The obscured quasar population from optical, mid-infrared, and X-ray
  surveys
**FULL TITLE**
ASP Conference Series, Vol. **VOLUME**, **YEAR OF PUBLICATION**
**NAMES OF EDITORS**
The obscured quasar population from optical,
mid-infrared, and X-ray surveys
C. Vignali
Dipartimento di Astronomia, Università degli Studi di Bologna, Italy
INAF – Osservatorio Astronomico di Bologna, Italy
A. Comastri
INAF – Osservatorio Astronomico di Bologna, Italy
D. M. Alexander
Department of Physics, Durham University, UK
Abstract. Over the last few years, optical, mid-infrared and X-ray surveys
have brought to light a significant number of candidate obscured AGN and,
among them, many Type 2 quasars, the long-sought after “big cousins” of lo-
cal Seyfert 2 galaxies. However, despite the large amount of multi-wavelength
data currently available, a proper census and a panchromatic view of the ob-
scured AGN/quasar population are still missing, mainly due to observational
limitations. Here we provide a review of recent results on the identification of
obscured AGN, focusing primarily on the population of Type 2 quasars selected
in the optical band from the Sloan Digital Sky Survey.
1. Introduction: the X-ray and mid-infrared hunt for obscured quasars
The quest for the identification of luminous and obscured Active Galactic Nuclei
(AGN), the so-called Type 2 quasars predicted by unification schemes of AGN
(e.g., Antonucci 1993) and required by many synthesis models of the X-ray
background (XRB; e.g., Gilli, Comastri & Hasinger 2007), has been the topic of
numerous investigations over the past few years. Although moderate-depth and
ultra-deep X-ray surveys (see Brandt & Hasinger 2005 for a review) have proven
effective to reveal a large number of Type 2 quasar candidates at high redshift
(e.g., Alexander et al. 2001; Mainieri et al. 2002; Mignoli et al. 2004), the
optical counterparts of these sources are typically faint and therefore represent
challenging targets to obtain spectroscopic redshifts and reliable classifications
based on “standard” optical emission-line ratio techniques. Furthermore, there
is evidence that despite the fact that ≈ 80% of the XRB has been resolved into
discrete sources by ultra-deep X-ray surveys in the 2–8 keV band (e.g., Bauer
et al. 2004; Hickox & Markevitch 2006), only ≈ 60% of the XRB has been
resolved above ≈ 6 keV (Worsley et al. 2004, 2005), indicating that while ultra-
deep X-ray surveys provide efficient identification of AGN activity, they do not
provide a complete census of the obscured AGN population. It is plausible that
http://arxiv.org/abs/0704.1298v1
2 Vignali, Comastri & Alexander
one of the “missing” XRB components is related to the population of Compton-
thick AGN (i.e., sources with column densities > 1/σT ≈ 10
24 cm−2, where σT
is the Thomson cross section; see Comastri 2004 for a review on Compton-thick
AGN) which are required by the AGN synthesis models of the XRB (Gilli et
al. 2007) but, as being heavily obscured, are difficult to discover and identify
in the X-rays by the current generation of X-ray telescopes; see, e.g., Tozzi et
al. (2006) for the Compton-thick AGN candidate selection in the Chandra Deep
Field-South.
In the presence of obscuration, the nuclear emission is expected to be re-
emitted at longer wavelengths and hence mid-infrared (MIR) observations can
be crucial to reveal obscured AGN emission. Recently, numerous attempts have
been made in this direction, fully exploiting the capabilities of the detectors on-
board Spitzer (e.g., Alonso-Herrero et al. 2006; Polletta et al. 2006; Donley et
al. 2007). Unfortunately, the AGN locus in the color-color diagrams obtained
from Spitzer photometry (e.g., Lacy et al. 2004; Stern et al. 2005) is often
contaminated by starburst galaxies, therefore further investigations and adjust-
ments are required to efficiently distinguish the obscured and elusive AGN from
the less intriguing unobscured population. The present limitations of Spitzer
diagnostic diagrams to select obscured AGN may be overcome either by refining
the selection criteria (e.g., Mart́ınez-Sansigre et al. 2005, 2006) or targeting the
optically faint or invisible source population with, e.g., MIR spectroscopy (e.g.,
Houck et al. 2005; Weedman et al. 2006a).
The next obvious step where most of the observational efforts will be con-
centrated in the years to come is to compare the obscured AGN selection criteria
adopted at different wavelengths and compute their efficiency in the detection
and identification of the most heavily obscured quasars. Parallel to these kinds
of studies, the analyses of the multi-wavelength (from MIR to X-rays) proper-
ties of obscured AGN and quasars will be crucial to investigate their emission
accurately (e.g., Weedman et al. 2006b), refine the current torus models and
templates (e.g., Silva, Maiolino & Granato 2004), and derive some fundamental
parameters such as the masses of the super-massive black holes (SMBHs) resid-
ing in these sources and their Eddington ratios (e.g., Pozzi et al. 2007). Finally,
the co-evolution of galaxies and SMBHs will be investigated up to high redshifts,
where a significant fraction of MIR–submillimeter-selected obscured AGN will
probably be found (e.g., Alexander et al. 2005a; Mart́ınez-Sansigre et al. 2005).
2. Are any optically selected Type 2 quasars out there?
Although primarily designed, in the AGN research field, for the discovery of
broad-line (Type 1) objects, the Sloan Digital Sky Survey (SDSS; York et al.
2000) has provided a sample of 291 high-ionization narrow emission-line AGN
in the redshift range 0.3–0.83 (Zakamska et al. 2003; small filled circles in
Fig. 1), many of which are identified as candidate Type 2 quasars on the basis of
their [O iii]5007Å luminosities. In the following, we summarize the main results
obtained over the last three years for a sub-sample of these optically selected
Type 2 quasars with ROSAT, Chandra, and XMM-Newton observations.
From the analysis of primarily archival ROSAT observations, Vignali,
Alexander & Comastri (2004, hereafter V04) were able to place constraints
The obscured quasar population 3
Figure 1. Logarithm of the measured L[O III] luminosities vs. redshifts
for all of the sources in the original sample of Type 2 quasar candidates from
Zakamska et al. (2003; small filled circles). At the right of the panel, the
2–10 keV luminosities, estimated using the correlation between the [O iii]
and 2–10 keV flux (Mulchaey et al. 1994), are shown. The key provides a
description of the X-ray observations. The grey region defines the locus of
the sources which still lie in the quasar regime (i.e., above 1044 erg s−1) even
taking into account the dispersion in the L[O III]–L2−10 keV correlation. After
completion of the Chandra AO8 observing cycle, X-ray information will be
available for the most extreme radio-quiet Type 2 quasars from the Zakamska
et al. (2003) sample.
on the X-ray emission of 17 SDSS Type 2 quasar candidates (open circles
in Fig. 1). Using the [O iii] line luminosity to predict the intrinsic X-ray
power of the AGN (following the correlation of Mulchaey et al. 1994), V04
found that at least 47% of the observed sample shows indications of X-ray
absorption, including the four highest luminosity sources with predicted
unobscured luminosities of ≈ 1045 erg s−1, hence well above the typically
adopted threshold of 1044 erg s−1 in the 2–10 keV band for Type 2 quasars.
In Vignali, Alexander & Comastri (2006, hereafter V06), the most up-
to-date results on the SDSS Type 2 quasar population were presented.
Using a combination of Chandra and XMM-Newton pointed and serendip-
itous observations (for a total of 16 sources; filled triangles in Fig. 1),
selected predominantly among the most luminous in [O iii], V06 detected
X-ray emission from ten sources. For seven of these AGN, basic/moderate-
quality X-ray spectral analyses constrained the column densities in the
range ≈ 1022 – a few 1023 cm−2 (filled triangles in Fig. 2). Once their ob-
served X-ray luminosities are corrected for the effect of absorption, there is
indication that the X-ray luminosity predictions based on the Mulchaey et
4 Vignali, Comastri & Alexander
Figure 2. Comparison of the 2–10 keV luminosity computed from the com-
pilation of V06 with that predicted assuming the Mulchaey et al. (1994) cor-
relation. The dotted line shows the 1:1 ratio between the two luminosities.
For the seven sources for which X-ray spectral fitting was possible using either
Chandra or XMM-Newton data, the X-ray luminosity has been de-absorbed
assuming the best-fit column density (filled triangles), while for the remain-
ing X-ray fainter AGN (open circles), some of which undetected, the X-ray
luminosity is derived from the X-ray flux with no correction for the unknown
absorption. Leftward arrows indicate upper limits on the observed X-ray
luminosity; the grey region shows the locus of heavily obscured, candidate
Compton-thick quasars, where the observed luminosity is less than 1% of the
predicted one.
al. (1994) correlation are consistent with the values obtained from X-ray
spectral fitting (all these seven sources lie close to the 1:1 line in Fig. 2).
Having calibrated the [O iii] line luminosity as an indicator of the intrinsic
X-ray emission on the seven sources with good X-ray photon statistics,
there are indications that the X-ray undetected sources and the sources
with a limited number of counts (open circles in Fig. 2) are possibly more
obscured than those found absorbed through direct X-ray spectral fitting,
as pointed out also by Ptak et al. (2006). This would imply that up to
≈ 50% of the population is characterized by column densities in excess
to 1023 cm−2, with a sizable number of Compton-thick quasars possibly
hiding among the X-ray faintest sources (see the quasars located in the
grey region in Fig. 2). This possibility is also suggested by the comparison
of the X-ray-to-[O iii] flux ratios of our sources vs. those obtained from a
large sample of mostly nearby Seyfert 2 galaxies (see Guainazzi et al. 2005
and Fig. 6 of V06).
Given the highly inhomogeneous selection and incompleteness of the sam-
ple presented by Zakamska et al. (2003), the number density of SDSS
The obscured quasar population 5
selected Type 2 quasars can be derived only roughly. In an attempt to
provide a first-order estimate, V06 obtained a value of ≈ 0.05 deg−2, while
Gilli et al. (2007) XRB synthesis models predict a surface density of Type 2
quasars of ≈ 0.15 deg−2 in the ≈ 0.3–0.8 redshift range. This comparison
indicates that the Zakamska et al. (2003) selection is relatively efficient
at finding obscured quasar activity; however, a combination of blank-field
X-ray surveys and optical selection techniques will provide a more com-
plete census.
Given the accurate analyses carried out by Zakamska et al. (2003) in the
original selection of the obscured SDSS AGN sample, it seems unlikely that
a significant population of starburst galaxies or low-luminosity AGN is hid-
ing among the sources with the highest [O iii] luminosity in the V06 sample.
Hence, it is likely that some of the Zakamska et al. (2003) quasars are obscured
by Compton-thick material in the X-ray band. Although effective and compar-
atively complete, the optical selection and requirement for [OIII] in the SDSS
spectra clearly limit obscured quasar searches to z < 1. Due to the likely optical
faintness of obscured quasars at z > 1, a multi-wavelength selection process in-
cluding targeted follow-up observations is likely to be required for a comparably
complete census of obscured quasar activity at high redshift.
3. What’s next in the study of SDSS Type 2 quasars?
The observations approved for Chandra Cycle 8 (10 ks pointing for 12 tar-
gets) will allow us to have an almost complete coverage of the most extreme
SDSS Type 2 quasar candidates (i.e., above a predicted 2–10 keV luminosity of
1044 erg s−1 also taking into account the dispersion in the L[O III]–L2−10 keV
correlation; open squares in Fig. 1). While direct X-ray spectral fitting will be
possible only for the most X-ray luminous SDSS Type 2 quasars, for the newly
observed faint X-ray sources, basic spectral analysis will be carried out by means
of the hardness-ratio technique. For the few-count or undetected sources, it will
be possible to derive the average source properties through stacking analysis;
use of this has been prevented thus far by the limited number of sources with
either Chandra or XMM-Newton constraints and the paucity of X-ray counts.
Luckily, it will be possible also to estimate the column density distribution of
SDSS Type 2 quasars and, by stacking the X-ray source spectra in different NH
bins, to search for faint spectral features, as shown by Alexander et al. (2005b)
for a sample of submillimeter galaxies.
Acknowledgments. The authors acknowledge support by the Italian Space
Agency (contract ASI–INAF I/023/05/0; CV and AC) and the Royal Society
(DMA). The authors wish to thank the people involved in the HELLAS2XMM
survey for useful discussions.
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