The bolometric output and host-galaxy properties of obscured AGN in the XMM-COSMOS survey
E. Lusso, A. Comastri, C. Vignali, G. Zamorani, E. Treister, D. Sanders, M. Bolzonella, A. Bongiorno, M. Brusa, F. Civano, R. Gilli, V. Mainieri, P. Nair, M. C. Aller, M. Carollo, A. M. Koekemoer, A. Merloni, J. R. Trump
aa r X i v : . [ a s t r o - ph . C O ] A ug Astronomy&Astrophysicsmanuscript no. 17175 c (cid:13)
ESO 2018November 11, 2018
The bolometric output and host-galaxy properties of obscuredAGN in the XMM-COSMOS survey
E. Lusso , ⋆ , A. Comastri , C. Vignali , , G. Zamorani , E. Treister , , D. Sanders , M. Bolzonella , A. Bongiorno ,M. Brusa , F. Civano , R. Gilli , V. Mainieri , P. Nair , M. C. Aller , M. Carollo , A. M. Koekemoer , A. Merloni , ,and J. R. Trump . Dipartimento di Astronomia, Universit`a di Bologna, via Ranzani 1, I-40127 Bologna, Italy. INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, I-40127 Bologna, Italy. Institute for Astronomy, 2680 Woodlawn Drive, University of Hawaii, Honolulu, HI 96822, USA. Max Planck Institut f¨ur extraterrestische Physik, Giessenbachstrasse 1, 85748 Garching, Germany. Excellence Cluster Universe, TUM, Boltzmannstr. 2, D-85748, Garching bei M¨unchen, Germany. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,Cambridge, MA 02138, USA. ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching bei M¨unchen, Germany. ETH Z¨urich, Physics Department, CH-8093, Z¨urich, Switzerland. Universidad de Concepci´on, Departamento de Astronom`ıa, Casilla 160-C, Concepci´on, Chile. University of California Observatories / Lick Observatory, University of California, Santa Cruz, CA 95064. Space Telescope Science Institute, Baltimore, Maryland 21218, USA.Accepted version August 24, 2011
ABSTRACT
We present a study of the multi-wavelength properties, from the mid-infrared to the hard X–rays, of a sample of 255 spectroscop-ically identified X–ray selected Type-2 AGN from the XMM-COSMOS survey. Most of them are obscured the X–ray absorbingcolumn density is determined by either X–ray spectral analyses (for the 45% of the sample), or from hardness ratios. Spectral EnergyDistributions (SEDs) are computed for all sources in the sample. The average SEDs in the optical band is dominated by the host-galaxylight, especially at low X–ray luminosities and redshifts. There is also a trend between X–ray and mid-infrared luminosity: the AGNcontribution in the infrared is higher at higher X–ray luminosities. We calculate bolometric luminosities, bolometric corrections, stel-lar masses and star formation rates (SFRs) for these sources using a multi-component modeling to properly disentangle the emissionassociated to stellar light from that due to black hole accretion. For 90% of the sample we also have the morphological classificationsobtained with an upgraded version of the Zurich Estimator of Structural Types (ZEST + ). We find that on average Type-2 AGN havelower bolometric corrections than Type-1 AGN. Moreover, we confirm that the morphologies of AGN host-galaxies indicate that thereis a preference for these Type-2 AGN to be hosted in bulge-dominated galaxies with stellar masses greater than 10 solar masses. Key words. galaxies: active – galaxies: evolution – quasars: general – methods: statistical
1. Introduction
The formation and growth of supermassive black holes(SMBHs) and their host-galaxies are related processes. Thisis supported by various observational signatures: the SMBHmass correlates with the mass of the bulge of the host-galaxy (Magorrian et al. 1998; Marconi & Hunt 2003), withthe velocity dispersion of the bulge (Ferrarese & Merritt2000; Tremaine et al. 2002), and with the luminosity of thebulge (Kormendy & Richstone 1995). From theoretical mod-els, AGN seem to be able to switch o ff cooling flows inclusters (Hoeft & Br¨uggen 2004) and star formation in galax-ies (Somerville et al. 2008), with the result that the SMBHmass is related to the host-galaxy bulge mass (or vice-versa).Feedback between an accreting SMBH and the host-galaxymay play an important role in galaxy formation and evolu-tion. Understand the role of feedback is a demanding prob-lem for both observers and theorists. Semi-analytical modelsand hydrodynamical simulations have been developed to attemptto link the formation and evolution of SMBHs to the struc- ⋆ [email protected] ture formation over cosmic time. These models invoke di ff erentmechanisms to fuel the central SMBHs and to build the host-galaxy bulges, such as major / minor mergers of galaxies (e.g.,Corbin 2000; Kau ff mann & Haehnelt 2000; Springel et al. 2005;Hopkins et al. 2006), smooth accretion of cold gas from fila-mentary structures (e.g., Kereˇs et al. 2009; Dekel et al. 2009),or accretion of recycled gas from dying stars (e.g., Ciotti et al.2010). Several works also consider radiative feedback whichcan reproduce two important phases of galaxy evolution, namelyan obscured-cold-phase, when the bulk of star formation andblack hole accretion occurs, and the following quiescent hotphase in which accretion remains highly sub-Eddington andunobscured (e.g., Sazonov et al. 2005; Lusso & Ciotti 2011).In some of these models, the obscured / unobscured AGN di-chotomy is more related to two di ff erent phases of galaxy evo-lution (Hopkins et al. 2008), rather than to an orientation e ff ect(i.e., unified model scheme).The obscured / unobscured time dependent AGN dichotomycould be related to the bimodality in the rest-frame color dis-tribution of host-galaxies (Rovilos & Georgantopoulos 2007;Nandra et al. 2007; Brusa et al. 2009; Silverman et al. 2009), E. Lusso et al: The bolometric output and host-galaxy properties of obscured AGN in the XMM-COSMOS survey namely the red-sequence (or “red-cloud”) and blue-cloud galax-ies. Broad-line AGN (if the morphology of the host-galaxy isavailable) are likely to be associated to galaxies belonging tothe blue-cloud, while obscured objects to red passive galaxies.The green valley should be populated by transition objects. Thepicture above is probably a too crude approximation. Moreover,one should note that red sequence galaxies may well be pas-sively evolving galaxies without significant star formation (e.g.,Rovilos & Georgantopoulos 2007; Nandra et al. 2007), ratherthan dusty starforming objects (e.g., Brusa et al. 2009).Disentangling the contribution of the nuclear AGN from thehost-galaxy properties in the broad band SED is fundamentalto constrain the physical evolution of AGN and to place theminto the context of galaxy evolution. In the standard picture theAGN energy output is powered by accretion onto SMBHs. Thedisk accretion emission is visible in the optical-UV as the blue-bump feature. The X–ray emission is believed to be due to ahot-electrons corona that surrounds the accretion disk, whilethe infrared emission is likely due to the presence of a dustytorus around the disk at few parsec from the center, which re-processes the nuclear radiation. According to the unified modelof AGN (e.g., Antonucci 1993; Urry & Padovani 1995), hotdust is located in the inner edge of the torus. However, recentstudies predict and observe exceptions to the unified model.From the theoretical point of view, an alternative solution tothe torus is the disk-wind scenario (e.g., Emmering et al. 1992;Elitzur & Shlosman 2006). From the observational side, AGNwithout any detectable hot dust emission (e.g., Jiang et al. 2010)and weak infrared emission (e.g., Hao et al. 2010) are predictedand observed. The vast majority of studies performed so far con-cern unobscured (Type-1) AGN for which their SED is wellknown from low- z ( h z i ∼ . z ( h z i ∼ . ffi cult toderive constraints on the colors, stellar populations, and mor-phologies of the host. On the other hand, for obscured (Type-2) AGN the host-galaxy light is the dominant component inthe optical / near-infrared SED, while it is di ffi cult to recoverthe AGN intrinsic nuclear emission. The lack of a proper char-acterization of the nuclear componentof the SED of obscuredType-2 AGN is a major limitation. As a consequence, the re-lations between stellar masses, SFR, morphologies and accre-tion luminosity remain poorly known. Since the relative con-tribution in the SED of the di ff erent components (AGN / host-galaxy) varies with wavelength, a proper decomposition can beobtained by an SED-fitting approach, complemented by a mor-phological analysis. This will provide a robust estimate of thenuclear emission (bolometric luminosities and bolometric cor-rections, absorption column density distributions, etc) and its re-lation with the host-galaxy properties (mass, star formation rates,morphological classification). The AGN structure is reflected inthe shape of the SED, specifically the big-blue bump and theinfrared-bump are related to the accretion disk and the surround-ing torus, respectively. Therefore, a densely sampled SED overa broad wavelength interval is mandatory to extract useful in-formation from SED fitting procedures, allowing to tightly con-strain physical parameters from multi-component modeling and,in particular, to properly disentangle the emission associated tostellar light from that due to accretion.The combination of sensitive X–ray and mid-IR observa-tories allows us to model the obscuring gas that in Type-2AGN hides the nuclear region from the near-IR to the UV. As supported by previous investigations, the reprocessed IRemission could be a good proxy of the intrinsic disk emission.Gandhi et al. (2009) confirm the correlation between the X–rayluminosity at [2-10]keV and the IR emission for a sample ofSeyfert galaxies (see also Lutz et al. 2004). Their data are thebest estimate of the nuclear (non stellar) IR flux in AGN to date.A highly significant correlation between L [2 − and the intrin-sic nuclear IR luminosity at 12.3 µ m is observed in the high qual-ity near-IR and X–ray data discussed by Gandhi et al. (2009).This reinforces the idea that “uncontaminated” mid-IR contin-uum is an accurate proxy for the intrinsic AGN emission.In this work we present the largest study of the multi-wavelength properties of an X–ray selected sample of ob-scured AGN using the XMM-Newton wide field survey inthe COSMOS field (XMM-COSMOS). Following a similar ap-proach to that of Pozzi et al. (2007) and Vasudevan et al. (2010),we use the infrared emission to evaluate the nuclear bolometricluminosity from a multi-component fit. The paper is aimed ata detailed characterization of a large sample of obscured AGNover a wide range of frequencies. The SEDs, morphology of thehost-galaxies, stellar masses, colors, bolometric luminosities andbolometric corrections for the sample of obscured AGN are pre-sented.This paper is organized as follows. In Sect. 2 we report theselection criteria for the sample used in this work. Section 3presents the multi-wavelength data-set, while in Sect. 4 themethod to compute average SED is described. Section 5 con-cerns the multi-component modeling used to disentangle the nu-clear emission from the stellar light. In Section 6 the methodused to compute intrinsic bolometric luminosites and bolomet-ric corrections for Type-2 AGN is described, while in Sect. 7 wehave applied the same method to a sample of Type-1 AGN. Thediscussion of our findings is given in Sect. 8, while in Sect. 9 wesummarize the most important results.We adopted a flat model of the universe with a Hubble con-stant H =
70 km s − Mpc − , Ω M = . Ω Λ = − Ω M (Komatsu et al. 2009).
2. The Data Set
The XMM-COSMOS catalog comprises 1822 point–like X–raysources detected by XMM-
Newton over an area of ∼ . Thetotal exposure time was ∼ . ∼
50 ks over a large fraction of the area (Hasinger et al.2007, Cappelluti et al. 2009). Following Brusa et al. (2010), weexcluded from our analysis 25 sources which turned out tobe a blend of two
Chandra sources. This leads to a total of1797 X–ray selected point-like sources. We restricted the anal-ysis to 1078 X–ray sources detected in the [2-10] keV bandat a flux larger than 2 . × − erg s − cm − (see Table 2 inCappelluti et al. 2009). The objects for which no secure opticalcounterpart could be assigned are often a ff ected by severe blend-ing problems, so that we consider in this analysis the 971 sources(hereafter 971-XMM) for which a secure optical counterpart canbe associated (see discussion in Brusa et al. 2010) .From the 971-XMM catalog we have selected 255 sources,which do not show broad (FWHM < − ) emission linesin their optical spectra (hereafter we will refer to them as the The multi-wavelength XMM-COSMOS catalog can be retrievedfrom: http: // / XMMCosmos / xmm53 release / , version1 st April 2010. The origin of spectroscopic redshifts for the 255 sources is asfollows: 11 objects from the SDSS archive, 2 from MMT observa-. Lusso et al: The bolometric output and host-galaxy properties of obscured AGN in the XMM-COSMOS survey 3
Fig. 1.
Plot of the [2 − i ∗ CFHTmagnitude for our sample of 255 Type-2 AGN. The red circlesrepresent sources with a de-absorbed 2–10 keV luminosity lowerthan 10 erg s − . The dashed lines represent a constant X–rayto optical flux ratio of Log ( X / O ) = ± i ∗ CFHT magnitude. The dashed lines limit the regiontypically occupied by AGN along the X–ray to optical flux ra-tio Log ( X / O ) = ± . Nine sources have a de-absorbed 2–10 keV luminosity lower than 10 erg s − , the conventionalthreshold below which the X–ray sources that can plausiblybe explained by moderate-strength starbursts, hot gas in ellip-tical galaxies, or other sources besides accretion onto a nuclearSMBH (Hornschemeier et al. 2001). The three sources inside thedashed lines have X–ray luminosities close to 10 erg s − , whilesix AGN (6 / < erg s − . Their inclusion in the analysis does not a ff ect themain results.The Type-2 AGN sample used in our analysis comprises 255X–ray selected AGN, all of them with spectroscopic redshifts,spanning a wide range of redshifts (0 . < z < . . ≤ Log L [2 − ≤ . tions (Prescott et al. 2006), 70 from the IMACS observation campaign(Trump et al. 2007), 156 from the zCOSMOS bright 20k sample (seeLilly et al. 2007), 7 from the zCOSMOS faint catalog and 9 from theKeck / DEIMOS campaign. Log ( X / O ) = Log f x + i ∗ / . + . mean redshift is h z i = .
76, while the mean Log L [2 − is43.34 with a dispersion of 0.64. For a sub-sample of 111 AGN we have an estimate of the col-umn density N H from spectral analysis (see Mainieri et al. 2007),while for 144 AGN absorption is estimated from hardness ra-tios (HR; see Brusa et al. 2010). For 25 sources for which acolumn density estimate is not available from HR we considerthe Galactic column density. Therefore, we can compute thede-absorbed X–ray luminosity at 0.5–2 keV (soft band) and 2–10 keV (hard band) for all sources in our sample. In Figure 2(right panel) we show the distribution of column densities thatranges from 3 × cm − to 1 . × cm − . The mean N H value is 8 . × cm − with a dispersion of 0.72 dex. The in-tegrated intrinsic un-absorbed luminosity is computed assuminga power-law spectrum with slope, Γ =
Γ = . h ∆ Log L [2 − i = . ± .
3. Rest-frame monochromatic fluxes andSpectral Energy Distributions
We used the catalog by Brusa et al. (2010) which includes multi-wavelength data from mid-infrared to hard X–rays: MIPS 160 µ m, 70 µ m and 24 µ m GO3 data (Le Floc’h et al. 2009), IRACflux densities (Sanders et al. 2007), u ∗ and i ∗ CFHT bands andnear-infrared K S -band data (McCracken et al. 2008), J UKIRT(Capak et al. 2008), HST / ACS F814W imaging of the COSMOSfield (Koekemoer et al. 2007), optical multiband photometry(SDSS, Subaru, Capak et al. 2007) and near- and far-ultravioletbands with GALEX (Zamojski et al. 2007).The number of X–ray sources detected at 160 µ m and 70 µ m is 18 and 42, respectively. For the undetected sources in thesebands we consider 5 σ upper limits of 65 mJy and 8 . µ m and 70 µ m , respectively. At 24 µ m the number of de-tected sources is 237. For the 18 undetected sources at 24 µ m ,we consider 5 σ upper limits of 80 µ Jy. All 255 sources are de-tected in the infrared in all IRAC bands, and only a few objectswere not detected in the optical and near-IR bands: we have only8 upper limits in the z + band; 1 upper limit in the B J and i ∗ bands; 2 upper limits in the u ∗ band; 4 upper limits in the K S CFHT band and 2 in the J UKIRT band. The observations inthe various bands are not simultaneous, as they span a time in-terval of about 5 years: 2001 (SDSS), 2004 (Subaru and CFHT)and 2006 (IRAC). Variability for absorbed sources is likely tobe a negligible e ff ect, but, in order to further reduce it, we se-lected the bands closest in time to the IRAC observations (i.e.,we excluded SDSS data, that in any case are less deep than otherdata available in similar bands). GALEX bands are not takeninto account because, given the large aperture they can includelight from close companions. All the data for the SED compu-tation were shifted to the rest frame, so that no K-correctionswere needed. Galactic reddening has been taken into account:we used the selective attenuation of the stellar continuum k ( λ )taken from Table 11 of Capak et al. (2007). Galactic extinctionis estimated from Schlegel et al. (1998) for each object in the971-XMM catalog. Count rates in the 0.5-2 keV and 2-10 keVare converted into monochromatic X–ray fluxes in the observedframe at 1 and 4 keV, respectively, using a Galactic column den-sity N H = . × cm − (see Cappelluti et al. 2009), and as- E. Lusso et al: The bolometric output and host-galaxy properties of obscured AGN in the XMM-COSMOS survey
Fig. 2.
Left panel:
Redshift distribution of the 255 Type-2 AGN considered in this work.
Center panel:
Intrinsic hard X–ray lumi-nosity distribution of the 255 Type-2 AGN considered in this work.
Right Panel:
Column density distribution of the 255 Type-2AGN ( black histogram ), of the 111 Type-2 AGN with an N H estimate from spectral analysis ( grey filled histogram ), and of the 144Type-2 AGN with an N H estimate from hardness ratios ( orange hatched histogram ). Fig. 3.
Mean ( orange crosses ) and median ( red points ) SED fromthe total sample of 255 Type-2 AGN. The blue points representthe rest-frame data, from infrared to X–ray, used to constructthe average SED, while the black lines represent the interpolatedSED.suming a photon index Γ x = Γ x = .
7, for the soft andhard band, respectively. We do not correct these X–ray fluxesfor the intrinsic column density. All sources are detected in the2-10 keV band by definition of the sample, while in the soft bandwe have 70 upper limits.
4. Average SED
We have computed the individual rest-frame SEDs for allsources in the sample, following the same approach as in L10,and we have normalized all of them at 1 µ m. After this nor-malization we divided the frequency interval from Log ν min toLog ν max using a fixed step ∆ Log ν . The minimum and max-imum frequency depends on both the data and the redshift ofthe source considered to compute the SED, in our case we have used Log ν min =
12 Hz and Log ν max =
20 Hz, witha ∆ Log ν = .
02. We averaged data in each given intervalLog ν l ≤ Log ν ≤ Log ν l + . The mean and median SEDs are ob-tained by taking the arithmetic mean and the median of logarith-mic luminosities, Log L , in each bin. It is important to note thatsources at di ff erent redshift contribute to the same bin. Becauseof the relatively wide range of redshifts, the lowest and the high-est frequency bins are populated by a variable number of points.This e ff ect may introduce relatively high fluctuations in the av-erage luminosity in those bins. In order to minimize these ef-fects we select a minimum number of 200 SEDs in each bin (thisnumber depends on the total number of sources in the sample).Then, we select the mean reference frequency, Log ν , of the binand use a binary-search algorithm to find all luminosities thatcorrespond at Log ν (if a source does not have a frequency thatcorrespond to Log ν , we choose the luminosity with the closerfrequency to Log ν ). Finally, all adjacent luminosities in eachbin are then connected to compute the final mean and medianSED. In Figure 3 the resulting mean and median SEDs are re-ported with orange crosses and red points, respectively. The datapoints are reported in order to show the dispersion with respectto 1 µ m. The average SED is characterized by a flat X–ray slope, h Γ = . i , while in the optical-UV the observed emission ap-pears to be consistent with the host-galaxy. The flattening of theX–ray slope is likely due to the fact that we do not correct forthe intrinsic absorption the fluxes at 1 and 4 keV. The averageSED in the mid-infrared is probably a combination of dust emis-sion from star-forming region and AGN emission reprocessedby the dust. Before trying to deconvolve each source using anSED-fitting code, we binned the total sample in X–ray and in-frared luminosites and redshift. We used the luminosity at 4 keVand 8 µ m to divide the total sample in 6 bins with the same num-ber of sources in each bin. The wavelength of the luminosityused to bin the sample is chosen to minimize the host-galaxycontribution. In the three panels of Figure 4 the resulting meanSEDs are shown. The redshift trend is directly connected to theluminosity trend, since at higher redshifts we are looking at themore luminous sources. The shapes of the average SEDs in theoptical bands are approximately the same in all luminosity andredshift bins. As expected, there is a stronger host-galaxy con-tribution at lower luminosity / redshift bins, where the averageSEDs have a typical galaxy shape. Moreover, there is a trendbetween X–ray and mid-infrared luminosity: the contribution in . Lusso et al: The bolometric output and host-galaxy properties of obscured AGN in the XMM-COSMOS survey 5 Fig. 4.
Average SEDs in the rest-frame Log ( ν L ν ) − Log ν plane. Right panel:
Mean SEDs computed binning in X–ray luminosityat 4 keV.
Center panel:
Mean SEDs computed binning in infrared luminosity at 8 µ m. Left Panel:
Mean SEDs computed binning inredshift. The color code refers to the di ff erent bins as labeled.the infrared is higher at higher X–ray luminosities. This e ff ectis already known for both Type-1 and Type-2 AGN using theintrinsic (non-stellar) emission from the AGN (e.g., Lutz et al.2004, Gandhi et al. 2009). The observed average SED for oursample is a combination of both host-galaxy and AGN light, butthe change in the average shape in the mid-infrared as a functionof the X–ray luminosity suggests that most luminous sources areprobably dominated by the AGN emission at those wavelengths.
5. SED-fitting
The purpose of performing SED fitting is to properly disentanglethe emission associated to stellar light from that due to accretionand constrain physical parameters. Since the relative contribu-tion of the di ff erent components varies with wavelength, a properdecomposition can be obtained through an SED-fitting approachproviding a robust estimate of the nuclear emission (bolometricluminosities and bolometric corrections) and its relation with thehost-galaxy properties (mass, star formation rates, morphologi-cal classification). A well sampled SED is mandatory; in par-ticular, far-infrared observations are fundamental to sample thestar-formation activity, while mid-infrared observations are nec-essary to sample the region of the SED where most of the bolo-metric luminosity of obscured AGN is expected to be re-emitted.In Fig. 5 the broad-band SEDs of four XMM-Newton Type-2AGN are plotted as examples. The lower two panels are repre-sentative of a full SED with all detections from the far-infraredto the optical. Unfortunately, there are very few detections at 160and 70 µ m, so that the more representative situation is shown inthe upper left panel of Fig. 5. The three components adoptedin the SED-fitting code, starburst , AGN torus and host-galaxy templates, are shown as a long-dashed line, solid line and dottedline, respectively. All the templates used in the SED-fitting codewill be described in the following Sections. The red line repre-sents the best-fit, while the black points represent the photomet-ric data used in the code, from low to high frequency: MIPS-Spitzer (160 µ m , 70 µ m and 24 µ m if available), 4 IRAC bands, KCFHT, J UKIRT, optical Subaru and CFHT bands.The observed data points from infrared to optical are fittedwith a combination of various SED templates (see Sect. 5.1) us-ing a standard χ minimization procedure χ = n filters X i = " F obs , i − A × F gal , i − B × F agn , i − C × F ir , i σ i (1) where F obs , i and σ i are the monochromatic observed flux andits error in the band i ; F gal , i , F agn , i and F ir , i are the monochro-matic template fluxes for the host-galaxy, the AGN and the star-burst component, respectively; A , B and C are the normaliza-tion constants for the host-galaxy, AGN and starbust compo-nent, respectively. The starburst component is used only whenthe source is detected at 160 µ m and 70 µ m . Otherwise, a twocomponents SED-fit is used. Sixteen is the maximum number ofbands adopted in the SED-fitting (only detection are considered),namely: 160 µ m , 70 µ m , 24 µ m , 8 . µ m , 5 . µ m , 4 . µ m , 3 . µ m , K S , J , z + , i ∗ , r + , g + , V J , B J and u ∗ . We used a set of 75 galaxy templates built from theBruzual & Charlot (2003, BC03 hereafter) code for spectral syn-thesis models, using the version with the “Padova 1994” tracks,solar metallicity and Chabrier IMF (Chabrier 2003). For the pur-poses of this analysis a set of galaxy templates representative ofthe entire galaxy population from ellipticals to starbursts is se-lected. To this aim, 10 exponentially decaying star formation his-tories with characteristic times ranging from τ = . τ smaller than 1 Gyr and ages larger than2 Gyr, whereas more actively star forming galaxies are repre-sented by models with longer values of τ and a wider range ofages from 0 . E ( B − V ), corresponding to a dust-screen model,with F o ( λ ) = F i ( λ )10 − . E ( B − V ) k ( λ ) , where F o and F i are the ob-served and the intrinsic fluxes, respectively. The extinction at awavelength λ is related to the colour excess E ( B − V ) and to thereddening curve k ( λ ) by A λ = k ( λ ) E ( B − V ) = k ( λ ) A V / R V , with R V = .
05 for the Calzetti’s law. The E ( B − V ) values rangebetween 0 and 1 with a step of 0 . E. Lusso et al: The bolometric output and host-galaxy properties of obscured AGN in the XMM-COSMOS survey
The nuclear SED templates are taken from Silva et al. (2004).They were constructed from a large sample of Seyfert galaxiesselected from the literature for which clear signatures of non-stellar nuclear emission were detected in the near-IR and mid-IR.After a proper subtraction of the stellar contribution, the nuclearinfrared data were interpolated with a radiative transfer code fordust heated by a nuclear source with a typical AGN spectrum,and including di ff erent geometries, dust distribution, variationof the radii, density and dust grain sizes to account for possibledeviations from a standard ISM extinction curve (see for moredetails Granato & Danese 1994; Maiolino et al. 2001).The infrared SEDs were then normalized by the intrinsic,unabsorbed X-ray flux in the 2–10 keV band, and are dividedinto 4 intervals of absorption: N H < cm − for Sy1, while10 < N H < cm − , 10 < N H < cm − and N H > cm − for Sy2 (see Fig. 1 in Silva et al. 2004). The maindi ff erences between the SEDs of Sy1s and Sy2s with 10 < N H < cm − are the absorption in the near-IR at about λ < µ m and the silicate absorption at λ = . µ m, which are presentin the Sy2 template. The shape of the SED in the mid-infraredwith 10 < N H < cm − is quite similar to that with 10 < N H < cm − . The Compton-thick SED ( N H > cm − )shows conspicuous absorption also at λ ∼ . µ m. If a source hasthe N H value available, this is used as a prior in the selection ofthe best-fit AGN template. We used two di ff erent starburst template libraries for the SED-fitting: Chary & Elbaz (2001) and Dale & Helou (2002). Thesetemplate libraries represent a wide range of SED shapes and lu-minosities and are widely used in the literature. Here, we brieflydescribe how each of these libraries was derived and discuss themain di ff erences between them.The Chary & Elbaz (2001) template library consists of 105templates based on the SEDs of four prototypical starburst galax-ies (Arp220 (ULIRG); NGC 6090 (LIRG); M82 (starburst); andM51 (normal star-forming galaxy)). They were derived using theSilva et al. (1998) models with the mid-infrared region replacedwith ISOCAM observations between 3 and 18 µ m (verifying thatthe observed values of these four galaxies were reproduced bythe templates). These templates were then divided into two por-tions (4–20 µ m and 20–1000 µ m) and interpolated between thefour to generate a set of libraries of varying shapes and lumi-nosities. The Dale et al. (2001) templates are also included inthis set to extend the range of shapes.The Dale & Helou (2002) templates are updated versions ofthe Dale et al. (2001) templates. This model involves three com-ponents, large dust grains in thermal equilibrium, small grainssemistochastically heated, and stochastically heated PAHs. Theyare based on IRAS / ISO observations of 69 normal star-forminggalaxies in the wavelength range 3–100 µ m. Dale & Helou(2002) improved upon these models at longer wavelengths usingSCUBA observations of 114 galaxies from the Bright GalaxySample (BGS, see Soifer et al. 1989), 228 galaxies observedwith ISOLWS (52–170 µ m; Brauher 2002), and 170 µ m obser-vations for 115 galaxies from the ISOPHOT Serendipity Survey(Stickel et al. 2000). All together, these 64 templates span theIR luminosity range 10 − L ⊙ . The total infrared templatesample used in our analysis is composed of 168 templates.
6. Bolometric luminosities and bolometriccorrections
The nuclear bolometric luminosities and bolometric correctionsare estimated, using an approach similar to Pozzi et al. (2007,see also Vasudevan et al. 2010; Pozzi et al. 2010), whereas theinfrared luminosity is used as a proxy of the intrinsic nuclearluminosity. The appropriate nuclear template from Silva et al.(2004) is selected based on the absorbing column density N H ,when available, or from the best-fit nuclear infrared template. Inorder to compute the hard X–ray bolometric correction we usedthe standard definition k bol = L bol L [2 − (2)where the L [2 − is the intrinsic X–ray luminosity and thebolometric luminosity is computed as the sum of the total in-frared and X–ray luminosity L bol = L IR + L X . (3)After performing the SED-fitting, only the nuclear component ofthe best-fit is integrated. Hence, the total IR luminosity L IR is ob-tained integrating the nuclear template between 1 and 1000 µ m.To convert this IR luminosity into the nuclear accretion diskluminosity, we applied the correction factors to account forthe torus geometry and the anisotropy (see Pozzi et al. 2007).The first correction is parameterized by the covering factor f .The covering factor is related to the geometry of the torus thatobscures the accretion disk emission in the optical-UV alongthe line of sight, and its value is estimated from the ratio ofobscured / unobscured quasars found by the X–ray backgroundsynthesis models (Gilli et al. 2007). This correction factor is ∼ .
5. This value correspond to a typical covering factor of f ∼ .
67, consistent with the results based on clumpy torus mod-els (Nenkova et al. 2008).The anisotropy factor is defined as the ratio of the luminosityof face-on versus edge-on AGN, where the obscuration is a func-tion of the column density N H . Therefore, SEDs in Silva et al.(2004) have been integrated in the 1–30 µ m range, after normal-izing these SEDs to the same luminosity in the 30–100 µ m range.The derived anisotropy values are 1.2–1.3 for 10 < N H < and 3–4 for N H > . The same values as in Vasudevan et al.(2010) are adopted: 1.3 for 10 < N H < and 3.5 for N H > .The total X–ray luminosity L X is estimated integrating in the0.5-100 keV range the X–ray SED. We have interpolated thede-absorbed soft and hard X–ray luminosities. Since we are in-tegrating at the rest-frame frequencies, the X–ray SED is ex-trapolated at higher and lower energies using the estimated X–ray slope, and introducing an exponential cut-o ff at 200 keV(Gilli et al. 2007, see also Sect. 3.1 in L10).
7. Robustness of the method
The robustness of the method used to estimate nuclear bolomet-ric luminosities and bolometric corrections from SED-fitting, forthe sample of Type-2 AGN, has been tested against the updatedsoft X–ray selected sample of Type-1 AGN discussed in L10.The Type-1 AGN sample in the L10 work was composed of 361spectroscopically classified broad-line AGN. The recent work byBrusa et al. (2010) has updated the spectroscopic classificationand increased the number of Type-1 AGN with spectroscopicredshift, so that the final sample is composed of 395 Type-1 . Lusso et al: The bolometric output and host-galaxy properties of obscured AGN in the XMM-COSMOS survey 7
Fig. 5.
Examples of SED decompositions. Black circles are the observed photometry in the rest-frame (from the far-infrared to theoptical-UV). The long-dashed, solid and dotted lines correspond respectively to the starburst, AGN and host-galaxy templates foundas the best fit solution. The red line represents the best-fit SED. The stellar mass and the SFR derived from the galaxy template arereported.AGN in the redshift range 0 . ≤ z ≤ .
255 with X–ray lu-minosities 42 . ≤ Log L [2 − ≤ .
23. We have computedbolometric and X–ray luminosities, and bolometric correctionsusing the same approach as in L10 for the Type-1 sample: bolo-metric luminosites are computed by integrating the rest-frameSEDs from 1 µ m up to the UV-bump. In order to compare theseestimates with the results from the SED-fitting code, we haveapplied to the same sample the method described in Sect. 5 and6 to estimate bolometric parameters. To be consistent with theselection criteria of the sample discussed in this paper, we haveconsidered only AGN with X–ray detection in the hard band, re-moving from the main sample 87 Type-1 AGN with an upperlimit at 2–10 keV. Moreover, for 2 sources the best-fit does notconsider an AGN component, so we cannot compute the bolo-metric luminosities for them. The final test sample is composedof 306 Type-1 AGN in the redshift range 0 . ≤ z ≤ .
626 andX–ray luminosities 42 . ≤ Log L [2 − ≤ . < N H < for 20 AGN that have N H in this range.We present the comparison between the values of L bol and k bol from L10 and this work in Fig. 6. The outlier in the bottomside of the plot, XID =
357 at redshift 2.151 has Log k bol = .
95 from L10 and Log k bol = .
04 using the new approach, andpresents large error bars in the 24 µ m detection, so that the totalbolometric luminosity, computed using the infrared luminosity,is probably underestimated. The outlier in the right end of thedistribution, XID = k bol = .
82 fromL10 and Log k bol = .
95 using the new approach) has detectionsat 160, 70 and 24 µ m, Log N H = .
68 and Log L [2 − = .
89. Probably this source is a star-forming galaxy, so that us-ing the L10 approach we included stellar emission in the esti-mate of the nuclear bolometric luminosity, thus overestimatingthe nuclear bolometric luminosity and, therefore, the bolometriccorrection. The last notable outlier in the top / left side of the dis-tribution, XID = k bol = .
35 from L10and Log k bol = .
27 using the new approach) presents a signif-icant host-galaxy contribution in the optical-UV and, therefore,the bolometric luminosity is likely to be underestimated in theL10 approach.Although the two methods are very di ff erent, the bolometricluminosity estimates agree remarkably well, with a 1 σ disper-sion of 0.20 dex after performing a 3.5 σ clipping method in or-der to avoid outliers. Bolometric luminosities from SED-fittingare on average slightly larger than those computed integratingthe rest-frame SED from 1 µ m to the X–ray (see the lower leftside in Fig. 6). This e ff ect is also present in the Vasudevan et al. E. Lusso et al: The bolometric output and host-galaxy properties of obscured AGN in the XMM-COSMOS survey
Fig. 6.
Upper panel: Comparison between the values of bolometric luminosity and bolometric correction from data presented inL10 and from this work. The three red triangles mark the outliers discussed in Sect. 7. Lower panel: Distribution of the di ff erencesbetween the values of bolometric luminosity and bolometric correction from data presented in L10 and from this work.(2010) work. A possible explanation is that SED-fitting underes-timates the host-galaxy contribution, or that the anisotropy andgeometry corrections are too large for some objects. The agree-ment between the two methods is overall quite satisfactory andin the following we will discuss our findings for the Type-2 sam-ple.
8. Results and discussion
Bolometric luminosities and bolometric corrections have beencomputed for the Type-2 AGN sample. Intrinsic soft and hardX–ray luminosities are estimated as described in Sect. 2.1. For15 sources we do not have an estimate of the AGN compo-nent from the SED-fitting, and we cannot compute the bolomet-ric luminosity for them. In Fig. 7 the distribution of the bolo-metric correction for the 240 Type-2 AGN sample and for the306 Type-1 AGN ( k bol for Type-1 AGN are computed usingthe SED-fitting code) are presented. Figure 8 shows bolomet-ric corrections for both the Type-1 and the Type-2 AGN sam-ples as a function of the hard X–ray luminosity. For both sam-ples, bolometric parameters are estimated from the SED-fittingas discussed in Sect. 5. The green and orange curves representthe bolometric corrections and their 1 σ dispersion as derivedby Hopkins et al. (2007) and Marconi et al. (2004), respectively.Type-2 AGN have, on average, smaller bolometric correctionsthan Type-1 AGN at comparable hard X–ray luminosity. Forexample, at 43 . ≤ Log L [2 − ≤ .
30 (vertical lines inFig. 8), where both AGN types are well represented, the me-
Fig. 7.
Distribution of the bolometric correction for the 240Type-2 AGN sample ( red hatched histogram ) and for the 306Type-1 AGN ( blue hatched histogram ).dian bolometric correction for the Type-2 AGN (134 objects) is h k bol i ∼ ±
1, to be compared with a median bolometric correc-tion h k bol i ∼ ± ff erent at the ∼ σ level and this is . Lusso et al: The bolometric output and host-galaxy properties of obscured AGN in the XMM-COSMOS survey 9 Fig. 8.
Hard X–ray bolometric correction against the intrinsic2–10 keV luminosity for 240 Type-2 AGN with AGN best-fit( red data ). The crosses represent the bolometric correction for306 Type-1 AGN, computed with the approach described inSect. 6. The green and blue lines represent the bolometric cor-rection and the 1 σ dispersion obtained by Hopkins et al. (2007)and Marconi et al. (2004), respectively. The red points and opensquares represent the 111 Type-2 AGN with N H from spectralanalyses and the 144 Type-2 AGN with N H from HR, respec-tively.consistent with the results in Vasudevan et al. (2010). The mean L [2 − for the Type-1 and Type-2 AGN within this luminosityrange di ff ers by a factor 1.8, and this could in principle explain atleast part of the di ff erence in the average bolometric correctionsfor the two samples of AGN. However, the significance of thedi ff erence is still present if we split this luminosity range in twoequal Log L [2 − bins and perform a Kolmogorov–Smirnovtest for the Type-1 and Type-2 AGN luminosity distributions ineach bin.Vasudevan & Fabian (2009) and Lusso et al. (2010) haveshown that hard X–ray bolometric corrections are correlatedwith the Eddington ratios ( λ Edd = L bol / L Edd ) for Type-1 AGN(see also Marconi et al. 2004; Kelly et al. 2008). The k bol − λ Edd relation suggests that there is a connection between the broad-band emission, mostly in the optical-UV, and the Eddingtonratio, which is directly linked to the ratio between mass ac-cretion rate and Eddington accretion rate. A high λ Edd corre-sponds to an enhanced optical-UV emission, which means aprominent big-blue bump and therefore a higher k bol . The dif-ference between the average bolometric corrections for Type-1 and Type-2 AGN could be due to lower mass accretion ratesin Type-2 AGN, assuming the same black hole mass distribu-tion for the two AGN populations (see Trump et al. 2011). Thecurrent theoretical framework of AGN / host-galaxy co-evolutionpredicts that obscured AGN are highly accreting objects andtheir black hole is rapidly growing. However, we note that thisis true for z = − . ≤ Log L bol ≤ . < z < The re-processed infrared emission can be used as a proxy of theaverage disc emission, since the timescale for transfer of energyfrom the disk to the outer edge of the torus into infrared emis-sion is of the order of several years in standard AGN picture;whereas optical, UV and X–ray variability in AGN is known tooccur on shorter timescales. The correlation between the 2–10keV X–ray emission and IR emission at 12.3 µ m for a sample ofSeyfert nuclei has been discussed in Gandhi et al. (2009), and itcould be used to estimate the intrinsic AGN power. Using X–raydata from the literature and new IR data from the Very LargeTelescope’s Imager and Spectrometer for mid-Infrared (VISIR),taken specifically for addressing the issue of nuclear emissionin local Seyferts, they found a tight correlation between intrin-sic, uncontaminated IR luminosity and X–ray luminosity in the2–10 keV rangeLog L . µ m = (0 . ± . + (1 . ± . L [2 − . (4)The relation is characterized by a small scatter with a standarddeviation of 0.23 dex. The expected nuclear mid-infrared lumi-nosity is computed from Eq. (4) using the estimate of the intrin-sic unabsorbed X–ray luminosity. From the observed rest-frameSED (AGN + host-galaxy) the luminosity at 12.3 µ m is com-puted. A comparison of the total observed luminosity at 12.3 µ mand that predicted by Eq. (4) is plotted in Figure 9 for four rep-resentative sources. In Figure 10 the distribution of the ratio r = Log (cid:16) L . µ m , obs / L . µ m , predicted (cid:17) is plotted for the Type-2AGN sample. The distribution of the ratio r has a mean which isshifted from zero by ∼ .
2. However, if we consider a gaussiandistribution centered at r = σ = .
23, i.e. the same disper-sion observed by Gandhi et al. (2009) in their local sample, themajority of the objects are found within 2 σ of the r distribution.The tail outside 2 σ and extending to high r includes 73 sources(with r & .
5) for which the predicted mid-infrared luminosity issignificantly lower than observed. The hard X–ray luminositiesof these 73 AGN are mainly in range Log L [2 − ∼ − ∼
30% of the objects) tailtoward high- r values: either the Gandhi relation, which was de-rived for a sample of local Seyfert galaxies, cannot be extendedto all the sources in our sample or the SED-fitting procedure mayoverestimate, in a fraction of these objects, the nuclear contribu-tion. In order to study the properties of these outliers, bolometriccorrections, morphologies, stellar masses and SFR are discussedin following. We call “low- r ” AGN all sources within 2 σ of the r distribution, while the “high- r ” AGN sample is populated by thesources deviating more than 2 σ (see Fig. 9 for some examples).A clear separation in bolometric corrections for these twosub-samples is found. This is shown in Figure 11 in which bolo-metric corrections are plotted as a function of the 2–10 keV lumi-nosity. At a given hard X–ray luminosity (43 ≤ Log L [2 − ≤
44) the low- r sample has a median bolometric correction of h k bol i ∼ ± r sample of h k bol i ∼ ± k bol are statistically dif-ferent at the ∼ σ level.Furthermore, in the high- r sample 24 Type-2 AGN out of 73have a detection at 70 µ m ( ∼ r sample, ∼ µ m ( ∼
12% considering the total high- r sample,and only ∼
1% for the low- r sample). This denotes that the dif-ference in the average bolometric corrections between the low- r Fig. 9.
Examples of SED decompositions. Black circles are the observed photometry in the rest-frame (from the far-infrared to theoptical-UV). The long-dashed, solid and dotted lines correspond respectively to the starburst, AGN and host-galaxy templates foundas the best fit solution. The red line represents the best-fit SED. The stellar mass and the SFR derived from the galaxy template arereported. The green point represents the nuclear mid-infrared luminosity using Eq. (4), while the cross represents the total observedluminosity at 12.3 µ m computed from the rest-frame SED. XID =
19 and 81 are examples of low- r AGN, while XID =
172 and 117represent high- r AGN.and high- r samples is probably due to the fact that a significantfraction of the infrared emission is attributable to an incorrectmodeling of the star-formation process, or the AGN contributionis somehow overestimated by the SED-fitting procedure.There is no significant di ff erence in the average nuclear ab-sorption between the low- r and the high- r sample, while thereis a possibly significant di ff erence in SFR and stellar masses.The median stellar mass in the high- r sample is h Log M ∗ i ∼ . M ⊙ with a dispersion of 0.30, while for the low- r sam-ple is h Log M ∗ i ∼ . M ⊙ with σ = .
30. The two av-erages are statistically di ff erent at the ∼ σ level. The me-dian SFR, as derived from the SED-fit, for the high- r sampleis h S FR i ∼ ± M ⊙ / yrs with a σ = .
30, while for the low- r sample is h S FR i ∼ ± M ⊙ / yrs with a σ = .
30 and the twoaverages are statistically di ff erent at the 4 . σ level.Overall, the SED-fitting for the 73 Type-2 AGN is likely tooverestimate the AGN emission in the infrared, which is proba-bly due to the infrared emission from star-forming regions. Theaverage bolometric correction for Type-2 AGN, excluding thesesources, would be even lower than what we have computed inthe previous Section. This reinforces the idea of lower bolomet-ric corrections for Type-2 AGN with respect to Type-1 AGN. The low bolometric corrections for Type-2 AGN could be alsoexplained if a fraction of the accretion disk bolometric output isnot re-emitted in the mid-infrared, but rather dissipated (e.g., byAGN-feedback). This would not be accounted in the bolomet-ric luminosity, and could provide a plausible explanation for low k bol especially if the low- r sample is considered. At this stage,this is just a speculation, and more work is needed to verify thispossibility. M ∗ , SFR,colors andmorphologies Galaxies show a colour bi-modality both in the local Universeand at higher redshift (up to z ∼
2; e.g., Strateva et al. 2001;Bell et al. 2004). This bi-modality (red-sequence and blue-cloudgalaxies) has been interpreted as an evidence for a dicothomyin their star formation and merging histories (e.g., Menci et al.2005, but see also Cardamone et al. 2010 for an alternative ex-planation). Color-magnitude and color-mass diagrams (e.g., rest-frame ( U − V ) versus stellar mass) have been used as tools ingalaxy evolution studies, and since many models invoke AGNfeedback as an important player in such evolution, it is inter- . Lusso et al: The bolometric output and host-galaxy properties of obscured AGN in the XMM-COSMOS survey 11 Table 1.
Properties of the Type-2 AGN sample.
XID Redshift Log L [2 − Log L bol k bol Log M ∗ SFR M U M V M J Morphological class a [erg s − ] [erg s − ] [M ⊙ ] [ M ⊙ / yrs]67 0.367 42.79 43.80 10.24 9.68 1.86 -18.63 -19.80 -20.72 065 0.979 43.83 44.85 10.62 10.19 38.60 -20.50 -21.63 -22.71 764 0.686 43.54 44.52 9.57 10.45 32.82 -20.17 -21.56 -23.05 1063 0.355 42.98 44.28 19.88 10.79 4.94 -20.61 -22.13 -23.26 254 0.350 42.58 43.38 6.32 11.18 0.11 -20.57 -22.62 -23.78 1245 0.121 41.90 42.91 10.31 9.39 0.00 -16.76 -18.70 -19.77 143 1.162 44.22 45.28 11.49 11.30 1.47 -21.55 -23.48 -24.61 219 0.659 43.67 44.76 12.44 11.07 35.88 -21.57 -22.88 -24.02 1117 0.936 43.47 45.59 132.11 11.24 199.40 -21.81 -23.34 -24.80 23116 0.874 43.49 44.50 10.17 10.56 3.77 -20.19 -21.81 -22.90 0112 0.762 43.65 44.65 9.97 10.93 0.62 -20.00 -22.13 -23.52 3104 0.623 44.08 45.11 10.75 10.79 0.45 -20.25 -22.18 -23.30 1101 0.927 43.70 44.68 9.56 10.92 0.61 -21.19 -22.93 -23.78 0100 0.270 42.61 43.54 8.59 10.21 0.00 -19.14 -20.98 -21.92 099 0.730 43.54 44.78 17.47 9.97 116.26 -21.61 -22.26 -23.15 1185 1.001 43.46 44.84 23.99 10.13 38.52 -20.15 -21.34 -22.56 881 0.915 44.11 45.05 8.65 11.18 0.00 -20.35 -22.59 -24.07 270 0.688 44.00 45.60 39.74 10.65 542.60 -20.87 -22.32 -24.31 3152 0.895 43.75 44.82 11.72 9.87 92.88 -21.06 -21.82 -22.85 7150 0.740 43.29 44.25 9.27 10.64 0.55 -19.38 -21.28 -22.50 3Notes—This table is presented entirely in the electronic edition; a portion is shown here for guidance. a The morphological classification of the Type-2 AGN hosts is coded from 0 to 23: 0 = elliptical, 1 = S0; 2 = bulge-dominated; 3 = intermediate-bulge; 4 = disk-dominated; 5 = irregular; 6 = compact / irregular; 7 = compact; 8 = unresolved / compact; 9 = blended; 10 = bulge-dominated / close-companion; 11 = intermediate-bulge / close-companion; 12 = S0 / close-companion; 23 = possible mergers. Fig. 10.
Histogram of the ratio between the total observed lumi-nosity at 12.3 µ m and the mid-infrared luminosity predicted byEq. (4). The red curve represents a gaussian with mean equal tozero and standard deviation 0.23. The 1 σ and 2 σ standard devi-ations of the correlation are also reported.esting to locate the hosts of Type-2 AGN in those diagrams.Using the galaxy component obtained from the best fit of theType-2 AGN, it is possible to derive rest-frame colors for thehost that, linked to the stellar mass and the morphology, canprovide hints on AGN feedback. Several studies found thatthe hosts of obscured AGN tend to be redder than the over- Fig. 11.
Hard X–ray bolometric correction against 2–10 keV lu-minosity for 240 Type-2 AGN with AGN best-fit. The 240 Type-2 sample is divided into subsamples: low- r AGN sample ( blackdata ) and high- r AGN sample ( yellow data ). The green and or-ange lines represent the bolometric correction and 1 σ dispersionobtained by Hopkins et al. (2007) and Marconi et al. (2004), re-spectively. In the de-absorbed hard X–ray luminosity range high-lighted by the solid lines, we have 167 low- r and 73 high- r (inthe infrared) sources.all galaxy population in the rest-frame ( U − V ) color (e.g.,Nandra et al. 2007). There are at least two possible and sig- Fig. 12.
The morphology distribution (using the ZEST + code)of the 233 AGN host-galaxies on the ( U − V ) colour-mass di-agram. We also plotted the 22 sources without morphologicalinformation. The ( U − V ) color and stellar masses are computedusing the SED-fitting code. We overplot the contours of about8700 galaxies in z COSMOS (colours and stellar masses from
Hyperz ). The morphology classification is labeled as follow: el-liptical (Ell), S0, bulge-dominated galaxy (BD), intermediate-bulge galaxy (IB), disk-dominated galaxy (DD), irregular (Irr),Compact, possible mergers (PM) and unresolved compact (UC).The red dashed line represents the red sequence cut defined byBorch et al. (2006), while the green short dashed line defines anapproximate green valley region, both lines are calculated at red-shift ∼ .
76, which is the average redshift of the main Type-2sample.nificantly di ff erent interpretations for this observational result:the observed red colors are mainly due to dust extinction, sothat a significant fraction of obscured AGN would live in mas-sive, dusty star-forming galaxies with red optical colors (e.g.,Brusa et al. 2009); or red sources are linked with passive sys-tems (e.g., Rovilos & Georgantopoulos 2007; Schawinski et al.2009; Cardamone et al. 2010). Therefore, accurate stellar massand SFR estimates, together with detailed galaxy morphologies,are of particular importance to discriminate between the two al-ternative possibilities.The very high resolution and sensitivity of ACS-HST imag-ing in the COSMOS survey provides resolved morphologiesfor several hundreds of thousands galaxies with i acs ≤
24 (seeScarlata et al 2007 for details). Galaxy morphologies were ob-tained with an upgraded version of the Zurich Estimator ofStructural Types (ZEST; Scarlata et al. 2007), known as ZEST + (Carollo et al. 2011, in prep). Relative to its predecessor,ZEST + includes additional measurements of non-parametricmorphological indices for characterising both structures andsubstructures. For consistency with the earlier versions, ZEST + uses a Principal Component Analysis (PCA) scheme in the6-dimensional space of concentration, asymmetry, clumpiness,M (second-order moment of the brightest 20% of galaxy pix-els), Gini coe ffi cient, and ellipticity. ZEST + classifies galax-ies in seven morphological types located in specific regionsof the 6-dimensional space: elliptical, S0, bulge-dominated disk, intermediate-bulge disk, disk-dominated, irregular, com-pact. The di ff erent types were then visually inspected. For 19objects ZEST + is unable to give any information on morphol-ogy because these sources lie o ff the edge of the ACS tiles and4 sources are blended. As a result of the ZEST + procedure andvisual inspection of the other 233 galaxies in our sample, we findthat 16 are ellipticals (Ell), 53 are S0s, 74 are bulge-dominated(BD) disks, 27 are intermediate-bulge (IB) disks, just 1 is disk-dominated (DD), 19 are irregular galaxies (Irr), 15 are compactgalaxies (i.e. the structural parameters computed for these galax-ies from the HST-ACS images are highly a ff ected by the in-strumental PSF) and 18 are unresolved compact galaxies (UC,i.e. essentially point-like sources). Ten galaxies show distortionsand potential signatures of ongoing or recent mergers (PM). Atthe typical magnitudes of the objects in our sample, the ZEST + classification is highly reliable for galaxies with redshift . ff ects can adversely a ff ect measurements ofZEST + parameters (note that only 4% of the main Type-2 AGNsample have z > . / late type galaxies) should be relatively robust. For high- z galaxies, resolution might also have an impact on the classifica-tion for mergers, and ACS images could not be deep enoughto distinguish merger features (see also Mainieri et al. 2011).Moreover, inclination might also a ff ect the morphological classi-fication (e.g., Nair & Abraham 2010). However, a detailed studyof systematics and biases in the morphological classification isbeyond the purposes of the present paper. In Table 1 we list themain properties of the sample.The rest-frame ( U − V ) color encompasses the 4000Å break,and it is particularly sentitive to age and metallicity variationsof stellar population in galaxies (e.g., Sandage & Visvanathan1978; Bell et al. 2004; Borch et al. 2006; Cardamone et al.2010). In Fig. 12 the distribution of the rest-frame ( U − V ) colors,which are computed directly from the best-fit galaxy template,and stellar masses (from the SED-fitting code) are reported forthe entire Type-2 AGN sample. In the same figure, the back-ground contours for a sample of ∼ i acs < .
5, 240 Type-2 are detected in the i acs band, 183 / i acs < .
5) are also plotted, wherecolours and stellar masses are computed using the Hyperz code(Bolzonella et al. 2000).AGN are known to reside in massive galaxies (e.g.,Silverman et al. 2009; Brusa et al. 2009) and this is fully con-firmed by the present analysis. The morphologies of the host-galaxies and the stellar masses indicate that there is a prefer-ence for these Type-2 AGN to be hosted in bulge-dominated andS0 galaxies ( ∼ M ⊙ .This result is consistent with the previous studies on Type-2AGN by Silverman et al. (2008, see also Kau ff mann et al. 2003;Bundy et al. 2008; Schawinski et al. 2011).It should be noted that no correction for the internal extinc-tion has been applied to the ( U − V ) colors of both backgroundgalaxies in zCOSMOS and Type-2 AGN hosts. This correctioncould be important as shown in Cowie & Barger (2008) (see alsoCardamone et al. 2010). In that work star formation and galac-tic stellar mass assembly are analyzed using a very large andhighly spectroscopically complete sample selected in the rest-frame NIR bolometric flux in the GOODS-N. They found thatapplying extinction corrections is critical when analyzing galaxycolors; nearly all of the galaxies in the green valley are 24 µ m sources, but after correcting for extinction, the bulk of the 24 µ m sources lie in the blue cloud. This correction introduces an av-erage shift in color of ∼ . / star- . Lusso et al: The bolometric output and host-galaxy properties of obscured AGN in the XMM-COSMOS survey 13 forming galaxies. However, to be consistent with the colors ofthe background galaxies no correction for intrinsic extinction isconsidered.AGN host-galaxies belong to the red-sequence if their ( U − V ) color is above the threshold (Borch et al. 2006):( U − V ) AB , rest − frame > . M ∗ − . z − .
39 (5)Sources in the green-valley are approximately defined shiftingthis relation by 0.25 downward towards bluer colors. With thesedefinitions, ∼
42% (108 / ∼
25% (63 / = SFR / M ∗ ). The inverse of the SSFR, S S FR − , is called “growth time” S S FR − = M ∗ / ˙ M ∗ , (6)and corresponds to the time required for the galaxy to double itsstellar mass, assuming its SFR remained constant. Actively star-forming galaxies are defined as sources with growth time smallerthan the age of the Universe at their redshift ( S S FR − < t Hubble ),while sources with
S S FR − larger than the age of the Universecan be considered passive galaxies (see also Fontana et al. 2009;Brusa et al. 2009). Figure 13 shows S S FR − as a function of thestellar mass in three di ff erent redshift bins for the AGN host-galaxies in the red-sequence, in the green-valley and in the blue-coud and for the zCOSMOS galaxies in same redshift ranges.The horizontal lines mark the age of the Universe at the tworedshift boundaries of the chosen intervals. At face value, al-most all the sources in the red-sequence have S S FR − largerthan the age of the Universe at their redshift, which is consis-tent with passive galaxies. However, the value of S S FR − hasto be considered only as an approximate indication of the star-formation activity; in fact, there is some possible evidence ofsome residual star-formation, in red-cloud AGN host-galaxies,as witnessed by their morphologies. In the red-sequence 8 and28 sources are classified as ellipticals and S0s, respectively; alltogether they represent 34% of the host-galaxy population inthe red-sequence. About 42% is represented by disk galaxies(both bulge-dominated and intemediate-bulge), which are prob-ably still forming stars but not at high rates. In fact, 15 over 108sources ( ∼ µ m and 5 have also adetection at 160 µ m ( ∼ U − V ) − ( V − J )color diagram). Near-infrared emission can distinguish betweenred-passive or dust-obscured galaxies: given a similar 0 . µ mflux, a star-forming galaxy has more emission near ∼ µ m thana passive galaxy. A sub-sample of galaxies is selected in thesame redshift range explored by Cardamone et al. (2010), wefind 92 AGN host-galaxies with 0 . ≤ z ≤ .
2. Fig. 14 showsboth inactive galaxies and AGN host-galaxies in the same red-shift range and the thresholds considered to divide galaxies inthe red-sequence and in the green-valley (we consider an aver-age redshift of 1 to define the threshold for the red-sequenceand the green-valley). Thirty-five out of 92 AGN hosts are foundto lie in the red-sequence ( ∼ Fig. 13.
Inverse of the SSFR rate as a function of the stellar massof the AGN host-galaxies in three di ff erent redshift bins for thezCOSMOS galaxies and for the Type-2 AGN sample in the red-sequence (red crosses), in the green-valley (green triangles) andin the blue-cloud (blue open circles). The horizontal lines markthe age of the Universe at the two redshift boundaries of the cho-sen intervals.( ∼ U − V ) − ( V − J ) color diagram for the 92 Type-2 AGN hosts ispresented. From a preliminary analysis of the rest frame ( U − V )against the rest-frame ( V − J ) color (see Fig. 15, but see alsoFig. 2 in Cardamone et al. 2010), only ∼
9% of the AGN host-galaxies in the red-sequence and ∼
30% of AGN host-galaxies inthe green-valley are moved in the region populated by dusty star-forming galaxies in the color-color diagram. To be comparedwith 20% AGN host-galaxies in the red-sequence and 75% AGNhost-galaxies in the green-valley found by Cardamone and col-laborators. The fractions of dust-obscured galaxies among thered-cloud and green-valley AGN in our sample, at 0 . ≤ z ≤ . ∼
9. Summary and Conclusions
A detailed analysis of the SEDs of 255 spectroscopically identi-fied hard X–ray selected Type-2 AGN from the XMM-COSMOSsurvey is presented. In obscured AGN, the optical-UV nuclearluminosity is intercepted along the line of sight by the dustytorus and reprocessed in the infrared, so what we see in theoptical-UV is mostly the light from the host-galaxy. On the onehand, this allows us to study the galaxy properties, on the otherhand it makes di ffi cult to estimate the nuclear bolometric power.An SED-fitting code has been developed with the main purposeof disentagling the various contributions (starburst, AGN, host-galaxy emission) in the observed SEDs using a standard χ min-imization procedure (the starburst component is only used in thecase of detection at 70 µ m). The code is based on a large set of Table 2.
AGN hosts and galaxies properties.
Sample N Red-sequence Green-valley Blue-cloud0 . ≤ z ≤ . . ≤ z ≤ .
232 (91%) P 16 (70%) PType-2 AGN 92 35 (38%) 3 (9%) D 23 (25%) 6 (30%) D 34 (37%)569 (97%) P 269 (70%) PGalaxies 1836 587 (32%) 18 (3%) D 385 (21%) 116 (30%) D 864 (47%)Note – P = Passive, D = Dusty.
Fig. 14.
Distribution of the stellar masses as a function of therest-frame ( U − V ) colors in the redshift range 0 . ≤ z ≤ . ∼
1. The points are color coded as in Fig. 12.starburst templates from Chary & Elbaz (2001) and Dale et al.(2001), and galaxy templates from the Bruzual & Charlot (2003)code for spectral synthesis models, while AGN templates aretaken from Silva et al. (2004). These templates represent a widerange of SED shapes and luminosities and are widely used inthe literature. The total (nuclear) AGN bolometric luminositiesare then estimated by adding the X–ray luminosities integratedover the 0.5-100 keV energy range to the infrared luminosity be-tween 1 and 1000 µ m. The total X–ray luminosity is computedintegrating the X–ray SED using the de-absorbed soft and hardX–ray luminosities. The SED is extrapolated to higher energiesusing the observed X–ray slope, and introducing an exponentialcut-o ff at 200 keV. The total infrared luminosity is evaluated in- Fig. 15.
Distribution of Type-2 AGN hosts in the rest-frame( U − V ) against the rest-frame ( V − J ) color. Color coded asin Fig. 13. Sources with the same best-fit galaxy template andthe same extinction lie in the same position in the color-colordiagram. Point size is keyed to the number of objects.tegrating the infrared AGN best-fit and then converted into thenuclear accretion disk luminosity applying the appropriate cor-rection factors to account for the geometry and the anisotropy ofthe torus emission. The reprocessed IR emission is consideredto be a good proxy of the intrinsic disk emission and this is sup-ported by previous investigations (Pozzi et al. 2007; Gandhi etal. 2009; Vasudevan et al. 2010). In the distribution of the ratio r = Log (cid:16) L . µ m , obs / L . µ m , predicted ; see Eq. 4) the majority ofthe objects are within 2 σ of the r distribution. The tail outside2 σ and extending to high r includes 73 sources (with r & . r ” AGN all sources within2 σ of the r distribution, while the “high- r ” AGN sample is rep-resented by the sources deviating more than 2 σ . . Lusso et al: The bolometric output and host-galaxy properties of obscured AGN in the XMM-COSMOS survey 15 Our main results are the following:1. The average observed SED is characterized by a flat X–rayslope, h Γ = . i , as expected for obscured AGN (not cor-rected for absorption), while in the optical-UV the observedlight appears to be consistent with the host-galaxy emission.The average SED in the mid-infrared is more likely a combi-nation of dust emission from star-forming region and AGNemission reprocessed by the dust.2. The full sample is split into four bins of di ff erent X–ray andinfrared luminosities and redshift. The shapes of the averageSEDs in the optical bands are approximately the same in allluminosity and redshift bins. There is a stronger host-galaxycontribution at lower luminosity / redshift bins, where the av-erage SEDs have a typical galaxy shape. Moreover, there is atrend between X–ray and mid-infrared luminosity: the con-tribution of the AGN in the infrared (around 8 − µ m) ishigher at higher X–ray luminosities.3. Type-2 AGN appear to have smaller bolometric correctionsthan Type-1 AGN. At the same hard X–ray luminosity,43 . ≤ Log L [2 − ≤ .
30, where both samples are wellrepresented, we find that the median bolometric correctionfor Type-2 AGN (134 objects) is h k bol i ∼ ±
1, to be com-pared with a median bolometric correction h k bol i ∼ ± ff erent at the ∼ σ level.4. A clear separation in bolometric corrections for the low- r and the high- r samples is found. The relation provided byGandhi and collaborators is valid for the majority of objects,while for 30% of the sample SED-fitting procedure may un-derestimate the non-nuclear contribution. At a given hard X–ray luminosity (43 ≤ Log L [2 − ≤
44) the low- r samplehas a median bolometric correction of h k bol i ∼ ± r sample of h k bol i ∼ ± k bol are statistically di ff erent atthe ∼ σ level.5. Host-galaxies morphologies and the stellar masses indicatethat Type-2 AGN are preferentially hosted in galaxies whichhave a bulge, irrespective of the strength of the bulge or if thegalaxy is on the red sequence or blue cloud, and with stellarmasses greater than 10 M ⊙ .6. Almost all the sources in the red-sequence have S S FR − larger than the age of the Universe at their redshift, whichis consistent with passive galaxies. Following the same ap-proach as in Cardamone and collaborators (i.e., combiningthe rest-frame ( U − V ) vs Log M ∗ and the rest-frame ( U − V )vs ( V − J ) color diagrams), we find that, consistent with theirresults, ∼
50% of AGN hosts lie in the passive region of thisdiagram. In contrast from Cardamone et al. (2010), only ∼
30% of AGN host-galaxies in the green-valley in our sampleare consistent with dust-obscured sources in 0 . ≤ z ≤ . ff erent depths (100 µ m and 160 µ m in the COSMOS field) and the upcoming ALMA sur-veys will allow us to gain an optimal multiwavelength coveragealso in the far-infrared. Acknowledgements.
We gratefully thank B. Simmons for her useful commentsand suggestions. In Italy, the XMM-COSMOS project is supported by ASI-INAF grants I / / /
0, I / /
06 and ASI / COFIS / WP3110 I / / /
0. ElisabetaLusso gratefully acknowledges financial support from the Marco Polo program,University of Bologna. In Germany the XMM-
Newton project is supported bythe Bundesministerium f¨ur Wirtshaft und Techologie / Deutsches Zentrum f¨ur Luft und Raumfahrt and the Max-Planck society. Support for the work of E.T.was provided by NASA through Chandra Postdoctoral Fellowship Award grantnumber PF8-90055, issued by the Chandra X-ray Observatory Center, which isoperated by the Smithsonian Astrophysical Observatory for and on behalf ofNASA under contract NAS8-03060. The entire COSMOS collaboration is grate-fully acknowledged.
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