Astronomy & Astrophysics manuscript no. 7092 c© ESO 2018
October 31, 2018
The HELLAS2XMM survey
X. The bolometric output of luminous obscured quasars:
The Spitzer perspective
F. Pozzi,1,2 C. Vignali,1,2 A. Comastri,2 L. Pozzetti,2 M. Mignoli,2 C. Gruppioni,2 G. Zamorani,2 C. Lari,3 F.
Civano,1,2 M. Brusa,4 F. Fiore,5 R. Maiolino,5 and F. La Franca6
1 Dipartimento di Astronomia, Università degli Studi di Bologna, Via Ranzani 1, I–40127 Bologna, Italy
2 INAF — Osservatorio Astronomico di Bologna, Via Ranzani 1, I–40127 Bologna, Italy
3 INAF — Istituto di Radioastronomia (IRA), Via Gobetti 101, I–40129 Bologna, Italy
4 Max Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstrasse 1, D-85748 Garching bei München, Germany
5 INAF — Osservatorio Astronomico di Roma, Via Frascati 33, I–00040 Monteporzio-Catone (RM), Italy
6 Dipartimento di Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, I–00146 Roma, Italy
Preprint online version: October 31, 2018
ABSTRACT
Aims. We aim at estimating the spectral energy distributions (SEDs) and the physical parameters related to the black holes harbored in eight
high X-ray-to-optical (FX/FR >10) obscured quasars at z > 0.9 selected in the 2–10 keV band from the HELLAS2XMM survey.
Methods. We use IRAC and MIPS 24 µm observations, along with optical and Ks-band photometry, to obtain the SEDs of the sources. The
observed SEDs are modeled using a combination of an elliptical template and torus emission (using the phenomenological templates of Silva
et al. 2004) for six sources associated with passive galaxies; for two point-like sources, the empirical SEDs of red quasars are adopted. The
bolometric luminosities and the MBH − LK relation are used to provide an estimate of the masses and Eddington ratios of the black holes
residing in these AGN.
Results. All of our sources are detected in the IRAC and MIPS (at 24 µm) bands. The SED modeling described above is in good agreement
with the observed near- and mid-infrared data. The derived bolometric luminosities are in the range ≈ 1045 − 1047 erg s−1, and the median
2–10 keV bolometric correction is ≈25, consistent with the widely adopted value derived by Elvis et al. (1994). For the objects with
elliptical-like profiles in the Ks band, we derive high stellar masses (0.8 − 6.2)×10
11 M⊙, black hole masses in the range (0.2 − 2.5)×10
9 M⊙,
and Eddington ratios L/LEdd < 0.1, suggesting a low-accretion phase.
Key words. quasars: general — galaxies: nuclei — galaxies: active
1. Introduction
Hard X-ray surveys have clearly revealed the important role
played by obscured Active Galactic Nuclei (AGN) to repro-
duce the cosmic X-ray background (XRB; e.g., Comastri et al.
1995) and have provided evidence that a significant fraction of
the accretion-driven energy density in the Universe resides in
obscured X-ray sources (e.g., Barger et al. 2005; Hopkins et al.
2006; Hickox & Markevitch 2006).
Until recently, the limited information on the broad-band
emission of the counterparts of obscured X-ray sources pre-
vented a reliable determination of their bolometric luminosi-
ties. The lack of a proper knowledge of the spectral energy dis-
tributions (SEDs) of obscured sources has led many authors to
adopt, in the computation of the bolometric luminosities, the
average value derived by Elvis et al. (1994), although in that
work the sample comprises mostly local unabsorbed quasars.
By the current work, we aim at providing a robust estimate of
the bolometric luminosity for obscured sources, which is an
essential parameter to derive the cosmic mass density of super-
massive black holes (SMBHs, i.e., following the Soltan 1982
approach). A reliable estimate of the bolometric luminosity of
obscured AGN is typically limited by the actual capabilities
of disentangling the nuclear emission (related to the accretion
processes) from that of the host galaxy which, unlike for un-
absorbed quasars, often dominates at optical and near-infrared
(near-IR) bands.
A significant fraction of high-redshift, luminous obscured
AGN (the so-called Type 2 quasars) may have escaped spec-
troscopic identification due to their faint optical counterparts,
thus preventing current studies from an accurate sampling of
obscured sources. Mid-infrared (mid-IR) observations appear
http://arxiv.org/abs/0704.0735v2
2 F. Pozzi, C. Vignali, A. Comastri et al.: Spitzer observations of luminous obscured quasars
Table 1. Properties of our targets
Source Id. 2–10 keV flux† R Ks Morph(Ks) X/O z
(10−14 erg cm−2 s−1)
Abell 2690#75 3.30 24.60 18.33 E 1.86 2.13a
PKS 0312−77#36 1.90 24.70 19.13 E 1.66 –
PKS 0537−28#91 4.20 23.70 18.99 E 1.60 –
PKS 0537−28#54 2.10 25.10 18.91 E 1.86 –
PKS 0537−28#111 2.10 24.50 17.64 E 1.62 –
Abell 2690#29 2.80 25.10 17.67 P 1.99 2.08b
PKS 0312−77#45 2.80 24.40 18.62 P 1.71 –
BPM 16274#69 2.27 24.08 17.87 E 1.49 1.35b
† 2-10 kev X-ray fluxes from Perola et al. (2004).
a Tentative spectroscopic redshift from near-IR spectroscopic observations (Maiolino et al. 2006).
b Spectroscopic redshift from near-IR spectroscopic observations (Maiolino et al. 2006).
to be fundamental for this class of objects, since they are only
marginally affected by dust obscuration and are able to recover
the nuclear emission. With mid-IR observations, we expect to
reveal the nuclear radiation re-processed by the torus of the
obscured active nuclei, which are often recognized as such by
means of their X-ray emission only. For these sources, the
soft X-ray emission, which is photo-electrically absorbed by
the gas, and the optical emission, extincted by the dusty cir-
cumnuclear medium, are expected to be downgraded in energy
and to emerge as thermally reprocessed radiation in the IR at
wavelengths in the range between a few and a few hundred µm
(Granato et al. 1997).
The potentialities of mid-IR observations were firstly
shown by Fadda et al. (2002), who detected with ISOCAM at
15 µm about two-thirds of the X-ray sources detected in the
5–10 keV band in the XMM-Newton Lockman Hole survey. A
similar high detection rate at 24 µm has been recently reported
by Rigby et al. (2004) and Franceschini et al. (2005) studying
the Spitzer counterparts of the Chandra sources in the CDF-
S and in the ELAIS-N1 field, respectively, within the SWIRE
survey (Lonsdale et al. 2004).
In this context, a new interesting class of objects is emerg-
ing from the current X-ray surveys: these sources are character-
ized by a high (>1) X-ray-to-optical flux ratio (hereafter X/O)1;
for comparison, unobscured Type 1 AGN have a broad distri-
bution peaked at X/O≈0. Objects with X/O &1 are about 20%
of the hard X-ray selected sources, and the fraction of these
sources seems to remain constant over ≈ 3 decades of X-ray
flux (Comastri & Fiore 2004). By definition, sources with high
X/O are among the faintest sources in the optical band. In the
shallow, large-area X-ray surveys (e.g., the HELLAS2XMM
survey, with F2−10 keV > 10
−14 erg cm−2 s−1 over ≈ 3 deg2;
Baldi et al. 2002), where the identification of a sizable sam-
ple of sources with X/O>1 has been possible (e.g., Fiore et al.
2003), the X/O selection criterion has proven to be effective in
selecting Type 2 quasars at high redshifts.
1 X/O is defined as log FX
= log FX +
+ 5.5.
We have performed a pilot program to study with Spitzer a
sample of eight sources selected in the 2–10 keV band from the
HELLAS2XMM survey on the basis of their high (>1) X/O and
large column densities (NH ≥10
22 cm−2). The sample observed
with Spitzer has been previously investigated in other bands.
The most surprising finding of the follow-up campaigns was
the association of these sources with luminous near-IR objects
(Mignoli et al. 2004), placing them into the class of Extremely
Red Objects (EROs, R − K ≥5).
The outline of the paper is as follows: in Sect. 2 we present
the sample selection; Spitzer data reduction and analysis are
discussed in Sect. 3, while in Sect. 4 we describe the analy-
sis of the SEDs. Finally, in Sect. 5 we estimate the bolometric
luminosities, the stellar masses of the host galaxies, the black
hole masses, and the Eddington ratios.
Throughout this paper we adopt the “concordance”
(WMAP) cosmology (H0=70 km s
−1 Mpc−1, ΩM=0.3, and
ΩΛ=0.7; Spergel et al. 2003). Magnitudes are expressed in the
Vega system.
2. Sample selection
The eight objects presented in this paper (see Table 1) were se-
lected among the 10 HELLAS2XMM high X/O ratio sources
detected in the Ks band with ISAAC at ESO-VLT; for details
on the association of the Ks-band counterpart to the X-ray
source, see Mignoli et al. (2004). Two sources of the origi-
nal sample were not selected for Spitzer observations: one
(PKS 0537−28#31) is associated with a disky galaxy, while
the other (BPM 16274#181) has an ambiguous Ks-band mor-
phological classification. All but one of the sources observed
by Spitzer belong to the first square degree field (122 X-ray
sources; see Fiore et al. 2003 for the spectroscopic and photo-
metric identification and Perola et al. 2004 for the X-ray spec-
tral analysis); the only exception is BPM 16274#69, which be-
longs to the sample of the second square degree (110 X-ray
sources; see Cocchia et al. 2007; Lanzuisi et al., in prepara-
tion).
F. Pozzi, C. Vignali, A. Comastri et al.: Spitzer observations of luminous obscured quasars 3
Fig. 1. ISAAC Ks images, centered on the Ks counterpart of the X-ray sources; each box is 30′′ wide. North is to the top and East
to the left. Contour levels of the 24 µm emission corresponding to [3, 4, 5, 7, 10, 20, 40]σ are superimposed to each image.
The selected sources, although faint in the optical band
(23.7<R<25.1), are all bright in the near-IR (Ks.19) and, all
but one, have R − Ks > 5, thus being EROs. Mignoli et al.
(2004) were able to study the surface brightness profiles of
these sources in the Ks band and obtained a morphological clas-
sification. Only two sources are classified as point-like objects,
while all of the others are extended, with clear detection of the
host galaxy and radial profiles consistent with those of ellipti-
cal galaxies. In this latter class of sources, the central AGN is
evident in the X-ray band, while in the near-IR the host galaxy
dominates. An upper limit to the contribution of a central un-
resolved source (i.e., the nuclear emission), ranging from 2%
to 12% of the galaxy emission, was obtained. Furthermore, us-
ing both the R − K colour and the morphological information,
a minimum photometric redshift, in the range 0.9–2.4, was es-
timated for these sources.
For three of the sources with the reddest colours
(Abell 2690#75, Abell 2690#29 and BPM 16274#69; see
Table 1), near-IR spectroscopic observations with ISAAC at
ESO-VLT were performed by Maiolino et al. (2006), thus al-
lowing for a spectroscopic identification for at least two of
these sources (one redshift measurement appears tentative).
The point-like source (Abell 2690#29) shows the typical rest-
frame optical spectrum of high-redshift dust-reddened quasars,
with a broad Hα line (Gregg et al. 2002). The other two ob-
served sources, both of them extended in the Ks band, have nar-
row emission-line spectra: one is a LINER-like object at z=1.35
and the second source has a spectrum with a single weak line,
tentatively associated with Hα at z=2.13. Consistently with the
morphological information, in the first source the AGN domi-
nates the emission, while in the other two sources the nuclear
spectrum is heavily diluted by the host galaxy starlight.
4 F. Pozzi, C. Vignali, A. Comastri et al.: Spitzer observations of luminous obscured quasars
Table 2. Spitzer flux densities
Source Id. 3.6 µm 4.5 µm 5.8 µm 8.0 µm 24 µm
Sν±∆Sν Sν±∆Sν Sν±∆Sν Sν±∆Sν Sν±∆Sν
Abell 2690#75 51 ± 5 56 ± 6 89 ± 11 139 ± 15 565 ± 62
PKS 0312−77#36 41 ± 4 44 ± 5 40 ± 8 71 ± 9 236 ± 30a
PKS 0537−28#91 28 ± 4 35 ± 4 42 ± 8 80 ± 10 301 ± 40
PKS 0537−28#54 31 ± 4 35 ± 4 50 ± 10 47 ± 8 279 ± 45
PKS 0537−28#111 88 ± 9 75 ± 8 41 ± 6 46 ± 7 148 ± 28
Abell 2690#29 141 ± 14 185 ± 19 260 ± 27 371 ± 38 1012 ±106a
PKS 0312−77#45 50 ± 6 62 ± 7 69 ± 10 78 ± 10 249 ± 35
BPM 16274#69 86 ± 9 92 ± 9 97 ± 11 120 ± 13 286 ± 34
The flux density is reported in units of µJy. a The 24 µm flux density is probably over-
estimated due to contamination from nearby sources and should be considered as an
upper limit.
3. Spitzer observations and data reduction
The whole sample of eight hard X-ray selected sources has
been observed by Spitzer (Werner et al. 2004), with IRAC
(Fazio et al. 2004) observations of 480 s integration time and
MIPS (Rieke et al. 2004) observations at 24 µm with a total
integration time of ≈ 1400 s per position. IRAC observations
were performed in photometry mode with frame time of 30 s
and dither pattern of 16 points. The MIPS 24 µm observations
were performed in MIPS photometry mode with frame time of
10 s, 10 cycles and small-field pattern. To reduce overheads,
the cluster option was used when possible.
For the IRAC bands, we used the final combined post-basic
calibrated data (BCD) mosaics produced by the Spitzer Science
Center (SSC) pipeline (Version S12.0–S13.01). At 24 µm,
we started the analysis from the BCD produced by the SSC
pipeline (Version S12.4.2–S13.01) and then we applied ad hoc
procedures to optimize the reduction, since some of our sources
were close to the detection limit (see Table 2). We remind that
BCD are individual frames already corrected for dark, flat field
and geometric distortion. We improved the quality of the BCD
by correcting each individual BCD for a residual flat field de-
pending on the scan mirror position (Fadda et al. 2006). The
residual flat fielding was obtained from our own data by aver-
aging the BCD corresponding to the same scan mirror position
and the same Astronomical Observation Request (AOR), con-
sidering all the different cluster positions. To each BCD, its
median level was subtracted before this operation. This pro-
cedure was possible since our observations are not dominated
by background fluctuations. The corrected BCD were co-added
and background-subtracted using the SSC MOPEX software
(Makovoz & Marleau 2005). The resulting mosaics were made
with 2.4′′ pixel size. The overall analysis at 24 µm produces
mosaics with a typical noise of ≈ 0.020 MJy/pixel (a factor of
2 lower in comparison with the SSC pipeline mosaics). The
noise map has been computed for each mosaic by scaling the
measured mean rms of the central part of the map according to
the inverse square root of the coverage map.
The flux densities of our targets in IRAC and MIPS bands
were measured on the signal maps using aperture photometry at
the position of the sources. The chosen aperture radius for the
IRAC bands is 2.45′′ and the adopted factors for the aperture
corrections are 1.21, 1.23, 1.38 and 1.58 (following the IRAC
Data Handbook, Version 3.0) at 3.6 µm, 4.5 µm, 5.8 µm and 8
µm, respectively.
The chosen aperture radius at 24 µm is 7.5′′; aperture correc-
tions were derived by examining the photometry of bright stars.
Taking into account an additional correction of 1.15 to match
the procedure used by the MIPS instrument team to derive cal-
ibration factors from standard star observations, the resulting
aperture correction is 1.57 (in agreement with the SWIRE team,
see Surace et al. 2005).
Table 2 reports the results of the Spitzer observations. To
compute the photometric uncertainties, we added in quadra-
ture the noise map and the systematic uncertainties (≈10%, see
MIPS and IRAC Data Handbook 2006, Version 3.0). The rel-
ative photometric uncertainties range from ≈10% in the best
cases (IRAC channels 1 and 2), up to ≈20% at 24 µm in the
worst ones.
All of the eight sources are clearly detected in both IRAC
and MIPS 24 µm bands. At 24 µm, the flux densities span an
order of magnitude, ranging between ≈1000 µJy and ≈150 µJy,
with the faintest source (PKS 0537−28#111) close to the 5σ
detection level.
In Fig. 1 the Ks-band images, along with the contour lev-
els of the 24 µm emission, are shown. At 24 µm, the sources
PKS 0312−77#36 and Abell 2690#29 appear to be confused.
In particular, in both cases, there is a second source at ≈ 8-10′′,
unrelated to the targets. The contribution of these sources to
the 24 µm flux density of our targets has been estimated by a
decomposition analysis (using the PSF fitting algorithm IMFIT
within the AIPS environment). Furthermore, both the sources
PKS 0312−77#36 and Abell 2690#29 present a second object
at ≈ 2′′, clearly visible in the Ks images, too close to our targets
for a decomposition analysis, given the 24 µm pixel size. Since
these close companions become increasingly fainter moving
from the Ks bands to the longer IRAC wavelengths, we have
attributed the entire flux density estimated from the decompo-
sition analysis to our targets (see Table 2). However, the 24 µm
flux densities for these two sources should be treated as upper
limits (see Fig. 2).
F. Pozzi, C. Vignali, A. Comastri et al.: Spitzer observations of luminous obscured quasars 5
We point out that the deblending procedure measures the
peak flux density. To convert the peak flux density into total
flux density, we have assumed that the deblended objects are
point-like sources and applied a correction factor of 8.9 derived
from the 24 µm Spitzer PSF and including the 1.15 calibration
factor.
4. Analysis of the spectral energy distributions
In this section we provide an analysis of the SEDs of our
sources, in order to derive the energy distribution of the nu-
clear component “cleaned” by the host galaxy contribution. As
anticipated in Sect. 1, the determination of the nuclear SEDs
over a large wavelength range is an essential step to estimate
the physical properties of the black hole, such as its bolometric
luminosity, mass, and accretion rate. Taking advantage of the
new Spitzer photometric points, simultaneously to the SED de-
termination we have estimated the photometric redshifts of our
sources. These new values are then compared with the min-
imum redshifts estimated by Mignoli et al. (2004) using only
the R and Ks bands and with the spectroscopic ones measured
in three cases by Maiolino et al. (2006, see §2).
Given the different morphological properties of our
sources, two approaches have been adopted, one for the sources
dominated in the Ks band by the host galaxy (elliptical-like
sources) and another one for the sources dominated by the nu-
clear component (point-like sources).
4.1. Elliptical-like sources
From the Ks-band morphological analysis, we know that at
least up to the observed 2.2 µm band the stellar contribu-
tion dominates the emission in these sources. At longer wave-
lengths, the nuclear component is expected to arise as repro-
cessed radiation of the primary emission, while the stellar
component is expected to drop (e.g., Bruzual & Charlot 2003;
Silva et al. 2004).
In the analysis presented in this work, we have decided
to follow a phenomenological approach, checking whether the
emission of our sources can be reproduced as the sum of two
components, one from the host galaxy and the other related to
the reprocessing of the nuclear emission by the dusty torus en-
visaged by unification schemes (Antonucci 1993). The shape
and relative strengths of the two components have to be con-
sistent with all our observed data sets (multi-band photometry,
Ks-band morphology and magnitude, Ks-band upper limit on
the nuclear component, and X-ray spectral analysis).
For the galaxy component, we adopted a set of six galaxy
templates, obtained from the synthetic spectra of GISSEL 2003
(Bruzual & Charlot 2003) assuming a simple stellar population
and spanning a wide range of ages, from 1 Gyr up to a “maxi-
mum age” model (z f orm = 20). The “maximum age” model has
been adopted by Mignoli et al. (2004) to derive the minimum
photometric redshift for the sources of the current sample.
For the nuclear component, we adopted the nuclear tem-
plates from Silva et al. (2004), based on the radiative transfer
models of Granato & Danese (1994). We chose these templates
since in the work of Silva et al. (2004) the radiative transfer
models are used to interpolate the observed nuclear IR data for
a sizable sample of local AGN. We must note, however, that
the nuclear observed SEDs are available only in the 2–20 µm
regime, where data from small-aperture instruments are avail-
able. At wavelengths above ≈ 20 µm, the SEDs are model ex-
trapolations.
Silva et al. (2004) found that the nuclear SEDs can be ex-
pressed as a function of two parameters, the hard X-ray (2–
10 keV) intrinsic luminosity, which provides the normalization
to the SED, and the column density NH , which gives the shape
to the SED (see Fig. 2 in Silva et al. 2004). The shapes of the
SEDs of the Seyfert galaxies are assumed to be valid also at
quasar luminosities.
In the attempt to provide a better estimate for the source
redshifts, we used the four torus templates as given by
Silva et al. (2004), which depend on the column densities NH ,
and we left the normalizations free to vary. The redshift interval
explored by this procedure is 0.5–3.0; the best-fit solution is ob-
tained when the algorithm, based on the χ2, finds a minimum
in the galaxy template, torus template and redshift parameter
space.
For four out of six sources the procedure finds a clear min-
imum χ2, which allows us to determine a photometric red-
shift with relatively good accuracy (see Table 3). For sources
BPM 16274#69 and PKS 0537−28#54, our procedure con-
strains only the lower bound of the redshift interval. For all
the six sources, the estimated redshifts are consistent with
the minimum redshifts of Mignoli et al. (2004). In case of the
source BPM 16274#69, where a secure spectroscopic redshift
is available, the minimum photometric redshift (zphot > 1.25)
is consistent with the spectroscopic one (z = 1.35). For source
Abell 2690#75, we find zphot = 1.30
+0.30
−0.20, which is signifi-
cantly lower than the spectroscopic value (z = 2.13) reported
by Maiolino et al. (2006). In this case, we choose to adopt the
photometric determination, since the spectroscopic redshift is
based on the tentative detection of a single line.
The results of the SED fitting and decomposition are shown
in Fig. 2. The dot-dashed line represents the best-fit galaxy tem-
plate, the dashed line is the best-fit nuclear template and the
thick solid line is the sum of the two components. The best-fit
galaxy templates are all typical of early-type galaxies with ages
between 3 and 6 Gyr. We find an overall agreement between the
SED templates and the data points. Moreover, in all but one of
the sources, the nuclear component derived from the best fit
is consistent with the upper limits derived from the analysis
of the Ks-band images (shown as downward-pointing arrows).
In Fig. 2 we also report as dotted line the SED of the nuclear
component normalized to the intrinsic (i.e., de-absorbed) X-ray
luminosity following the prescriptions of Silva et al. (2004),
where, at a given NH , the normalization depends only on the in-
trinsic hard X-ray luminosity. The overall agreement between
the SEDs normalized to the X-ray luminosity and the best-fit
SEDs is extremely interesting, being consistent within a factor
of ≈ 2–3.
In Table 3 we report, along with the photometric redshifts,
the column densities NH and the de-absorbed L2−10 keV lumi-
nosities. We derive rest-frame NH column densities in the range
1022.0–1023.4 cm−2 and 2–10 keV luminosities in the range
6 F. Pozzi, C. Vignali, A. Comastri et al.: Spitzer observations of luminous obscured quasars
Fig. 2. Rest-frame SEDs of the elliptical sources (black filled circles) compared with the best-fit model obtained as the sum (solid
line) of an early-type galaxy (dot-dashed line) and a nuclear component (dashed line). For comparison, the nuclear component
as derived from the X-ray normalization is also reported (dotted line). The nuclear Ks-band upper limits (downward-pointing
arrows) were derived from the morphological analysis carried out by Mignoli et al. (2004). The 24 µm upper limit of source
PKS 0312−77#36 takes into account a possible contribution from a companion source. zs means that the redshift is spectroscopic
(see §2 for details), while zphot means that the redshift is photometric (see §4.1).
1043.8–1044.7 erg s−1, placing these sources among the Type 2
quasar population.
4.2. Point-like sources
From the Ks-band morphological analysis, Abell 2690#29 and
PKS 0312−77#45 (see Mignoli et al. 2004 and Table 1) show
a completely different appearance at 2.2 µm in comparison
with the sample of extended objects; these two sources have
their near-IR emission mostly dominated by an unresolved
source. The dominant role played by the AGN is supported
for Abell 2690#29 also by its near-IR spectrum, where a broad
Hα emission line is detected (see Maiolino et al. 2006 and §2).
F. Pozzi, C. Vignali, A. Comastri et al.: Spitzer observations of luminous obscured quasars 7
Fig. 3. Rest-frame SEDs of the two point-like sources (black filled circles) compared to an extinguished quasar template (dashed
line) and the best-fit red quasar template (solid line). The extinguished quasar template is obtained from the unobscured quasar
template of Elvis et al. (1994) using the SMC extinction law with E(B − V) = 0.7 and scaled to fit the R − Ks colour (for
comparison, the unobscured quasar template is also shown as a dotted line). The red quasar template is taken from Polletta et al.
(2006). The 24 µm flux density upper limit for the source Abell 2690#29 takes into account a possible contribution from a
companion source. zs means that the redshift is spectroscopic (see §2 for details), while zphot means that the redshift is photometric
(see §4.2).
Unfortunately, the near-IR spectroscopy information is absent
for PKS 0312−77#45 (see Table 1).
We first tried to reproduce their observed SEDs by redden-
ing the composite template spectrum of bright Type 1 quasars
of Elvis et al. (1994) with several extinction laws. Reddening
has been applied as prescribed by Calzetti (1997) for a dust-
screen model and by Pei (1992) for the Small Magellanic
Cloud (SMC) galaxy. The two prescriptions produce similar
effects at λ > 0.5 µm, but the SMC law produces redder spec-
tra at shorter wavelengths for the same amount of extinction.
Reddened templates with an SMC law reproduce quite well the
optical spectra of dust-reddened quasars in the Sloan Digital
Sky Survey (SDSS; see Richards et al. 2003), while, using the
Calzetti (1997) law, Polletta et al. (2006) were able to repro-
duce the SEDs of X-ray sources in the Spitzer SWIRE survey.
The procedure of reddening a typical Type 1 quasar does
not provide a satisfactory fit to the photometric data points of
our sources. In Fig. 3 we show the results obtained when the
prescription of Mignoli et al. (2004) for the extinction [SMC
extinction law and E(B − V) = 0.7] is adopted.
The dashed line shows the reddened quasar template nor-
malized to fit the R−Ks colour. For comparison, the unobscured
quasar template is also shown (dotted line). Although the R−Ks
colour is obviously reproduced, the overall SED is not well re-
produced, since the observed IRAC and 24 µm flux densities
are systematically lower than predicted (up to a factor of 10 at
24 µm).
The discrepancy between the data points and the reddened
Type 1 quasar template might be due to the application to ac-
tive galactic nuclei of an extinction curve derived from galax-
ies. The different behaviour of AGNs from galaxies can be
attributed to different dust distribution (i.e., torus shape in
AGNs), and gas-to-dust ratios, which can lead to an unusual
dust reddening curve for AGNs.
Since a reddened quasar template does not reproduce the
shape of our data, we adopted the red quasar template from
Polletta et al. (2006), which is a composite spectrum: in the
optical/near-IR band, it is the spectrum of the red quasar
FIRST J013435.7−093102 from Gregg et al. (2002), while the
average of several bright quasars from the Palomar-Green (PG)
sample (Schmidt & Green 1983) with consistent optical data
has been used in the IR. The Polletta et al. (2006) template re-
produces the observed data points significantly better, allowing
for the observed sharp decrease from 0.2 to 0.7 µm in the source
rest frame (see Fig. 3 where the Polletta et al. 2006 spectrum is
shown as a solid line).
For source Abell 2690#29, which has a spectroscopic red-
shift, the SED normalization has been obtained through a best-
fit procedure. For source PKS 0312−77#45, where only a min-
imum redshift was available prior to this analysis, we have left
free to vary both the normalization and the redshift.
In Table 3 we report the derived redshifts, column densi-
ties NH and de-absorbed L2−10 keV also for these sources which,
similarly to the elliptical-like sources, belong to the Type 2
quasar population.
5. Physical parameters
5.1. Bolometric correction
Once the SED of the nuclear component has been determined,
the following step is to estimate the bolometric luminosity Lbol,
which is a quantity directly related to the central black hole
activity.
The bolometric luminosity Lbol can be estimated from
the luminosity in a given band b, Lb, by applying a suit-
able bolometric correction kbol,b = Lbol/Lb. For the X-ray se-
lected sources, the bolometric luminosity is typically estimated
8 F. Pozzi, C. Vignali, A. Comastri et al.: Spitzer observations of luminous obscured quasars
from the luminosity in the 2–10 keV band (kbol,2−10 keV =
Lbol/L2−10 keV ). In previous works, several authors used the
bolometric correction obtained by Elvis et al. (1994) for lumi-
nous, mostly nearby quasars, i.e., kbol,2−10 keV≈ 30. However,
these corrections could be affected by the following uncer-
tainties: firstly, they are average corrections obtained from
a few dozens of bright quasars; secondly, as discussed in
Marconi et al. (2004), these corrections could overestimate the
bolometric luminosities since they are based on the integral
of the observed SEDs of bright unobscured AGN, without
removing the IR bump (hence counting twice a fraction of
≈ 30% of the intrinsic optical–UV radiation). At lower lumi-
nosities (typical of Seyfert galaxies, i.e., 1042 − 1044 erg s−1), a
lower value for this correction (kbol,2−10 keV≈ 10) was suggested
(e.g., Fabian 2004). For heavily obscured luminous sources,
only few objects have been studied in detail; in particular, for
two SWIRE Compton-thick (i.e., log NH & 24 cm
−2) AGN
Polletta et al. (2006) found kbol,0.3−8 keV ≈ 3 and ≈ 100.
In this work, thanks to the multi-band observations and ef-
forts in disentangling the AGN and the host components, we
try to derive directly the nuclear bolometric luminosity of our
sources without assuming any average correction. We estimate
Lbol by adding the X-ray luminosity integrated over the entire
X-ray range (L0.5−500 keV , not corrected for absorption) to the
IR luminosity (L1−1000 µm).
L0.5−500 keV has been estimated from the observed L2−10 keV
luminosity assuming a single power-law spectrum with Γ=1.9
(typical for AGN emission; see, e.g., Fig. 6 of Vignali et al.
2005 and references therein) plus absorption (where the col-
umn densities are taken from the X-ray spectral analysis; see
Perola et al. 2004 and Lanzuisi et al., in preparation) and an
exponential cut-off at 200 keV. The median value found for the
ratio L0.5−500 keV/L2−10 keV is ≈ 4. The IR luminosity has been
estimated by integrating the SED from 1 µm to 1000 µm using
only the nuclear component for the AGN hosted in the ellipti-
cal galaxies (see §4.1), and the Polletta et al. (2006) template
for the point-like sources.
Before computing the bolometric output of our sources, the
derived IR luminosities must be properly corrected to account
for the geometry of the torus and its orientation. The first cor-
rection is related to the covering factor f (which represents the
fraction of the primary optical–UV radiation intercepted by the
torus), while the second correction is due to the anisotropy of
the IR emission, which is a function of the viewing angle (see
Pier & Krolik 1993 and Granato & Danese 1994 for further de-
tails).
We estimated the first correction (≈ 1.5) from the ratio
of obscured (Compton thin + Compton thick) to unobscured
quasars as required by the most recent X-ray background syn-
thesis model (see Gilli et al. 2007) in the luminosity range of
our sources. A correction of ≈ 1.5 implies an average covering
factor f≈0.67 which, in a simple torus geometry, corresponds
to an angle θ≈48◦ between the perpendicular to the equatorial
plane and the edge of the torus.
A first-order estimate of the anisotropy factor has been
computed from the Silva et al. (2004) templates as the ratio
(R) of the luminosity of a face-on vs. an edge-on AGN, whose
obscuration is parametrized as a function of NH. The integra-
tion has been performed in the 1–30 µm range, after normal-
izing the two SEDs to the same luminosity in the 30–100 µm
range, where the anisotropy is thought to be negligible. The
derived anisotropy factors are large only for the Silva et al.
(2004) template with higher column density (R ≈3–4 for NH =
1024.5 cm−2); since all of our targets are characterized by lower
obscuration, such corrections do not affect our IR luminosities
significantly (R ≈1.2–1.3 for NH ≈ 10
22.0 − 1023.4 cm−2). In
conclusion, the final combined corrections to be applied to the
observed IR luminosities of our sources, given their column
densities, are in the range ≈ 1.8–2.0. After adding the X-ray lu-
minosities, the IR correction factors would translate in a mean
correction factor of ≈ 1.7 in the computation of the bolometric
luminosities.
In Table 3 the derived bolometric luminosities are reported
along with the full range (L0.5−500 keV) of X-ray luminosities,
the IR (L1−1000 µm) luminosities and the bolometric corrections
(kbol,2−10 keV ). We note that our L1−1000 µm estimates (hence Lbol)
are robust despite the choice of our SEDs. By comparing the
L1−1000 µm obtained using the Silva et al. (2004) model with the
L1−1000 µm obtained adopting other recent average quasar SEDs
(i.e., Richards et al. 2006), we have verified that the uncertain-
ties in L1−1000 µm are within the ≈ 10% level. The bolometric
output of our targets is dominated by the IR reprocessed emis-
sion, the primary X-ray radiation (L0.5−500 keV) accounting only
for .15% of the total luminosity.
The derived median (mean) value of kbol,2−10 keV is ≈ 25
(35±9; see Fig. 4 and column 8 of Table 3), consistent with the
value kbol,2−10 keV≈ 30 from Elvis et al. (1994) widely adopted
in past works. However, as pointed out also by Elvis et al.
(1994), the bolometric corrections span a wide range of values
(≈ 12–100); as a consequence, the adoption of a mean value
could lead to inaccurate results.
We note that in the extreme case where no corrections for
the covering factor and the anisotropy of the torus are applied,
we would obtain a median kbol,2−10 keV of ≈ 16.
In Fig. 4 the derived Lbol as a function of L2−10 keV is shown;
the dot-dashed line joins the expected values from the analysis
of Marconi et al. (2004), where kbol,2−10 keV is derived by con-
structing an AGN reference template taking into account how
the spectral index αox (Zamorani et al. 1981) varies as a func-
tion of the luminosity (Vignali et al. 2003). Although the bolo-
metric luminosities estimated for our objects are on average
lower than those expected on the basis of Marconi et al. (2004)
relation, they are however consistent with a trend of higher
kbol,2−10 keV for objects with higher X-ray luminosity. If we fit
our objects in the Lbol − L2−10keV plane with the same slope as
the Marconi et al. (2004) relation, the difference in normaliza-
tion is ≈ 50%.
5.2. Galaxy and black hole masses, and black hole
Eddington ratios
For the AGN hosted in elliptical galaxies we are able to infer
both the galaxy and the black hole masses. The galaxy masses
are estimated, assuming a Salpeter (1955) initial mass func-
tion (IMF), from the Ks luminosities taking into account that
ozzi,C
ignali,A
astri
itzer
observations
inous
obscured
quasars
Table 3. Inferred rest-frame properties of our targets
Source Id. za NbH L
2−10 keV L
0.5−500 keV L
1−1000 µm Lbol Lbol/L2−10 keV L
K Mstar M
BH (Lbol/LEdd)
(1022 cm−2) (1044 erg s−1) (1045 erg s−1) (1045 erg s−1) (1045 erg s−1) (1011Lk,⊙) (10
11M⊙) (10
Abell 2690#75 1.30+0.30
−0.20 6.9 3.2 1.3 9.7 11.0 34.7 5.2 3.5 1.3 0.065
PKS 0312-77#36 0.90+0.05
−0.15 1.0 0.7 0.2 1.2 1.5 20.6 1.0 0.8 0.2 0.058
PKS 0537-28#91 1.30+0.40
−0.70 25.8 5.3 2.6 4.4 7.1 13.4 2.8 1.5 0.7 0.084
PKS 0537-28#54 >1.30 1.6 2.0 0.6 3.9 4.6 23.0 3.0 2.0 0.7 0.049
PKS 0537-28#111 1.20+0.20
−0.10 9.1 1.7 0.7 1.4 2.1 12.3 7.8 6.2 2.1 0.008
Abell 2690#29 2.08 2.1 8.4 2.8 78.4 81.2 97.0 4.17 – – –
PKS 0312-77#45 1.85+0.20
−0.30 8.0 6.2 2.6 27.2 29.8 47.9 1.65 – – –
BPM 16274#69 1.35 2.5 2.4 0.8 5.9 6.7 28.2 8.8 6.0 2.5 0.022
a Photometric redshifts as derived from the analysis presented in this paper (see §4.1). For source PKS 0537-28#54, only a minimun redshift was estimated; for sources Abell 2690#29
and BPM 16274#69, the spectroscopic redshifts measured by Maiolino et al. (2006) are reported.
b The column densities, measured through X-ray spectral fitting (see Perola et al. 2004 and Lanzuisi et al., in preparation, for details), were “matched” to the redshift used in the SED
best-fitting procedure (using the relation NH(z)=NH(z = 0)×(1 + z)
2.6).
c Absorption-corrected X-ray luminosity.
d The 0.5–500 keV luminosities have been derived from the observed 2–10 keV luminosities as described in the text. The luminosities are not corrected for absorption.
e The 1–1000 µm luminosities have been derived from the integral of the nuclear SEDs (including the corrections described in the text). The values reported for the point-like sources
refer to the Polletta et al. (2006) red quasar template (see 4.2 for details).
f The Ks-band luminosities refer to the nuclear component for the point-like sources (Abell 2690#29 and PKS 0312−77#45) and to the host-galaxy starlight for the AGN hosted in the
elliptical galaxies.
g,h The reported MBH and Lbol/LEdd have been computed from the local LK −MBH relation (Marconi & Hunt 2003), under the hypothesis of an evolution of the MBH/Mstar ratio of a factor
two with redshift (see §5.2 for details).
10 F. Pozzi, C. Vignali, A. Comastri et al.: Spitzer observations of luminous obscured quasars
Fig. 4. Bolometric luminosity vs. absorption-corrected 2–
10 keV luminosity for the six AGN hosted in the elliptical
galaxies (circles) and the two point-like AGN (triangles). The
filled symbols refer to the values corrected for the covering fac-
tor and torus anisotropy, while the empty symbols refer to the
bolometric luminosity without applying these corrections. The
dot-dashed line represents the correlation from Marconi et al.
(2004). The three dashed lines represent the loci of kbol,x=10,
30 and 100.
Fig. 5. Black hole mass (MBH) vs. bolometric luminosity (Lbol)
for the AGN hosted in the elliptical galaxies. The Eddington
luminosity for a given MBH is reported in the right-hand axis.
The black hole masses have been estimated from the local
LK − MBH relation (Marconi & Hunt 2003) under two differ-
ent hypotheses (see 5.2): (1) evolution by a factor of two of
the MBH/Mstar ratio with redshift in comparison to the local
values (black filled circles); (2) no evolution of the MBH/Mstar
ratio (empty circles). The three dashed lines represent the loci
of Lbol/LEdd = 0.01, 0.033 and 0.1 (from left to right).
Mstar/LK for an old stellar population can vary from ≈ 0.5 to
≈ 0.9 (for ages between 3 and 6 Gyr; Bruzual & Charlot 2003).
We can derive Mstar directly from LK since for these sources
the Ks-band emission is dominated by the galaxy starlight. The
rest-frame LK have been derived using the appropriate SED
templates (see Sect. 4.1). The inferred stellar masses are in
the range (0.8–6.2)×1011 M⊙, implying that our obscured AGN
are hosted by massive elliptical galaxies at high redshifts. In
Table 3 both the LK and Mstar values are reported; we note
that the different assumption of the Chabrier (2003) IMF would
produce a factor ≈ 1.7 lower masses (di Serego Alighieri et al.
2005).
To estimate the black hole masses, we take advantage of the
local MBH − LK relation (Marconi & Hunt 2003) which, taking
into account the Mstar/LK values, is expression of the intrin-
sic MBH − Mstar relation. Given the challenging measurements
of high-redshift black hole masses, the behaviour of this rela-
tion with redshift is still matter of debate and different authors,
using different techniques, have found different results.
Woo et al. (2006) and Peng et al. (2006) derive a signifi-
cant evolution of the MBH − Mstar relation with redshift, be-
ing the MBH/Mstar ratio larger, at high redshift, up to a factor
≈ 4 in comparison to the local value. In the Woo et al. (2006)
analysis the discrepancy with respect to the local value is al-
ready present at z = 0.36, while Peng et al. (2006) find an av-
erage MBH/Mstar a factor &4 times larger than the local value
at z > 1.7, while at lower redshifts (1 . z . 1.7) they de-
rive a ratio which is at most two times higher than the local
value, and maybe consistent with marginal or no evolution. On
the other hand, Shields et al. (2006) and Hopkins et al. (2006)
suggest that the MBH/Mstar ratio is not significantly higher (at
most a factor of two) than that measured locally up to z . 2.
Given these uncertainties about the evolution of the MBH −
Mstar relation with redshift, we have estimated the black hole
masses for our objects under two different hypotheses: (1) the
MBH/Mstar ratio is higher than locally by a factor two in the
redshift range (0.9 . z . 1.4) of our sources; (2) the MBH/Mstar
ratio does not evolve with redshift.
In both cases, our results imply very massive black hole
masses (see Fig. 5), in the range ≈ 2.0 × 108 − 2.5× 109 M⊙ in
the former hyphothesis, with a factor of two lower values under
the second hypothesis.
Our estimated MBH are consistent with the results derived
by McLure & Dunlop (2004) studying a large sample of Type 1
SDSS quasars and deriving the black hole masses from virial
methods; most of their black hole masses are in the range
1.5×108–2.5×109 M⊙ in the redshift interval of our sample (see
Fig. 1 of McLure & Dunlop 2004).
From the comparison of the bolometric luminosities com-
puted in the previous section (see Table 3) with the Eddington
luminosities calculated from the black hole masses esti-
mated above, we derive that our obscured AGN are radiat-
ing at a relatively low fraction of their Eddington luminos-
ity (λ ≈ 0.008–0.084 and λ ≈ 0.015–0.170 under the two hy-
potheses; see Fig. 5 and Table 3). This finding confirms and
extends to a larger sample the results found by Maiolino et al.
(2006) for two sources of our sample and by Brusa et al. (2005)
for a sample of EROs in the “Daddi Field”. As suggested by
Maiolino et al. (2006), the data indicate that our very massive
black holes may have already passed their rapidly accreting
phase and are reaching their final masses at low accretion rates.
F. Pozzi, C. Vignali, A. Comastri et al.: Spitzer observations of luminous obscured quasars 11
Fig. 6. Bolometric luminosity as a fraction of the Eddington lu-
minosity vs. redshift for the whole sample of SDSS quasars of
McLure & Dunlop (2004), plotted as small crosses. The large
circles indicate the six HELLAS2XMM AGN hosted in ellipti-
cal galaxies (symbols as in Fig. 5).
The estimated radiating efficiencies are significantly lower
than the average Lbol/LEdd ≈ 0.4 inferred by Marconi et al.
(2004). However, since in the Marconi et al. (2004) model only
the phases of significant black hole growth are considered, our
results are not in contrast with the proposed model but sug-
gest that our targets belong to the tail of the sources charac-
terized by low accretion rates. Consistently, our data (black
filled and empty circles representing the evolution and no-
evolution hypothesis, respectively) lie in the lower envelope of
the Eddington ratio distribution found by McLure & Dunlop
(2004) for their large SDSS quasar sample. This is shown in
Fig. 6, where our data are overlaid on the SDSS data points.
This suggests that the SDSS quasar survey and the HELLAS
survey probe different regimes of AGN activity: the SDSS sam-
ples the brightest sources in the sky (R <∼ 20), most likely char-
acterized by a high accretion rate, while our targets (X-ray se-
lected, optically faint, i.e., R > 24, and obscured), are asso-
ciated with a different evolutionary phase. We argue that the
SMBH in our targets has already reached its final mass and the
observed emission is witnessing a late stage of the accretion
activity.
6. Conclusions
We have performed with Spitzer a pilot program to study a sam-
ple of eight Type 2 (i.e., luminous and obscured) quasars at
high redshift, selected from the HELLAS2XMM survey. Three
sources have a measured spectroscopic redshift (two secure and
one tentative) from near-IR spectroscopy; the remaining ob-
jects have an estimated minimum redshift obtained from the
R − K colours. On the basis of their Ks-band morphological
properties, the sample is divided into two classes: sources with
radial profiles typical of elliptical galaxies and point-like ob-
jects. The most important results can be summarized as fol-
lows:
• All of the eight sources have been clearly detected in both
IRAC and MIPS 24 µm bands.
• The Spitzer observations have allowed us to detect the nu-
clear component (often hidden at short wavelengths by the
host galaxy) as thermal IR re-processed emission from the
circumnuclear torus. While for the two point-like sources
the nuclear component dominates at all frequencies, for the
six sources with elliptical-like radial profile the contribu-
tion from the strong stellar continuum is dominant up to
the first IRAC bands, but the torus emission accounts for
the entire emission at 24 µm.
• Taking advantage of the new Spitzer data, the nuclear SEDs
of the sources have been modeled and new photometric
redshifts have been estimated, following two approaches:
for the elliptical sources, the nuclear emission has been
“cleaned” from the host galaxy contribution adopting a
two-component model (galaxy plus nuclear component),
constrained using all the extensive observed data sets. For
the point-like sources, the SEDs appear inconsistent with
an extinguished Type 1 quasar template, being well repro-
duced by an empirical SED of red quasars (Polletta et al.
2006). We find an overall agreement between the SED tem-
plates and the data points, and the derived photometric red-
shifts are consistent with the spectroscopic ones for two
sources.
• Using the model components to extrapolate the nuclear
SEDs in the far-IR regime, we derived the bolometric lumi-
nosities (being in the range ≈ 1045−1047 erg s−1) by adding
the IR luminosities to the full range of X-ray luminosities.
In this computation, we have considered and discussed the
corrections to be applied to the observed IR luminosities to
take into account the covering factor of the torus and the
anisotropy of the IR emission. The median 2–10 keV bolo-
metric correction is ≈ 25, consistent with the value typically
assumed in literature.
• For the elliptical sources, thanks to the independent esti-
mates of the stellar light and nuclear bolometric luminos-
ity, the physical parameters of the central black holes have
been estimated using the MBH − LK relation and exploring
different hypotheses for the evolution of the MBH/Mstar ra-
tio with redshift. Under the hyphothesis that the MBH/Mstar
ratio is a factor of two higher at z ≈ 1.2 than locally, our
luminous, obscured AGN have masses in the range (0.2-
2.5)×109 M⊙, reside in massive [(0.8-6.2)×10
11 M⊙] high-
redshift ellipticals and are characterized by low Eddington
ratios (λ≈ 0.008–0.084). Through our direct estimate of the
IR luminosity, we confirm the conclusion of Maiolino et al.
(2006) that these black holes may have already passed their
rapidly accretion phase.
Acknowledgements. The authors acknowledge partial support by the
Italian Space agency under the contract ASI–INAF I/023/05/0. The
authors thank R. Gilli, M. Polletta and L.Silva for useful discussions,
and R. J. McLure for kindly providing us with the data points of Fig. 6.
We thank the anonymous referee for the useful comments.
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	Introduction
	Sample selection
	Spitzer observations and data reduction
	Analysis of the spectral energy distributions
	Elliptical-like sources
	Point-like sources
	Physical parameters
	Bolometric correction
	Galaxy and black hole masses, and black hole Eddington ratios
	Conclusions

  Aims: We aim at estimating the spectral energy distributions (SEDs) and the
physical parameters related to the black holes harbored in eight high
X-ray-to-optical (F_X/F_R>10) obscured quasars at z>0.9 selected in the 2--10
keV band from the HELLAS2XMM survey.
  Methods: We use IRAC and MIPS 24 micron observations, along with optical and
Ks-band photometry, to obtain the SEDs of the sources. The observed SEDs are
modeled using a combination of an elliptical template and torus emission (using
the phenomenological templates of Silva et al. 2004) for six sources associated
with passive galaxies; for two point-like sources, the empirical SEDs of red
quasars are adopted. The bolometric luminosities and the M_BH-L_K relation are
used to provide an estimate of the masses and Eddington ratios of the black
holes residing in these AGN.
  Results: All of our sources are detected in the IRAC and MIPS (at 24 micron)
bands. The SED modeling described above is in good agreement with the observed
near- and mid-infrared data. The derived bolometric luminosities are in the
range ~10^45-10^47 erg s^-1, and the median 2--10 keV bolometric correction is
~25, consistent with the widely adopted value derived by Elvis et al. (1994).
For the objects with elliptical-like profiles in the K_s band, we derive high
stellar masses (0.8-6.2)X10^11 Mo, black hole masses in the range
(0.2-2.5)X10^9 Mo, and Eddington ratios L/L_Edd<0.1, suggesting a low-accretion
phase.

Introduction
Hard X-ray surveys have clearly revealed the important role
played by obscured Active Galactic Nuclei (AGN) to repro-
duce the cosmic X-ray background (XRB; e.g., Comastri et al.
1995) and have provided evidence that a significant fraction of
the accretion-driven energy density in the Universe resides in
obscured X-ray sources (e.g., Barger et al. 2005; Hopkins et al.
2006; Hickox & Markevitch 2006).
Until recently, the limited information on the broad-band
emission of the counterparts of obscured X-ray sources pre-
vented a reliable determination of their bolometric luminosi-
ties. The lack of a proper knowledge of the spectral energy dis-
tributions (SEDs) of obscured sources has led many authors to
adopt, in the computation of the bolometric luminosities, the
average value derived by Elvis et al. (1994), although in that
work the sample comprises mostly local unabsorbed quasars.
By the current work, we aim at providing a robust estimate of
the bolometric luminosity for obscured sources, which is an
essential parameter to derive the cosmic mass density of super-
massive black holes (SMBHs, i.e., following the Soltan 1982
approach). A reliable estimate of the bolometric luminosity of
obscured AGN is typically limited by the actual capabilities
of disentangling the nuclear emission (related to the accretion
processes) from that of the host galaxy which, unlike for un-
absorbed quasars, often dominates at optical and near-infrared
(near-IR) bands.
A significant fraction of high-redshift, luminous obscured
AGN (the so-called Type 2 quasars) may have escaped spec-
troscopic identification due to their faint optical counterparts,
thus preventing current studies from an accurate sampling of
obscured sources. Mid-infrared (mid-IR) observations appear
http://arxiv.org/abs/0704.0735v2
2 F. Pozzi, C. Vignali, A. Comastri et al.: Spitzer observations of luminous obscured quasars
Table 1. Properties of our targets
Source Id. 2–10 keV flux† R Ks Morph(Ks) X/O z
(10−14 erg cm−2 s−1)
Abell 2690#75 3.30 24.60 18.33 E 1.86 2.13a
PKS 0312−77#36 1.90 24.70 19.13 E 1.66 –
PKS 0537−28#91 4.20 23.70 18.99 E 1.60 –
PKS 0537−28#54 2.10 25.10 18.91 E 1.86 –
PKS 0537−28#111 2.10 24.50 17.64 E 1.62 –
Abell 2690#29 2.80 25.10 17.67 P 1.99 2.08b
PKS 0312−77#45 2.80 24.40 18.62 P 1.71 –
BPM 16274#69 2.27 24.08 17.87 E 1.49 1.35b
† 2-10 kev X-ray fluxes from Perola et al. (2004).
a Tentative spectroscopic redshift from near-IR spectroscopic observations (Maiolino et al. 2006).
b Spectroscopic redshift from near-IR spectroscopic observations (Maiolino et al. 2006).
to be fundamental for this class of objects, since they are only
marginally affected by dust obscuration and are able to recover
the nuclear emission. With mid-IR observations, we expect to
reveal the nuclear radiation re-processed by the torus of the
obscured active nuclei, which are often recognized as such by
means of their X-ray emission only. For these sources, the
soft X-ray emission, which is photo-electrically absorbed by
the gas, and the optical emission, extincted by the dusty cir-
cumnuclear medium, are expected to be downgraded in energy
and to emerge as thermally reprocessed radiation in the IR at
wavelengths in the range between a few and a few hundred µm
(Granato et al. 1997).
The potentialities of mid-IR observations were firstly
shown by Fadda et al. (2002), who detected with ISOCAM at
15 µm about two-thirds of the X-ray sources detected in the
5–10 keV band in the XMM-Newton Lockman Hole survey. A
similar high detection rate at 24 µm has been recently reported
by Rigby et al. (2004) and Franceschini et al. (2005) studying
the Spitzer counterparts of the Chandra sources in the CDF-
S and in the ELAIS-N1 field, respectively, within the SWIRE
survey (Lonsdale et al. 2004).
In this context, a new interesting class of objects is emerg-
ing from the current X-ray surveys: these sources are character-
ized by a high (>1) X-ray-to-optical flux ratio (hereafter X/O)1;
for comparison, unobscured Type 1 AGN have a broad distri-
bution peaked at X/O≈0. Objects with X/O &1 are about 20%
of the hard X-ray selected sources, and the fraction of these
sources seems to remain constant over ≈ 3 decades of X-ray
flux (Comastri & Fiore 2004). By definition, sources with high
X/O are among the faintest sources in the optical band. In the
shallow, large-area X-ray surveys (e.g., the HELLAS2XMM
survey, with F2−10 keV > 10
−14 erg cm−2 s−1 over ≈ 3 deg2;
Baldi et al. 2002), where the identification of a sizable sam-
ple of sources with X/O>1 has been possible (e.g., Fiore et al.
2003), the X/O selection criterion has proven to be effective in
selecting Type 2 quasars at high redshifts.
1 X/O is defined as log FX
= log FX +
+ 5.5.
We have performed a pilot program to study with Spitzer a
sample of eight sources selected in the 2–10 keV band from the
HELLAS2XMM survey on the basis of their high (>1) X/O and
large column densities (NH ≥10
22 cm−2). The sample observed
with Spitzer has been previously investigated in other bands.
The most surprising finding of the follow-up campaigns was
the association of these sources with luminous near-IR objects
(Mignoli et al. 2004), placing them into the class of Extremely
Red Objects (EROs, R − K ≥5).
The outline of the paper is as follows: in Sect. 2 we present
the sample selection; Spitzer data reduction and analysis are
discussed in Sect. 3, while in Sect. 4 we describe the analy-
sis of the SEDs. Finally, in Sect. 5 we estimate the bolometric
luminosities, the stellar masses of the host galaxies, the black
hole masses, and the Eddington ratios.
Throughout this paper we adopt the “concordance”
(WMAP) cosmology (H0=70 km s
−1 Mpc−1, ΩM=0.3, and
ΩΛ=0.7; Spergel et al. 2003). Magnitudes are expressed in the
Vega system.
2. Sample selection
The eight objects presented in this paper (see Table 1) were se-
lected among the 10 HELLAS2XMM high X/O ratio sources
detected in the Ks band with ISAAC at ESO-VLT; for details
on the association of the Ks-band counterpart to the X-ray
source, see Mignoli et al. (2004). Two sources of the origi-
nal sample were not selected for Spitzer observations: one
(PKS 0537−28#31) is associated with a disky galaxy, while
the other (BPM 16274#181) has an ambiguous Ks-band mor-
phological classification. All but one of the sources observed
by Spitzer belong to the first square degree field (122 X-ray
sources; see Fiore et al. 2003 for the spectroscopic and photo-
metric identification and Perola et al. 2004 for the X-ray spec-
tral analysis); the only exception is BPM 16274#69, which be-
longs to the sample of the second square degree (110 X-ray
sources; see Cocchia et al. 2007; Lanzuisi et al., in prepara-
tion).
F. Pozzi, C. Vignali, A. Comastri et al.: Spitzer observations of luminous obscured quasars 3
Fig. 1. ISAAC Ks images, centered on the Ks counterpart of the X-ray sources; each box is 30′′ wide. North is to the top and East
to the left. Contour levels of the 24 µm emission corresponding to [3, 4, 5, 7, 10, 20, 40]σ are superimposed to each image.
The selected sources, although faint in the optical band
(23.7<R<25.1), are all bright in the near-IR (Ks.19) and, all
but one, have R − Ks > 5, thus being EROs. Mignoli et al.
(2004) were able to study the surface brightness profiles of
these sources in the Ks band and obtained a morphological clas-
sification. Only two sources are classified as point-like objects,
while all of the others are extended, with clear detection of the
host galaxy and radial profiles consistent with those of ellipti-
cal galaxies. In this latter class of sources, the central AGN is
evident in the X-ray band, while in the near-IR the host galaxy
dominates. An upper limit to the contribution of a central un-
resolved source (i.e., the nuclear emission), ranging from 2%
to 12% of the galaxy emission, was obtained. Furthermore, us-
ing both the R − K colour and the morphological information,
a minimum photometric redshift, in the range 0.9–2.4, was es-
timated for these sources.
For three of the sources with the reddest colours
(Abell 2690#75, Abell 2690#29 and BPM 16274#69; see
Table 1), near-IR spectroscopic observations with ISAAC at
ESO-VLT were performed by Maiolino et al. (2006), thus al-
lowing for a spectroscopic identification for at least two of
these sources (one redshift measurement appears tentative).
The point-like source (Abell 2690#29) shows the typical rest-
frame optical spectrum of high-redshift dust-reddened quasars,
with a broad Hα line (Gregg et al. 2002). The other two ob-
served sources, both of them extended in the Ks band, have nar-
row emission-line spectra: one is a LINER-like object at z=1.35
and the second source has a spectrum with a single weak line,
tentatively associated with Hα at z=2.13. Consistently with the
morphological information, in the first source the AGN domi-
nates the emission, while in the other two sources the nuclear
spectrum is heavily diluted by the host galaxy starlight.
4 F. Pozzi, C. Vignali, A. Comastri et al.: Spitzer observations of luminous obscured quasars
Table 2. Spitzer flux densities
Source Id. 3.6 µm 4.5 µm 5.8 µm 8.0 µm 24 µm
Sν±∆Sν Sν±∆Sν Sν±∆Sν Sν±∆Sν Sν±∆Sν
Abell 2690#75 51 ± 5 56 ± 6 89 ± 11 139 ± 15 565 ± 62
PKS 0312−77#36 41 ± 4 44 ± 5 40 ± 8 71 ± 9 236 ± 30a
PKS 0537−28#91 28 ± 4 35 ± 4 42 ± 8 80 ± 10 301 ± 40
PKS 0537−28#54 31 ± 4 35 ± 4 50 ± 10 47 ± 8 279 ± 45
PKS 0537−28#111 88 ± 9 75 ± 8 41 ± 6 46 ± 7 148 ± 28
Abell 2690#29 141 ± 14 185 ± 19 260 ± 27 371 ± 38 1012 ±106a
PKS 0312−77#45 50 ± 6 62 ± 7 69 ± 10 78 ± 10 249 ± 35
BPM 16274#69 86 ± 9 92 ± 9 97 ± 11 120 ± 13 286 ± 34
The flux density is reported in units of µJy. a The 24 µm flux density is probably over-
estimated due to contamination from nearby sources and should be considered as an
upper limit.
3. Spitzer observations and data reduction
The whole sample of eight hard X-ray selected sources has
been observed by Spitzer (Werner et al. 2004), with IRAC
(Fazio et al. 2004) observations of 480 s integration time and
MIPS (Rieke et al. 2004) observations at 24 µm with a total
integration time of ≈ 1400 s per position. IRAC observations
were performed in photometry mode with frame time of 30 s
and dither pattern of 16 points. The MIPS 24 µm observations
were performed in MIPS photometry mode with frame time of
10 s, 10 cycles and small-field pattern. To reduce overheads,
the cluster option was used when possible.
For the IRAC bands, we used the final combined post-basic
calibrated data (BCD) mosaics produced by the Spitzer Science
Center (SSC) pipeline (Version S12.0–S13.01). At 24 µm,
we started the analysis from the BCD produced by the SSC
pipeline (Version S12.4.2–S13.01) and then we applied ad hoc
procedures to optimize the reduction, since some of our sources
were close to the detection limit (see Table 2). We remind that
BCD are individual frames already corrected for dark, flat field
and geometric distortion. We improved the quality of the BCD
by correcting each individual BCD for a residual flat field de-
pending on the scan mirror position (Fadda et al. 2006). The
residual flat fielding was obtained from our own data by aver-
aging the BCD corresponding to the same scan mirror position
and the same Astronomical Observation Request (AOR), con-
sidering all the different cluster positions. To each BCD, its
median level was subtracted before this operation. This pro-
cedure was possible since our observations are not dominated
by background fluctuations. The corrected BCD were co-added
and background-subtracted using the SSC MOPEX software
(Makovoz & Marleau 2005). The resulting mosaics were made
with 2.4′′ pixel size. The overall analysis at 24 µm produces
mosaics with a typical noise of ≈ 0.020 MJy/pixel (a factor of
2 lower in comparison with the SSC pipeline mosaics). The
noise map has been computed for each mosaic by scaling the
measured mean rms of the central part of the map according to
the inverse square root of the coverage map.
The flux densities of our targets in IRAC and MIPS bands
were measured on the signal maps using aperture photometry at
the position of the sources. The chosen aperture radius for the
IRAC bands is 2.45′′ and the adopted factors for the aperture
corrections are 1.21, 1.23, 1.38 and 1.58 (following the IRAC
Data Handbook, Version 3.0) at 3.6 µm, 4.5 µm, 5.8 µm and 8
µm, respectively.
The chosen aperture radius at 24 µm is 7.5′′; aperture correc-
tions were derived by examining the photometry of bright stars.
Taking into account an additional correction of 1.15 to match
the procedure used by the MIPS instrument team to derive cal-
ibration factors from standard star observations, the resulting
aperture correction is 1.57 (in agreement with the SWIRE team,
see Surace et al. 2005).
Table 2 reports the results of the Spitzer observations. To
compute the photometric uncertainties, we added in quadra-
ture the noise map and the systematic uncertainties (≈10%, see
MIPS and IRAC Data Handbook 2006, Version 3.0). The rel-
ative photometric uncertainties range from ≈10% in the best
cases (IRAC channels 1 and 2), up to ≈20% at 24 µm in the
worst ones.
All of the eight sources are clearly detected in both IRAC
and MIPS 24 µm bands. At 24 µm, the flux densities span an
order of magnitude, ranging between ≈1000 µJy and ≈150 µJy,
with the faintest source (PKS 0537−28#111) close to the 5σ
detection level.
In Fig. 1 the Ks-band images, along with the contour lev-
els of the 24 µm emission, are shown. At 24 µm, the sources
PKS 0312−77#36 and Abell 2690#29 appear to be confused.
In particular, in both cases, there is a second source at ≈ 8-10′′,
unrelated to the targets. The contribution of these sources to
the 24 µm flux density of our targets has been estimated by a
decomposition analysis (using the PSF fitting algorithm IMFIT
within the AIPS environment). Furthermore, both the sources
PKS 0312−77#36 and Abell 2690#29 present a second object
at ≈ 2′′, clearly visible in the Ks images, too close to our targets
for a decomposition analysis, given the 24 µm pixel size. Since
these close companions become increasingly fainter moving
from the Ks bands to the longer IRAC wavelengths, we have
attributed the entire flux density estimated from the decompo-
sition analysis to our targets (see Table 2). However, the 24 µm
flux densities for these two sources should be treated as upper
limits (see Fig. 2).
F. Pozzi, C. Vignali, A. Comastri et al.: Spitzer observations of luminous obscured quasars 5
We point out that the deblending procedure measures the
peak flux density. To convert the peak flux density into total
flux density, we have assumed that the deblended objects are
point-like sources and applied a correction factor of 8.9 derived
from the 24 µm Spitzer PSF and including the 1.15 calibration
factor.
4. Analysis of the spectral energy distributions
In this section we provide an analysis of the SEDs of our
sources, in order to derive the energy distribution of the nu-
clear component “cleaned” by the host galaxy contribution. As
anticipated in Sect. 1, the determination of the nuclear SEDs
over a large wavelength range is an essential step to estimate
the physical properties of the black hole, such as its bolometric
luminosity, mass, and accretion rate. Taking advantage of the
new Spitzer photometric points, simultaneously to the SED de-
termination we have estimated the photometric redshifts of our
sources. These new values are then compared with the min-
imum redshifts estimated by Mignoli et al. (2004) using only
the R and Ks bands and with the spectroscopic ones measured
in three cases by Maiolino et al. (2006, see §2).
Given the different morphological properties of our
sources, two approaches have been adopted, one for the sources
dominated in the Ks band by the host galaxy (elliptical-like
sources) and another one for the sources dominated by the nu-
clear component (point-like sources).
4.1. Elliptical-like sources
From the Ks-band morphological analysis, we know that at
least up to the observed 2.2 µm band the stellar contribu-
tion dominates the emission in these sources. At longer wave-
lengths, the nuclear component is expected to arise as repro-
cessed radiation of the primary emission, while the stellar
component is expected to drop (e.g., Bruzual & Charlot 2003;
Silva et al. 2004).
In the analysis presented in this work, we have decided
to follow a phenomenological approach, checking whether the
emission of our sources can be reproduced as the sum of two
components, one from the host galaxy and the other related to
the reprocessing of the nuclear emission by the dusty torus en-
visaged by unification schemes (Antonucci 1993). The shape
and relative strengths of the two components have to be con-
sistent with all our observed data sets (multi-band photometry,
Ks-band morphology and magnitude, Ks-band upper limit on
the nuclear component, and X-ray spectral analysis).
For the galaxy component, we adopted a set of six galaxy
templates, obtained from the synthetic spectra of GISSEL 2003
(Bruzual & Charlot 2003) assuming a simple stellar population
and spanning a wide range of ages, from 1 Gyr up to a “maxi-
mum age” model (z f orm = 20). The “maximum age” model has
been adopted by Mignoli et al. (2004) to derive the minimum
photometric redshift for the sources of the current sample.
For the nuclear component, we adopted the nuclear tem-
plates from Silva et al. (2004), based on the radiative transfer
models of Granato & Danese (1994). We chose these templates
since in the work of Silva et al. (2004) the radiative transfer
models are used to interpolate the observed nuclear IR data for
a sizable sample of local AGN. We must note, however, that
the nuclear observed SEDs are available only in the 2–20 µm
regime, where data from small-aperture instruments are avail-
able. At wavelengths above ≈ 20 µm, the SEDs are model ex-
trapolations.
Silva et al. (2004) found that the nuclear SEDs can be ex-
pressed as a function of two parameters, the hard X-ray (2–
10 keV) intrinsic luminosity, which provides the normalization
to the SED, and the column density NH , which gives the shape
to the SED (see Fig. 2 in Silva et al. 2004). The shapes of the
SEDs of the Seyfert galaxies are assumed to be valid also at
quasar luminosities.
In the attempt to provide a better estimate for the source
redshifts, we used the four torus templates as given by
Silva et al. (2004), which depend on the column densities NH ,
and we left the normalizations free to vary. The redshift interval
explored by this procedure is 0.5–3.0; the best-fit solution is ob-
tained when the algorithm, based on the χ2, finds a minimum
in the galaxy template, torus template and redshift parameter
space.
For four out of six sources the procedure finds a clear min-
imum χ2, which allows us to determine a photometric red-
shift with relatively good accuracy (see Table 3). For sources
BPM 16274#69 and PKS 0537−28#54, our procedure con-
strains only the lower bound of the redshift interval. For all
the six sources, the estimated redshifts are consistent with
the minimum redshifts of Mignoli et al. (2004). In case of the
source BPM 16274#69, where a secure spectroscopic redshift
is available, the minimum photometric redshift (zphot > 1.25)
is consistent with the spectroscopic one (z = 1.35). For source
Abell 2690#75, we find zphot = 1.30
+0.30
−0.20, which is signifi-
cantly lower than the spectroscopic value (z = 2.13) reported
by Maiolino et al. (2006). In this case, we choose to adopt the
photometric determination, since the spectroscopic redshift is
based on the tentative detection of a single line.
The results of the SED fitting and decomposition are shown
in Fig. 2. The dot-dashed line represents the best-fit galaxy tem-
plate, the dashed line is the best-fit nuclear template and the
thick solid line is the sum of the two components. The best-fit
galaxy templates are all typical of early-type galaxies with ages
between 3 and 6 Gyr. We find an overall agreement between the
SED templates and the data points. Moreover, in all but one of
the sources, the nuclear component derived from the best fit
is consistent with the upper limits derived from the analysis
of the Ks-band images (shown as downward-pointing arrows).
In Fig. 2 we also report as dotted line the SED of the nuclear
component normalized to the intrinsic (i.e., de-absorbed) X-ray
luminosity following the prescriptions of Silva et al. (2004),
where, at a given NH , the normalization depends only on the in-
trinsic hard X-ray luminosity. The overall agreement between
the SEDs normalized to the X-ray luminosity and the best-fit
SEDs is extremely interesting, being consistent within a factor
of ≈ 2–3.
In Table 3 we report, along with the photometric redshifts,
the column densities NH and the de-absorbed L2−10 keV lumi-
nosities. We derive rest-frame NH column densities in the range
1022.0–1023.4 cm−2 and 2–10 keV luminosities in the range
6 F. Pozzi, C. Vignali, A. Comastri et al.: Spitzer observations of luminous obscured quasars
Fig. 2. Rest-frame SEDs of the elliptical sources (black filled circles) compared with the best-fit model obtained as the sum (solid
line) of an early-type galaxy (dot-dashed line) and a nuclear component (dashed line). For comparison, the nuclear component
as derived from the X-ray normalization is also reported (dotted line). The nuclear Ks-band upper limits (downward-pointing
arrows) were derived from the morphological analysis carried out by Mignoli et al. (2004). The 24 µm upper limit of source
PKS 0312−77#36 takes into account a possible contribution from a companion source. zs means that the redshift is spectroscopic
(see §2 for details), while zphot means that the redshift is photometric (see §4.1).
1043.8–1044.7 erg s−1, placing these sources among the Type 2
quasar population.
4.2. Point-like sources
From the Ks-band morphological analysis, Abell 2690#29 and
PKS 0312−77#45 (see Mignoli et al. 2004 and Table 1) show
a completely different appearance at 2.2 µm in comparison
with the sample of extended objects; these two sources have
their near-IR emission mostly dominated by an unresolved
source. The dominant role played by the AGN is supported
for Abell 2690#29 also by its near-IR spectrum, where a broad
Hα emission line is detected (see Maiolino et al. 2006 and §2).
F. Pozzi, C. Vignali, A. Comastri et al.: Spitzer observations of luminous obscured quasars 7
Fig. 3. Rest-frame SEDs of the two point-like sources (black filled circles) compared to an extinguished quasar template (dashed
line) and the best-fit red quasar template (solid line). The extinguished quasar template is obtained from the unobscured quasar
template of Elvis et al. (1994) using the SMC extinction law with E(B − V) = 0.7 and scaled to fit the R − Ks colour (for
comparison, the unobscured quasar template is also shown as a dotted line). The red quasar template is taken from Polletta et al.
(2006). The 24 µm flux density upper limit for the source Abell 2690#29 takes into account a possible contribution from a
companion source. zs means that the redshift is spectroscopic (see §2 for details), while zphot means that the redshift is photometric
(see §4.2).
Unfortunately, the near-IR spectroscopy information is absent
for PKS 0312−77#45 (see Table 1).
We first tried to reproduce their observed SEDs by redden-
ing the composite template spectrum of bright Type 1 quasars
of Elvis et al. (1994) with several extinction laws. Reddening
has been applied as prescribed by Calzetti (1997) for a dust-
screen model and by Pei (1992) for the Small Magellanic
Cloud (SMC) galaxy. The two prescriptions produce similar
effects at λ > 0.5 µm, but the SMC law produces redder spec-
tra at shorter wavelengths for the same amount of extinction.
Reddened templates with an SMC law reproduce quite well the
optical spectra of dust-reddened quasars in the Sloan Digital
Sky Survey (SDSS; see Richards et al. 2003), while, using the
Calzetti (1997) law, Polletta et al. (2006) were able to repro-
duce the SEDs of X-ray sources in the Spitzer SWIRE survey.
The procedure of reddening a typical Type 1 quasar does
not provide a satisfactory fit to the photometric data points of
our sources. In Fig. 3 we show the results obtained when the
prescription of Mignoli et al. (2004) for the extinction [SMC
extinction law and E(B − V) = 0.7] is adopted.
The dashed line shows the reddened quasar template nor-
malized to fit the R−Ks colour. For comparison, the unobscured
quasar template is also shown (dotted line). Although the R−Ks
colour is obviously reproduced, the overall SED is not well re-
produced, since the observed IRAC and 24 µm flux densities
are systematically lower than predicted (up to a factor of 10 at
24 µm).
The discrepancy between the data points and the reddened
Type 1 quasar template might be due to the application to ac-
tive galactic nuclei of an extinction curve derived from galax-
ies. The different behaviour of AGNs from galaxies can be
attributed to different dust distribution (i.e., torus shape in
AGNs), and gas-to-dust ratios, which can lead to an unusual
dust reddening curve for AGNs.
Since a reddened quasar template does not reproduce the
shape of our data, we adopted the red quasar template from
Polletta et al. (2006), which is a composite spectrum: in the
optical/near-IR band, it is the spectrum of the red quasar
FIRST J013435.7−093102 from Gregg et al. (2002), while the
average of several bright quasars from the Palomar-Green (PG)
sample (Schmidt & Green 1983) with consistent optical data
has been used in the IR. The Polletta et al. (2006) template re-
produces the observed data points significantly better, allowing
for the observed sharp decrease from 0.2 to 0.7 µm in the source
rest frame (see Fig. 3 where the Polletta et al. 2006 spectrum is
shown as a solid line).
For source Abell 2690#29, which has a spectroscopic red-
shift, the SED normalization has been obtained through a best-
fit procedure. For source PKS 0312−77#45, where only a min-
imum redshift was available prior to this analysis, we have left
free to vary both the normalization and the redshift.
In Table 3 we report the derived redshifts, column densi-
ties NH and de-absorbed L2−10 keV also for these sources which,
similarly to the elliptical-like sources, belong to the Type 2
quasar population.
5. Physical parameters
5.1. Bolometric correction
Once the SED of the nuclear component has been determined,
the following step is to estimate the bolometric luminosity Lbol,
which is a quantity directly related to the central black hole
activity.
The bolometric luminosity Lbol can be estimated from
the luminosity in a given band b, Lb, by applying a suit-
able bolometric correction kbol,b = Lbol/Lb. For the X-ray se-
lected sources, the bolometric luminosity is typically estimated
8 F. Pozzi, C. Vignali, A. Comastri et al.: Spitzer observations of luminous obscured quasars
from the luminosity in the 2–10 keV band (kbol,2−10 keV =
Lbol/L2−10 keV ). In previous works, several authors used the
bolometric correction obtained by Elvis et al. (1994) for lumi-
nous, mostly nearby quasars, i.e., kbol,2−10 keV≈ 30. However,
these corrections could be affected by the following uncer-
tainties: firstly, they are average corrections obtained from
a few dozens of bright quasars; secondly, as discussed in
Marconi et al. (2004), these corrections could overestimate the
bolometric luminosities since they are based on the integral
of the observed SEDs of bright unobscured AGN, without
removing the IR bump (hence counting twice a fraction of
≈ 30% of the intrinsic optical–UV radiation). At lower lumi-
nosities (typical of Seyfert galaxies, i.e., 1042 − 1044 erg s−1), a
lower value for this correction (kbol,2−10 keV≈ 10) was suggested
(e.g., Fabian 2004). For heavily obscured luminous sources,
only few objects have been studied in detail; in particular, for
two SWIRE Compton-thick (i.e., log NH & 24 cm
−2) AGN
Polletta et al. (2006) found kbol,0.3−8 keV ≈ 3 and ≈ 100.
In this work, thanks to the multi-band observations and ef-
forts in disentangling the AGN and the host components, we
try to derive directly the nuclear bolometric luminosity of our
sources without assuming any average correction. We estimate
Lbol by adding the X-ray luminosity integrated over the entire
X-ray range (L0.5−500 keV , not corrected for absorption) to the
IR luminosity (L1−1000 µm).
L0.5−500 keV has been estimated from the observed L2−10 keV
luminosity assuming a single power-law spectrum with Γ=1.9
(typical for AGN emission; see, e.g., Fig. 6 of Vignali et al.
2005 and references therein) plus absorption (where the col-
umn densities are taken from the X-ray spectral analysis; see
Perola et al. 2004 and Lanzuisi et al., in preparation) and an
exponential cut-off at 200 keV. The median value found for the
ratio L0.5−500 keV/L2−10 keV is ≈ 4. The IR luminosity has been
estimated by integrating the SED from 1 µm to 1000 µm using
only the nuclear component for the AGN hosted in the ellipti-
cal galaxies (see §4.1), and the Polletta et al. (2006) template
for the point-like sources.
Before computing the bolometric output of our sources, the
derived IR luminosities must be properly corrected to account
for the geometry of the torus and its orientation. The first cor-
rection is related to the covering factor f (which represents the
fraction of the primary optical–UV radiation intercepted by the
torus), while the second correction is due to the anisotropy of
the IR emission, which is a function of the viewing angle (see
Pier & Krolik 1993 and Granato & Danese 1994 for further de-
tails).
We estimated the first correction (≈ 1.5) from the ratio
of obscured (Compton thin + Compton thick) to unobscured
quasars as required by the most recent X-ray background syn-
thesis model (see Gilli et al. 2007) in the luminosity range of
our sources. A correction of ≈ 1.5 implies an average covering
factor f≈0.67 which, in a simple torus geometry, corresponds
to an angle θ≈48◦ between the perpendicular to the equatorial
plane and the edge of the torus.
A first-order estimate of the anisotropy factor has been
computed from the Silva et al. (2004) templates as the ratio
(R) of the luminosity of a face-on vs. an edge-on AGN, whose
obscuration is parametrized as a function of NH. The integra-
tion has been performed in the 1–30 µm range, after normal-
izing the two SEDs to the same luminosity in the 30–100 µm
range, where the anisotropy is thought to be negligible. The
derived anisotropy factors are large only for the Silva et al.
(2004) template with higher column density (R ≈3–4 for NH =
1024.5 cm−2); since all of our targets are characterized by lower
obscuration, such corrections do not affect our IR luminosities
significantly (R ≈1.2–1.3 for NH ≈ 10
22.0 − 1023.4 cm−2). In
conclusion, the final combined corrections to be applied to the
observed IR luminosities of our sources, given their column
densities, are in the range ≈ 1.8–2.0. After adding the X-ray lu-
minosities, the IR correction factors would translate in a mean
correction factor of ≈ 1.7 in the computation of the bolometric
luminosities.
In Table 3 the derived bolometric luminosities are reported
along with the full range (L0.5−500 keV) of X-ray luminosities,
the IR (L1−1000 µm) luminosities and the bolometric corrections
(kbol,2−10 keV ). We note that our L1−1000 µm estimates (hence Lbol)
are robust despite the choice of our SEDs. By comparing the
L1−1000 µm obtained using the Silva et al. (2004) model with the
L1−1000 µm obtained adopting other recent average quasar SEDs
(i.e., Richards et al. 2006), we have verified that the uncertain-
ties in L1−1000 µm are within the ≈ 10% level. The bolometric
output of our targets is dominated by the IR reprocessed emis-
sion, the primary X-ray radiation (L0.5−500 keV) accounting only
for .15% of the total luminosity.
The derived median (mean) value of kbol,2−10 keV is ≈ 25
(35±9; see Fig. 4 and column 8 of Table 3), consistent with the
value kbol,2−10 keV≈ 30 from Elvis et al. (1994) widely adopted
in past works. However, as pointed out also by Elvis et al.
(1994), the bolometric corrections span a wide range of values
(≈ 12–100); as a consequence, the adoption of a mean value
could lead to inaccurate results.
We note that in the extreme case where no corrections for
the covering factor and the anisotropy of the torus are applied,
we would obtain a median kbol,2−10 keV of ≈ 16.
In Fig. 4 the derived Lbol as a function of L2−10 keV is shown;
the dot-dashed line joins the expected values from the analysis
of Marconi et al. (2004), where kbol,2−10 keV is derived by con-
structing an AGN reference template taking into account how
the spectral index αox (Zamorani et al. 1981) varies as a func-
tion of the luminosity (Vignali et al. 2003). Although the bolo-
metric luminosities estimated for our objects are on average
lower than those expected on the basis of Marconi et al. (2004)
relation, they are however consistent with a trend of higher
kbol,2−10 keV for objects with higher X-ray luminosity. If we fit
our objects in the Lbol − L2−10keV plane with the same slope as
the Marconi et al. (2004) relation, the difference in normaliza-
tion is ≈ 50%.
5.2. Galaxy and black hole masses, and black hole
Eddington ratios
For the AGN hosted in elliptical galaxies we are able to infer
both the galaxy and the black hole masses. The galaxy masses
are estimated, assuming a Salpeter (1955) initial mass func-
tion (IMF), from the Ks luminosities taking into account that
ozzi,C
ignali,A
astri
itzer
observations
inous
obscured
quasars
Table 3. Inferred rest-frame properties of our targets
Source Id. za NbH L
2−10 keV L
0.5−500 keV L
1−1000 µm Lbol Lbol/L2−10 keV L
K Mstar M
BH (Lbol/LEdd)
(1022 cm−2) (1044 erg s−1) (1045 erg s−1) (1045 erg s−1) (1045 erg s−1) (1011Lk,⊙) (10
11M⊙) (10
Abell 2690#75 1.30+0.30
−0.20 6.9 3.2 1.3 9.7 11.0 34.7 5.2 3.5 1.3 0.065
PKS 0312-77#36 0.90+0.05
−0.15 1.0 0.7 0.2 1.2 1.5 20.6 1.0 0.8 0.2 0.058
PKS 0537-28#91 1.30+0.40
−0.70 25.8 5.3 2.6 4.4 7.1 13.4 2.8 1.5 0.7 0.084
PKS 0537-28#54 >1.30 1.6 2.0 0.6 3.9 4.6 23.0 3.0 2.0 0.7 0.049
PKS 0537-28#111 1.20+0.20
−0.10 9.1 1.7 0.7 1.4 2.1 12.3 7.8 6.2 2.1 0.008
Abell 2690#29 2.08 2.1 8.4 2.8 78.4 81.2 97.0 4.17 – – –
PKS 0312-77#45 1.85+0.20
−0.30 8.0 6.2 2.6 27.2 29.8 47.9 1.65 – – –
BPM 16274#69 1.35 2.5 2.4 0.8 5.9 6.7 28.2 8.8 6.0 2.5 0.022
a Photometric redshifts as derived from the analysis presented in this paper (see §4.1). For source PKS 0537-28#54, only a minimun redshift was estimated; for sources Abell 2690#29
and BPM 16274#69, the spectroscopic redshifts measured by Maiolino et al. (2006) are reported.
b The column densities, measured through X-ray spectral fitting (see Perola et al. 2004 and Lanzuisi et al., in preparation, for details), were “matched” to the redshift used in the SED
best-fitting procedure (using the relation NH(z)=NH(z = 0)×(1 + z)
2.6).
c Absorption-corrected X-ray luminosity.
d The 0.5–500 keV luminosities have been derived from the observed 2–10 keV luminosities as described in the text. The luminosities are not corrected for absorption.
e The 1–1000 µm luminosities have been derived from the integral of the nuclear SEDs (including the corrections described in the text). The values reported for the point-like sources
refer to the Polletta et al. (2006) red quasar template (see 4.2 for details).
f The Ks-band luminosities refer to the nuclear component for the point-like sources (Abell 2690#29 and PKS 0312−77#45) and to the host-galaxy starlight for the AGN hosted in the
elliptical galaxies.
g,h The reported MBH and Lbol/LEdd have been computed from the local LK −MBH relation (Marconi & Hunt 2003), under the hypothesis of an evolution of the MBH/Mstar ratio of a factor
two with redshift (see §5.2 for details).
10 F. Pozzi, C. Vignali, A. Comastri et al.: Spitzer observations of luminous obscured quasars
Fig. 4. Bolometric luminosity vs. absorption-corrected 2–
10 keV luminosity for the six AGN hosted in the elliptical
galaxies (circles) and the two point-like AGN (triangles). The
filled symbols refer to the values corrected for the covering fac-
tor and torus anisotropy, while the empty symbols refer to the
bolometric luminosity without applying these corrections. The
dot-dashed line represents the correlation from Marconi et al.
(2004). The three dashed lines represent the loci of kbol,x=10,
30 and 100.
Fig. 5. Black hole mass (MBH) vs. bolometric luminosity (Lbol)
for the AGN hosted in the elliptical galaxies. The Eddington
luminosity for a given MBH is reported in the right-hand axis.
The black hole masses have been estimated from the local
LK − MBH relation (Marconi & Hunt 2003) under two differ-
ent hypotheses (see 5.2): (1) evolution by a factor of two of
the MBH/Mstar ratio with redshift in comparison to the local
values (black filled circles); (2) no evolution of the MBH/Mstar
ratio (empty circles). The three dashed lines represent the loci
of Lbol/LEdd = 0.01, 0.033 and 0.1 (from left to right).
Mstar/LK for an old stellar population can vary from ≈ 0.5 to
≈ 0.9 (for ages between 3 and 6 Gyr; Bruzual & Charlot 2003).
We can derive Mstar directly from LK since for these sources
the Ks-band emission is dominated by the galaxy starlight. The
rest-frame LK have been derived using the appropriate SED
templates (see Sect. 4.1). The inferred stellar masses are in
the range (0.8–6.2)×1011 M⊙, implying that our obscured AGN
are hosted by massive elliptical galaxies at high redshifts. In
Table 3 both the LK and Mstar values are reported; we note
that the different assumption of the Chabrier (2003) IMF would
produce a factor ≈ 1.7 lower masses (di Serego Alighieri et al.
2005).
To estimate the black hole masses, we take advantage of the
local MBH − LK relation (Marconi & Hunt 2003) which, taking
into account the Mstar/LK values, is expression of the intrin-
sic MBH − Mstar relation. Given the challenging measurements
of high-redshift black hole masses, the behaviour of this rela-
tion with redshift is still matter of debate and different authors,
using different techniques, have found different results.
Woo et al. (2006) and Peng et al. (2006) derive a signifi-
cant evolution of the MBH − Mstar relation with redshift, be-
ing the MBH/Mstar ratio larger, at high redshift, up to a factor
≈ 4 in comparison to the local value. In the Woo et al. (2006)
analysis the discrepancy with respect to the local value is al-
ready present at z = 0.36, while Peng et al. (2006) find an av-
erage MBH/Mstar a factor &4 times larger than the local value
at z > 1.7, while at lower redshifts (1 . z . 1.7) they de-
rive a ratio which is at most two times higher than the local
value, and maybe consistent with marginal or no evolution. On
the other hand, Shields et al. (2006) and Hopkins et al. (2006)
suggest that the MBH/Mstar ratio is not significantly higher (at
most a factor of two) than that measured locally up to z . 2.
Given these uncertainties about the evolution of the MBH −
Mstar relation with redshift, we have estimated the black hole
masses for our objects under two different hypotheses: (1) the
MBH/Mstar ratio is higher than locally by a factor two in the
redshift range (0.9 . z . 1.4) of our sources; (2) the MBH/Mstar
ratio does not evolve with redshift.
In both cases, our results imply very massive black hole
masses (see Fig. 5), in the range ≈ 2.0 × 108 − 2.5× 109 M⊙ in
the former hyphothesis, with a factor of two lower values under
the second hypothesis.
Our estimated MBH are consistent with the results derived
by McLure & Dunlop (2004) studying a large sample of Type 1
SDSS quasars and deriving the black hole masses from virial
methods; most of their black hole masses are in the range
1.5×108–2.5×109 M⊙ in the redshift interval of our sample (see
Fig. 1 of McLure & Dunlop 2004).
From the comparison of the bolometric luminosities com-
puted in the previous section (see Table 3) with the Eddington
luminosities calculated from the black hole masses esti-
mated above, we derive that our obscured AGN are radiat-
ing at a relatively low fraction of their Eddington luminos-
ity (λ ≈ 0.008–0.084 and λ ≈ 0.015–0.170 under the two hy-
potheses; see Fig. 5 and Table 3). This finding confirms and
extends to a larger sample the results found by Maiolino et al.
(2006) for two sources of our sample and by Brusa et al. (2005)
for a sample of EROs in the “Daddi Field”. As suggested by
Maiolino et al. (2006), the data indicate that our very massive
black holes may have already passed their rapidly accreting
phase and are reaching their final masses at low accretion rates.
F. Pozzi, C. Vignali, A. Comastri et al.: Spitzer observations of luminous obscured quasars 11
Fig. 6. Bolometric luminosity as a fraction of the Eddington lu-
minosity vs. redshift for the whole sample of SDSS quasars of
McLure & Dunlop (2004), plotted as small crosses. The large
circles indicate the six HELLAS2XMM AGN hosted in ellipti-
cal galaxies (symbols as in Fig. 5).
The estimated radiating efficiencies are significantly lower
than the average Lbol/LEdd ≈ 0.4 inferred by Marconi et al.
(2004). However, since in the Marconi et al. (2004) model only
the phases of significant black hole growth are considered, our
results are not in contrast with the proposed model but sug-
gest that our targets belong to the tail of the sources charac-
terized by low accretion rates. Consistently, our data (black
filled and empty circles representing the evolution and no-
evolution hypothesis, respectively) lie in the lower envelope of
the Eddington ratio distribution found by McLure & Dunlop
(2004) for their large SDSS quasar sample. This is shown in
Fig. 6, where our data are overlaid on the SDSS data points.
This suggests that the SDSS quasar survey and the HELLAS
survey probe different regimes of AGN activity: the SDSS sam-
ples the brightest sources in the sky (R <∼ 20), most likely char-
acterized by a high accretion rate, while our targets (X-ray se-
lected, optically faint, i.e., R > 24, and obscured), are asso-
ciated with a different evolutionary phase. We argue that the
SMBH in our targets has already reached its final mass and the
observed emission is witnessing a late stage of the accretion
activity.
6. Conclusions
We have performed with Spitzer a pilot program to study a sam-
ple of eight Type 2 (i.e., luminous and obscured) quasars at
high redshift, selected from the HELLAS2XMM survey. Three
sources have a measured spectroscopic redshift (two secure and
one tentative) from near-IR spectroscopy; the remaining ob-
jects have an estimated minimum redshift obtained from the
R − K colours. On the basis of their Ks-band morphological
properties, the sample is divided into two classes: sources with
radial profiles typical of elliptical galaxies and point-like ob-
jects. The most important results can be summarized as fol-
lows:
• All of the eight sources have been clearly detected in both
IRAC and MIPS 24 µm bands.
• The Spitzer observations have allowed us to detect the nu-
clear component (often hidden at short wavelengths by the
host galaxy) as thermal IR re-processed emission from the
circumnuclear torus. While for the two point-like sources
the nuclear component dominates at all frequencies, for the
six sources with elliptical-like radial profile the contribu-
tion from the strong stellar continuum is dominant up to
the first IRAC bands, but the torus emission accounts for
the entire emission at 24 µm.
• Taking advantage of the new Spitzer data, the nuclear SEDs
of the sources have been modeled and new photometric
redshifts have been estimated, following two approaches:
for the elliptical sources, the nuclear emission has been
“cleaned” from the host galaxy contribution adopting a
two-component model (galaxy plus nuclear component),
constrained using all the extensive observed data sets. For
the point-like sources, the SEDs appear inconsistent with
an extinguished Type 1 quasar template, being well repro-
duced by an empirical SED of red quasars (Polletta et al.
2006). We find an overall agreement between the SED tem-
plates and the data points, and the derived photometric red-
shifts are consistent with the spectroscopic ones for two
sources.
• Using the model components to extrapolate the nuclear
SEDs in the far-IR regime, we derived the bolometric lumi-
nosities (being in the range ≈ 1045−1047 erg s−1) by adding
the IR luminosities to the full range of X-ray luminosities.
In this computation, we have considered and discussed the
corrections to be applied to the observed IR luminosities to
take into account the covering factor of the torus and the
anisotropy of the IR emission. The median 2–10 keV bolo-
metric correction is ≈ 25, consistent with the value typically
assumed in literature.
• For the elliptical sources, thanks to the independent esti-
mates of the stellar light and nuclear bolometric luminos-
ity, the physical parameters of the central black holes have
been estimated using the MBH − LK relation and exploring
different hypotheses for the evolution of the MBH/Mstar ra-
tio with redshift. Under the hyphothesis that the MBH/Mstar
ratio is a factor of two higher at z ≈ 1.2 than locally, our
luminous, obscured AGN have masses in the range (0.2-
2.5)×109 M⊙, reside in massive [(0.8-6.2)×10
11 M⊙] high-
redshift ellipticals and are characterized by low Eddington
ratios (λ≈ 0.008–0.084). Through our direct estimate of the
IR luminosity, we confirm the conclusion of Maiolino et al.
(2006) that these black holes may have already passed their
rapidly accretion phase.
Acknowledgements. The authors acknowledge partial support by the
Italian Space agency under the contract ASI–INAF I/023/05/0. The
authors thank R. Gilli, M. Polletta and L.Silva for useful discussions,
and R. J. McLure for kindly providing us with the data points of Fig. 6.
We thank the anonymous referee for the useful comments.
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	Introduction
	Sample selection
	Spitzer observations and data reduction
	Analysis of the spectral energy distributions
	Elliptical-like sources
	Point-like sources
	Physical parameters
	Bolometric correction
	Galaxy and black hole masses, and black hole Eddington ratios
	Conclusions
