The obscured X-ray source population in the HELLAS2XMM survey: the
  Spitzer view
The obscured X-ray source population in the
HELLAS2XMM survey: the Spitzer view
Cristian Vignali∗,†, Francesca Pozzi∗, Andrea Comastri†, Lucia Pozzetti†,
Marco Mignoli†, Carlotta Gruppioni†, Giovanni Zamorani†, Carlo Lari∗∗,
Francesca Civano∗, Marcella Brusa‡, Fabrizio Fiore§ and Roberto
Maiolino§
∗Dipartimento di Astronomia, Università di Bologna, Via Ranzani 1, I–40127 Bologna, Italy
†INAF – Osservatorio Astronomico di Bologna, Via Ranzani 1, I–40127 Bologna, Italy
∗∗INAF – Istituto di Radioastronomia (IRA), Via Gobetti 101, I–40129 Bologna, Italy
‡Max Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstrasse 1, D–85748
Garching bei München, Germany
§INAF – Osservatorio Astronomico di Roma, Via Frascati 33, I–00040 Monteporzio-Catone (RM),
Italy
Abstract. Recent X-ray surveys have provided a large number of high-luminosity, obscured Active
Galactic Nuclei (AGN), the so-called Type 2 quasars. Despite the large amount of multi-wavelength
supporting data, the main parameters related to the black holes harbored in such AGN are still poorly
known. Here we present the preliminary results obtained for a sample of eight Type 2 quasars in
the redshift range ≈ 0.9–2.1 selected from the HELLAS2XMM survey, for which we used Ks-band,
Spitzer IRAC and MIPS data at 24 µm to estimate bolometric corrections, black hole masses, and
Eddington ratios.
Keywords: Galaxies: active – Galaxies: nuclei – X-rays: galaxies
PACS: 98.54.-h, 98.58.Jg
INTRODUCTION
Over the last six years, the X-ray surveys carried out by Chandra and XMM-Newton
(e.g., [1, 2, 3]; see [4] for a review) have provided remarkable results in resolving a
significant fraction of the cosmic X-ray background (XRB; [5, 6]), up to ≈ 80% in the
2–8 keV band (e.g., [7, 8]). Despite the idea that a large fraction of the accretion-driven
energy density in the Universe resides in obscured X-ray sources has been widely sup-
ported and accepted (e.g., [9, 10]), until recently only limited information was available
to properly characterize the broad-band emission of the counterparts of the X-ray ob-
scured sources and provide a reliable estimate of their bolometric output.
In this context, Spitzer data have provided a major step forward the understanding
of the broad-band properties of the X-ray source populations. If, on one hand, Spitzer
data have allowed to pursue the “pioneering” studies of [11] on the spectral energy
distributions (SEDs) of broad-line (Type 1), unobscured quasars at higher redshifts (e.g.,
[12]), on the other hand they have produced significant results in the definition of the
SEDs of narrow-line (Type 2), obscured AGN (e.g., [13]).
In this work we aim at providing a robust determination of the bolometric luminosity
for hard X-ray selected obscured AGN. This result can be achieved by effectively
http://arxiv.org/abs/0704.1300v1
disentangling the nuclear emission related to the active nucleus from the host galaxy
starlight, which represents the dominant component (at least for our obscured sources)
at optical and near-infrared (near-IR) wavelengths.
SAMPLE SELECTION AND Ks-BAND PROPERTIES
The sources presented in this work were selected from the HELLAS2XMM survey ([3])
which, at the 2–10 keV flux limit of ≈ 10−14 erg cm−2 s−1, covers ≈ 1.4 square degrees
of the sky using XMM-Newton archival pointings ([14]). Approximately 80% of the
HELLAS2XMM sources have a spectroscopic optical classification in the final source
catalog (see [15] for details). In particular, we selected eight sources from the original
sample of [16] which are characterized by faint (23.7–25.1) R-band magnitudes and
bright Ks-band counterparts (≈ 17.6–19.1); all of our sources are therefore classified
as extremely red objects (EROs, R − Ks > 5 in Vega magnitudes). From the good-
quality Ks-band images, [16] were able to study the surface brightness profiles of these
sources, obtaining a morphological classification. While two sources are associated with
point-like objects, the remaining six sources are extended, showing a profile typical of
elliptical galaxies. In this latter class of sources, the active nucleus, although evident in
the X-ray band, appears hidden or suppressed at optical and near-IR wavelengths, where
the observed emission is clearly dominated by the host galaxy starlight. The relatively
good constraints on the nuclear emission in the near-IR represent a starting point for the
analysis of the Spitzer IRAC and MIPS data.
Due to the faint R-band magnitudes of our sources, optical spectroscopy was not
feasible even with the 8-m telescope facilities; however, the bright near-IR counterparts
of our sources allowed us to obtain spectroscopic redshifts in the Ks band with ISAAC at
VLT for two sources: one point-like AGN is classified as a Type 1.9 quasar at a redshift
of 2.09, while one extended source has line ratios typical of a LINER at z=1.35 (see [17]
for further details on these classifications). For the remaining sources, the redshift has
been estimated using the optical and near-IR magnitudes, along with the morphological
information, as extensively described in §5.1 of [16]; all of the redshifts are in the range
≈ 0.9–2.1. The large column densities [≈ 1022 – a few×1023 cm−2] and the 2–10 keV
luminosities [≈ (1 − 8)× 1044 erg s−1, once corrected for the absorption] place our
sources in the class of the high-luminosity, obscured AGN, the so-called Type 2 quasars
(see, e.g., [18] and references therein).
SPITZER DATA
For our sample of eight sources, we obtained IRAC observations of 480 s integration
time and MIPS observations at 24 µm for a total integration time per position of
≈ 1400 s. All of the sources are detected in the four IRAC bands and in MIPS; the
faintest source in MIPS has a 24 µm flux density of ≈ 150 µJy (≈ 5σ detection; see
[19] for further details on data reduction and cleaning procedures).
ANALYSIS OF THE TYPE 2 QUASAR SPECTRAL ENERGY
DISTRIBUTIONS
A reliable determination of the bolometric output of our AGN sample requires that the
nuclear component, directly related to the accretion processes, is disentangled from the
emission of the host galaxy, which provides a dominant contribution in the optical and
Ks bands (in the case of extended sources, see [16]). To achieve this goal, we constructed
SEDs for all our sources over the optical, near- and mid-IR range. At the same time, we
used Spitzer data to improve our previous estimates on the source redshift when possible.
In the following, we consider the sample of six extended sources and two point-like
objects separately, since a different approach has been adopted for the two sub-samples.
Extended sources
As already pointed out, from the Ks-band morphological analysis carried out by [16],
we know that at least up to 2.2 µm (observed frame) the stellar contribution is mostly
responsible for the emission of these sources. At longer wavelengths, the emission of
the active nucleus is expected to arise as reprocessed radiation of the primary emission,
while the emission from the galaxy should drop significantly, assuming reasonable
elliptical templates. Although many models have been developed in the past to deal
with circum-nuclear dust emission (including the effects of the torus geometry and
opening angle, grain size distribution and density), in our study we adopted a more
phenomenological approach. To reproduce the observed data, we used a combination of
two components, one for the host galaxy and another related to the reprocessing of the
nuclear emission.
For the galaxy component, we adopted a set of early-type galaxy templates obtained
from the synthetic spectra of [20], assuming a simple stellar population spanning a large
range of ages (see [19] for details). For the nuclear component, we adopted the templates
of [21], which are based on the interpolation of the observed nuclear IR data (at least, up
to ≈ 20 µm) of a sample of local AGN through the radiative transfer models of [22]. The
strength of such an approach is that the nuclear templates depend upon two quantities,
the intrinsic 2–10 keV luminosity (which provides the normalization of the SED) and
the column density (responsible for the shape of the SED), and these are known directly
from the X-ray spectra ([23]), once the redshift is known.
We also used all the available information, extended over the Spitzer wavelength
range, to place better constraints on the source redshift than those reported in [16].
Overall, we find a good agreement with the redshifts presented in [16], although Spitzer
allows us to provide estimates with lower uncertainties; only for one source the redshift
is significantly lower (z ≈ 1 instead of ≈ 2) and likely more reliable.
The data are well reproduced by the sum of the two components; the emission from
the galaxy progressively becomes less important at wavelengths above ≈ 4 µm (in the
source rest frame), where the nuclear reprocessed emission starts emerging significantly
(see Fig. 1, left panel), being dominant in MIPS at 24 µm. Furthermore, the latter is
fully consistent with the upper limits provided in the Ks band by [16].
FIGURE 1. Rest-frame SEDs for two representative Type 2 quasars of the current sample: an obscured
AGN hosted by an elliptical galaxy (on the left) and a point-like AGN (on the right). (Left) The observed
data (filled circles) are reproduced by summing up (solid line) the contribution of an early-type galaxy
template (dot-dashed line) to the reprocessed nuclear component (dashed line). The dotted line shows
the nuclear component obtained from the templates of [21], normalized using the X-ray luminosity and
column density (i.e., without fitting the data; see text and [19] for details). The downward-pointing arrow
indicates the constraint on the nuclear emission derived from the Ks-band data ([16]). The combination of
the two templates is also consistent with the R-Ks color. (Right) The observed data (filled circles) are well
reproduced by the red quasar template from [13] (solid line).
Point-like sources
For the two point-like sources, we adopted a different strategy, since their emission
in the near-IR is dominated by the unresolved AGN. To reproduce their observed SEDs,
we extincted a Type 1 quasar template from [11] with several extinction laws, but we
were not able to find a satisfactory solution. Then we used the recently published red
quasar template from [13] and found good agreement with the data (Fig. 1, right panel),
consistently with the results obtained for some obscured AGN in the ELAIS-S1 field
([24]). As in the AGN sub-sample described above, most of the uncertainty lies in the
far-IR, where a proper study of the SEDs would require MIPS data at 70 and 160 µm.
BOLOMETRIC CORRECTIONS
The determination of the SEDs is meant to be the first step toward the estimate of
the bolometric luminosities (Lbol) of obscured AGN. The bolometric luminosities can
be estimated from the luminosity in a given band by applying a suitable bolometric
correction kbol; typically, to convert the 2–10 keV luminosity into Lbol, kbol≈ 30 is
assumed, although this value was derived from the average of few dozens of bright,
mostly low-redshift Type 1 quasars ([11]). For obscured sources, only few estimates are
present in literature (e.g., [13]). We derived kbol by integrating the quasar SEDs over the
X-ray (0.5–500 keV) and IR (1–1000 µm) intervals; in the X-ray band, we converted
the 2–10 keV luminosity assuming a power law with photon index Γ = 1.9 (typical for
AGN emission) and the observed column density ([23]).
To derive the bolometric corrections, we accounted for both the covering factor of
the absorbing material (i.e., the opening angle of the torus) and the anisotropy of the IR
emission. According to unification models of AGN, the former effect should be directly
related to the observed fraction of Type 2/Type 1 AGN which, in the latest models of
[6], is ≈ 1.5 in the luminosity range of our sample. Furthermore, the torus is likely to
re-emit a fraction of the intercepted radiation in a direction which does not lie along
our line-of-sight; the correction for this anisotropy, according to the templates of [21], is
≈ 10–20% (given the column densities of our sources). Once these corrections are taken
into account, we obtain 〈kbol〉 ≈ 35 (median kbol≈ 26), similar to the average value of
[11]; the Type 1.9 quasar at z=2.09 has the highest kbol (≈ 97); see [19] for a discussion
on the uncertainties in these estimates.
BLACK HOLE MASSES AND EDDINGTON RATIOS
For the six AGN hosted by elliptical galaxies, we can derive both the galaxy and
black hole masses. Since the near-IR emission is dominated by the galaxy starlight,
we computed the rest-frame LK assuming the appropriate SED templates and then the
galaxy masses using M⋆/LK ≈ 0.5− 0.9 ([20]); all of our AGN are hosted by massive
galaxies (≈ 1−6×1011 M⊙).
To estimate the black hole masses, we used the local MBH–LK ([25]) which, along
with the M⋆/LK values, provides a MBH–M⋆ relation. Despite several attempts in the
recent literature to investigate whether and how the black hole mass vs. stellar mass
relation evolves with cosmic time, there is no consensus yet. In this work, we assume
the findings of [26], who found that in the redshift range covered by our sources, the
MBH-M⋆ relation evolves by a factor of ≈ 2 with respect to the local value; see [19]
for an extensive discussion. Under this hypothesis, we obtain black hole masses for the
six obscured quasars hosted by elliptical galaxies of ≈ 2.0×108 −2.5×109 M⊙; these
values are broadly consistent with the average black hole masses obtained by [27] for
the Sloan Digital Sky Survey (SDSS) Type 1 quasars (using optical and ultra-violet mass
scaling relationships) in our redshift range (≈ 3.5×108 −8.6×108 M⊙).
As a final step, we derived the Eddington ratios, defined as Lbol/LEdd, where LEdd is
the Eddington luminosity. We note that the uncertainties related to these estimates are
clearly large, due to the uncertainties of the approach adopted to derive the bolometric
luminosities (through the templates of [21]) and the black hole masses (see above).
The average Eddington ratio is ≈ 0.05, suggesting that our obscured quasars may have
already passed their rapidly accreting phase and are reaching their final masses at low
Eddington rates. The Eddington ratios of our sources are significantly lower than those
derived for the SDSS Type 1 quasars in the same redshift range (≈ 0.3–0.4, see [27]).
SUMMARY
We used optical, near-IR, and Spitzer IRAC and MIPS (at 24 µm) data to unveil the re-
processed nuclear emission of eight hard X-ray selected Type 2 quasars at z ≈ 0.9−2.1.
From proper modelling of the nuclear SEDs, we derived a median (average) bolometric
correction of ≈ 26 (≈ 35). For the six obscured sources dominated by the host galaxy
starlight up to near-IR wavelengths, we also derived black hole masses of the order of
2.0×108−2.5×109 M⊙ and relatively low Eddington ratios (≈ 0.05), suggestive of a
low-activity accretion phase.
ACKNOWLEDGMENTS
The authors acknowledge partial financial support by the Italian Space Agency under
the contract ASI–INAF I/023/05/0.
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astro-ph/0612023
arXiv:0704.0735
	Introduction
	Sample selection and Ks-band properties
	Spitzer data
	Analysis of the Type 2 quasar spectral energy distributions
	Extended sources
	Point-like sources
	Bolometric corrections
	Black hole masses and Eddington ratios
	Summary
