One of the main topics in modern observational cosmology and, in
particular, in X-ray astrophysics is the study of the obscured
Active Galactic Nuclei (AGN), in particular those at high redshift,
the co-called Type 2 quasars.
This kind of research can be pursued efficiently through X-ray
surveys and, over the last few years, with infrared observations,
whose complementarity to X-rays has been probed clearly by the
Spitzer satellite. The population of obscured AGN is
relevant for many cosmological reasons: at first, these sources
have a key role in the X-ray background radiation; secondly, most
of the energy density in the Universe, associated to accretion
phenomena, is due to obscured AGN. Although many investigations
have been carried out recently with the Chandra and
XMM-Newton satellites, the number and properties of the most
heavily obscured AGN, the so-called Compton thick, remain to be
defined; for these AGN, the X-ray obscuration is such to prevent
the nuclear radiation from being observed directly below 10 keV,
where typically only the scattered/reflected component can be
observed. A valuable observational strategy to pick up these
obscured sources, also at high redshift, consists of a combined
X-ray plus infrared selection.
Another relevant topic in modern astrophysics is the study of the
first AGN to form in the Universe, in particular of their growth,
relation with their environment, and evolution of their properties
over cosmic time. At present, only the most luminous AGN (quasars)
at high redshift are essentially known, since the discovery of the
weaker high-redshift AGN population is observationally, both in
current optical and X-ray surveys, more challenging. Several
theoretical models predict a large number of AGN at the highest
redshifts, when the age of the Universe was less than 1 Gyr. At
present, an extensive search for and characterization of high-z AGN
is ongoing in Bologna using a multi-wavelength approach.
Furthermore, high-redshift AGN are being searched for also in z~6
luminous quasar fields as over-densities of massive dark matter
halos in the 'young' Universe.
High-energy astrophysics has been characterized, over the last
decade, by numerous studies with the purpose of finding luminous
and obscured AGN at high redshift, the so-called Type 2 quasars.
These objects, predicted by the Unified Models, play a key role in
modeling the X-ray background radiation (XRB). There are evidences
that a significant fraction (50-80%) of the energy density in the
Universe associated to accretion phenomena onto super-massive black
holes (SMBHs) is linked to obscured AGN. Moreover, comparing the
fraction of resolved XRB below 6 keV (80-90%) and at energies 6-10
keV (50-60%), a further population of heavily obscured, maybe
Compton-thick AGN (with column densities above 10^24 cm^-2) can be
assumed. These sources are probably not revealed efficiently in
current X-ray surveys, even in the deepest ones. The study of these
sources cannot be conducted without a proper and large
multi-wavelength coverage in order to characterize, from a physical
and morphological point of view, the obscured AGN, and to estimate
how many of these sources are lost in surveys conducted in only one
band (e.g., the X-ray band) because of selection effects.
In this regard, the deep narrow-field X-ray surveys, coupled with
the shallower ones on larger fields carried out by Chandra
and XMM-Newton, have allowed for a more comprehensive
understanding of the issues related to the obscured AGN population
at high redshift. Similarly, recent observations in the near and
mid-infrared with the Spitzer satellite have played a key
role in defining selection criteria to pick up obscured quasars at
high redshift in an alternative way wrt those offered by X-ray
surveys.
The overall picture is that only a multi-wavelength approach (e.g.,
by selecting sources with large mid-infrared to optical flux ratio)
we will be able to have a more accurate view of all the physical
issues related to the obscured AGN population.
In this research field, there is growing evidence that a fraction
of sources at high redshift has very red colors; with R-K>5,
these sources,
called Extremely Red Objects (EROs), are a mixed bag with a
relevant fraction of AGN, whose extinction in the
optical/near-infrared is associated to the obscuration observed in
X-rays. Given the good quality of the data for many of these
sources, it is now possible to perform accurate and systematic
studies of these objects, in order to estimate how many EROs have
properties consistent with accretion processes. For the faintest
X-ray EROs, "stacking analysis" is able to provide the average
properties. The cosmological implication of these studies are
multiple, in primis because, if they host an AGN, they
probably represent a co-evolution phase between the SMBH
(AGN) and the host galaxy, responsible for most of the emission in
the optical and near-infrared.
The evolution of AGN is another issue of strong impact in modern
astrophysics. Although it is clear that they evolve, many doubts
arise when their evolution is connected with their environment. In
this regard, the study of luminous quasars at z>4, when the age
of the Universe is less than 1 Gyr, has been a widely studied topic
over the last decade. X-ray emission is a common property of AGN
and can be revealed, thanks to current X-ray detectors on-board
Chandra and XMM-Newton, up to very high redshifts.
X-ray radiation can probe the innermost regions of AGN, thus
providing a detailed picture of the emission mechanisms in act in
the process of energy production. The current availability of large
samples of quasars, especially thanks to the Sloan Digital Sky
Survey, has provided a significant number of sources for X-ray
follow-up observations aimed at investigating how their emission
evolves over cosmic time and the dependence upon their UV/optical
properties. These studies are supported, in the local Universe, by
detailed X-ray spectroscopic studies using XMM-Newton and
Chandra data. Similar studies at the highest redshifts will
be possible only with the next generation of large collecting-area
telescopes like the recently approved (by ESA) Athena
mission.
