07 <0.41 0.89+0.32 189.4/236
−0.46 −0.25 −0.11 −0.03 −0.59
54514 New 0.7072 265 1.29+0.28 4.18+0.37 44.48+0.09 43.57+0.04 1.58±0.48 0.50+0.34 403.9/416
−0.26 −0.44 −0.11 −0.05 −0.32
54490 284 0.9082 147 1.12+0.40 2.25+0.47 44.74+0.15 43.48+0.08 <1.50 0.54+0.19 258.7/261
−0.27 −0.35 −0.16 −0.10 −0.39
60342 576 0.9413 72 1.01+0.69 1.00+0.12 44.11+0.23 43.10+0.05 <1.22 <0.71 195.4/196
−0.30 −0.11 −0.17 −0.06
5511 New 1.0233 202 1.08+0.57 1.52+0.13 44.55+0.18 43.55+0.03 6.78±2.50 <0.59 401.3/422
−0.34 −0.12 −0.17 −0.03
60361 New 1.733 120 3.92+... 1.01+0.10 44.75+... 43.48+0.04 0.59±0.45 1.47+0.50 335.2/382
−2.84 −0.09 −0.57 −0.05 −0.76
70082 747 2.4293,∗ 102 1.20+0.87 1.47+0.09 44.95+0.25 43.54+0.11 <2.01 <1.301 402.0/418
−0.47 −0.13 −0.25 −0.09
70145 708 2.5483 123 1.28+0.54 0.87+0.23 44.63+0.16 43.79+0.05 2.72±0.93 0.89+0.67 140.1/138
−0.40 −0.11 −0.18 −0.06 −0.57
Notes.Sourcesareordered byincreasingredshift.Column: (1)XMM-NewtonIDnumber; (2)Chandra ID;(3)redshift ((1) spectroscopic from
SDSS;(2) spectroscopicfromzCOSMOS-20k;(3) photometricfromSalvatoetal.(2011),(∗) tentativespectroscopicredshiftofz = 2.710from
zCOSMOS-deep,seeSect.6.1);(4)total0.3−10keVnetcounts;(5)columndensityinunitsof1024 cm−2;(6)2−10keVobservedfluxinunits
of10−14ergcm−2s−1;(7)Logofthe2−10keVrestframeabsorptioncorrectedluminosity,inunitsofergs−1.Theerrorsareobtainedtakinginto
account theerrorsontheobservedluminosityandonthecolumndensity;(8)Logofthe2−10keVrestframeobservedluminosity, inunitsof
ergs−1;(9)intensityofthescatteredcomponent,relativetotheprimarypower-law;(10)equivalentwidthoftheFeKαline,inunitsofkeV,from
thePlfit;(11)bestfitCstat/d.o.f.
the Chandra counterpart, if already available in the catalog of which the pn+MOS+Chandrajoint fit has been performedare
Elviset al. (2009), Col. 3 show the redshift(andits origin)for showninblack,whiletheremainingareingray.Itiswellknown
eachsource,whileCol.4 showsthetotalnumberofnetcounts that,astheEWismeasuredagainstasuppressedcontinuum,its
(pn+MOS+Chandra). The rest of the columns show the same valueincreaseswithincreasingcolumndensity,fromafewtens
parametersofTable1,thistimeobtainedfromthejointfitofall of eV in type-1 AGN (Bianchi et al. 2007), to values of sev-
theinstruments. eralhundredeVtoover1keVinCTsources(Mattetal.1997;
Finally, we co-added all the available counts, following Guainazzietal.2000).TheredshadedareainFig.6(left)shows
Iwasawaetal.(2012).WeshowinFig.4theresultingtotalspec- theexpectedFeKαEWasafunctionofN ascomputedinthe
H
tra for all the CTK sources. The spectra are de-convolvedfor torusmodelof Ikeda etal. (2009), for a torushalf-openingan-
f
each instrumental response, then merged together, and shown gle of 30◦ and an inclination angle in the range 60−84◦. The
in units of photons/keV/s/cm2. In this way we show the intrin- green shaded area shows the predictions from the torus model
sicspectrum,freefromthedistortingeffectsoftheinstrumental of Murphy & Yaqoob (2009) for a torus half-opening angle
response,withoutimposinganypreferredmodel,and therefore of 60◦ and similar inclination angles (60−84.26◦). The num-
providinga moreobjectivevisualizationof the fluxedspectra4. berofdetectedlinesinoursampleissmall(andallaboveN =
H
Allsourcesappeartobestronglyabsorbed,uptorestframeen- 1024cm−2).Thereforewedecidedtoinclude55highlyobscured
ergiesof7−10keV.Furthermore,morethanhalfofthesources sources(incyan)takenfromasampleof88localSeyfertgalax-
show some evidence of an emission line in correspondence of iesanalyzedinFukazawaetal.(2011)observedwithSuzaku,to
theexpectedrestframeFe Kαline energy(dashedline ineach putinacontextourresult.Thetwosamplespopulatedifferently
panel).Indeed,sixoutoftenCTK sourceshaveadetectionof the EW-N plane, but are in very good agreement where they
f H
the Fe Kα line in the joint fit (Col. 10 of Table 2). Our result overlap (e.g. around N ∼ 1024 cm−2). All together, these ob-
H
isstrengthenedbythefactthatalloursourceshavean N con- servationalresultsshowthat,whileforCompton-thinabsorbers
H
strainedtobe>1023.7cm−2withinthe90%error-bar. the observedEW lies between the predictionsof the two mod-
Figure 5showsthe finaldistributionof 2−10keVintrinsic, els, and closer to the prediction of the Ikeda et al. model at
absorption corrected luminosity vs. column density, for all the NH ∼ 1024 cm−2 (i.e. EW ∼ 0.5−1 keV), the increase in EW
sources in the sample (the CTK in red and CTN in green). saturates above 1024 cm−2, and the EW remains in the range
f f
Asexpectedbythe factthatthese sourcesare highlyobscured, ∼0.5−1.5keVevenforNH =4×1024cm−2,closertothepredic-
and that the XMM-COSMOS survey has a shallow X-ray lim- tionsoftheMurphy&Yaqoobmodel.
iting flux, almost all sources are in the QSO regime (L2−10 > Figure 6 (right) shows the fraction of the soft component,
1044 ergs−1), with a few exceptions due to the very low red- with respect to the primary power-law, as a function of N . If
H
shift sources. For the most obscured source of the CTK sam- the soft component is indeed produced by scattered light, and
f
ple (XID 60361) the upper boundary of the N distribution is assumingthatthecoveringfactorofthetorusincreaseswithob-
H
notconstrained,duetotheverylowqualityofthespectrum(see scuration,thescatteredfractionisexpectedtodecreasewithin-
Fig.4).Asaresult,alsothe2−10keVabsorptioncorrectedlumi- creasing N (Brightman & Ueda 2012). There is indeed some
H
nosityhasnoupperboundary.However,assumingasupperlimit evidencethat,aboveN =1.5×1024cm−2,thetypicalscattered
H
ofNH =1025cm−2,wecanestimatetheresultingLog(L2−10)up- fraction is significantly lower (below 1%) with respect to less
perlimittobe∼45.4ergs−1. severely obscured sources. However, given the limited sample
Figure6 (left)showsthe restframeequivalentwidthof the available,andthelargenumberofupperlimitsonthescattered
FeKαlineasafunctionofthecolumndensity N .Sourcesfor fraction,plusthelimiteddynamicalrangeprobed,(especiallyin
H
N )wecannotdrawanyfirmconclusioninthiscase.Asimilar
H
4 SeeAppendixCforanexampleofaspectrumplusmodelplot. butmore significanttrend of decreasingscattered fractionwith
A137,page8of23
G.Lanzuisietal.:ComptonthickAGNintheXMM-COSMOSsurvey
Fig.4.Fluxed,mergedpn+MOS+Chandra spectraoftheCTK sources, ordered byincreasingredshift.TheXMMsourceIDand redshiftare
f
labeledineachpanel.Thedashedlinemarkstheexpectedlocationofthe6.4keVFeKαline.
increasing N has been found in Brightman et al. (2014), in a from SED fitting. This information can then be used in order
H
muchlargersampleofCTsources,intworedshiftbins. totestourselectionmethodforhighlyobscuredsources.
The absorption-correctedX-ray luminosity, after the appli-
cation of a bolometric correction k , is generally considered
4.2.Bolometricluminosity bol
agoodmeasureoftheAGNbolometricluminositybecauseitis
Given the wealth of multi-wavelength data available in the lessaffectedbyobscuration/reprocessingthanotherwavelengths
COSMOSfield,andthegreateffortalreadyputinthedataanal- TheX-raybolometriccorrection,rangingfromfactor10to100,
ysis(seeSect.2.1),wecanrelyontheindependentmeasurement isknowntobe luminositydependent(i.e.brighterobjectshave
of the bolometric luminosity (L ), available for our sources largerX-rayk ,Marconietal.2004;Hopkinsetal.2007;L12).
bol bol
A137,page9of23
A&A573,A137(2015)
an overestimate of the obscuration would lead to an overesti-
mate of the X-ray luminosity, that is finally multiplied by the
10−100factorofthek .Therefore,wecaninterpretthefactof
bol
having a good agreement between the two quantities (within a
factor of ∼2), as a furtherconfirmationof the reliability of our
N estimates.
H
5. SMBHandhostpropertiesofhighlyobscured
sources
AnimportantgoalofsearchesforhighlyobscuredAGN,inthe
contextofAGN/galaxyco-evolutionmodels,istounderstandif
they occupy a special place in the proposed evolutionarytrack
that goes from gas inflow (driven by mergers or internal pro-
cesses, e.g. Hernquist 1989; Ciotti & Ostriker 1997; Hopkins
et al. 2006) to highlyaccreting/highlystar-formingsystems, to
unobscuredquasarinredellipticalsorifsuchevolutionarytrack
existsatall.Thekeytotestthesemodelsresidesinlookingfor
the distributionof physicalSMBH andhostproperties,suchas
BHmassandEddingtonratio(λEdd6),stellarmass(M∗)andspe-
Fig.5. 2−10 keV rest frame, absorption corrected luminosity vs. cific star formation rate (SFR/M∗, sSFR). For example, if this
column density. In red are shown CTK sources, and in green evolutionaryscenariois true,we expecttosee these highlyob-
f
CTN sources. scuredsystemstohavelowerBHmasses,accreteclosetoorpos-
f
siblyabovetheEddingtonlimit,andresideinhostswithgreater
starformationcomparedtounobscuredAGN.Ontheotherhand,
Evenifthisgeneraltrendisrobust,differentk −L relations ifthesesourcesareseenasobscuredonlyforgeometricaleffects,
bol bol
have been derived in the literature for different sample selec- nosuchtrendsareexpected.
tions of AGN, and the scatter is usually large. We decided to
useforoursourcesthe 2−10keVkbol−Lbol relationcomputed 5.1.MBH,λEddandM∗
inL12,whichisderivedfromalargesampleofX-rayselected
type-2AGN from the same XMM-COSMOS catalog. The un- Because the sample of highlyobscuredsourcesis rather small,
certaintiesink isestimatedtobe∼0.20dex.TheX-raybased weanalyzedCTK andCTN sourcestogether.Westresshow-
bol f f
L is shownin they-axisof Fig.7,togetherwith the L that everthatthedistributionoftheparametersdiscussedinthissec-
bol bol
wouldbeobtainedusingtheobservedL2−10 (error-bars). tionisverysimilarbetweenCTKf andCTNf.Wehaveavailable
On theotherhand,itis possibletoestimate the AGNbolo- M (andhenceλ )for25outof39oftheobscuredsources,
BH Edd
metricluminosity,assumingthattheAGNmid-IRluminosityis fromL12.ThesewereobtainedrescalingtheM∗,obtainedfrom
anindirectprobe(throughabsorptionandisotropicre-emission) the SED fitting and corrected for the bulge-to-total mass frac-
of the accretion disk optical/UV luminosity. The accretion lu- tion, f (whenmorphologicalinformationwasavailable,from
bulge
minosityis thencomputedthroughthe SED fitting,by firstde- theZESTcatalog,Scarlataetal.2007;Sargentetal.2007).The
composingthehostandAGNemissionandthenintegratingthe scalingrelationofHäring&Rix(2004)wasadopted,takingalso
AGN IR luminosity L , e.g. between 1 and 1000 μm, correct- intoaccountitsredshiftevolutionasestimatedinMerlonietal.
IR
ingtheIRemissiontoaccountfortheobscuringtorusgeometry (2010).Typicaluncertaintiesareestimatedtobe∼0.5dex.
and optical thickness (see Pozzi et al. 2007, 2010). The bolo- For the remaining sources, we rely on the M∗ computed
metric luminosity is then obtained adding the absorption cor- in B12 or D147, and estimated the M following the same
BH
rectedtotalX-rayluminosity,computedinthe0.5−500keVen- stepsdescribedinL12.Forallthesourceswithoutmorphologi-
ergy range5. We collected SED based bolometric luminosities calinformation,fromwhichanestimateofthe f needtobe
bulge
fornineoutoftenCTK sourcesand19outof29CTN either inferred, the M is considered to be an upper limit. We also
f f BH
from L12 or D14. There is a good agreementbetween the two derived the λ for all our sources, using the M described
Edd BH
quantities,withasmallsystematicshiftinthedirectionofhigher above, and the X-ray based bolometric luminosities illustrated
X-ray based L . The agreementis even more striking consid- inSect.4.2.
bol
ering the large differencesbetween the Lbol estimated from the We then comparedthe distributionof these quantities, with
observedandabsorptioncorrectedL2−10,shownwiththevertical theoneofacontrolsampleofunobscuredAGN.MBHforalarge
error-bars.Thedottedlinesinthefiguremarksthe1σdispersion sampleoftype-1AGNinXMM-COSMOShavebeencomputed
of0.31dex. using virial estimators, in several papers. We use the compila-
We stressthatthecontributionoftheX-ray,absorptioncor- tion published in Rosario et al. (2013) for ∼289 type-1 AGN.
rected luminosity to the L from SED fitting is very small To compare these distributions, the two samples have to be
bol
(∼15%), and therefore the SED based Lbol is largely indepen- matched in intrinsic X-ray luminosity (42.7 <∼ L2−10 <∼ 45.6)
dentofthe N asestimatedfromtheX-ray.Ontheotherhand,
H
theX-raybasedLbol isextremelysensitivetotheNH,giventhat 6 DefinedasLbol/LEdd,whereLEddistheEddingtonluminosityassoci-
atedwithagivenBHmass,i.e.LEdd =1.3×1038ergs−1perM(cid:7).
5 L0.5−500 isestimatedfromtheobserved L2−10 assumingapower-law 7 We verified that, when in common, the M∗ estimates between
spectrumwithΓ=1.9andanexponentialcut-offat200keV.Thevalue the3catalogs arealways ingood agreement, within0.3 dex, withno
foundfortheratioL0.5−500/L2−10is=4.1. systematicshift.
A137,page10of23