Thanks to the Chandra and XMM–Newton surveys, the hard X-ray sky is
now probed down to a flux limit where the bulk of the X-ray background is
almost completely resolved into discrete sources, at least in the 2–8 keV band.
Extensive programs of multiwavelength follow-up observations showed that the large
majority of hard X–ray selected sources are identified with Active Galactic Nuclei (AGN)
spanning a broad range of redshifts, luminosities and optical properties. A sizable fraction
of relatively luminous X-ray sources hosting an active, presumably obscured, nucleus
would not have been easily recognized as such on the basis of optical observations because
characterized by “peculiar” optical properties. In my PhD thesis, I will focus the attention
on the nature of two classes of hard X-ray selected “elusive” sources: those characterized
by high X-ray-to-optical flux ratios and red optical-to-near-infrared colors, a fraction of
which associated with Type 2 quasars, and the X-ray bright optically normal galaxies,
also known as XBONGs. In order to characterize the properties of these classes of elusive
AGN, the datasets of several deep and large-area surveys have been fully exploited.
The first class of “elusive” sources is characterized by X-ray-to-optical flux ratios (X/O)
significantly higher than what is generally observed from unobscured quasars and Seyfert
galaxies. The properties of well defined samples of high X/O sources detected at bright
X–ray fluxes suggest that X/O selection is highly efficient in sampling high–redshift
obscured quasars. At the limits of deep Chandra surveys (∼10−16 erg cm−2 s−1), high
X/O sources are generally characterized by extremely faint optical magnitudes, hence
their spectroscopic identification is hardly feasible even with the largest telescopes. In
this framework, a detailed investigation of their X-ray properties may provide useful
information on the nature of this important component of the X-ray source population.
The X-ray data of the deepest X-ray observations ever performed, the Chandra deep
fields, allows us to characterize the average X-ray properties of the high X/O population.
The results of spectral analysis clearly indicate that the high X/O sources represent the
most obscured component of the X–ray background. Their spectra are harder (G ∼ 1)
than any other class of sources in the deep fields and also of the XRB spectrum (G ≈ 1.4).
In order to better understand the AGN physics and evolution, a much better knowledge
of the redshift, luminosity and spectral energy distributions (SEDs) of elusive AGN
is of paramount importance. The recent COSMOS survey provides the necessary
multiwavelength database to characterize the SEDs of a statistically robust sample of
obscured sources. The combination of high X/O and red-colors offers a powerful
tool to select obscured luminous objects at high redshift. A large sample of X-ray
emitting extremely red objects (R−K >5) has been collected and their optical-infrared
properties have been studied. In particular, using an appropriate SED fitting procedure,
the nuclear and the host galaxy components have been deconvolved over a large range of
wavelengths and ptical nuclear extinctions, black hole masses and Eddington ratios have
been estimated. It is important to remark that the combination of hard X-ray selection and
extreme red colors is highly efficient in picking up highly obscured, luminous sources at
high redshift.
Although the XBONGs do not present a new source population, the interest on the nature
of these sources has gained a renewed attention after the discovery of several examples
from recent Chandra and XMM–Newton surveys. Even though several possibilities
were proposed in recent literature to explain why a relatively luminous (LX = 1042 −
1043erg s−1) hard X-ray source does not leave any significant signature of its presence
in terms of optical emission lines, the very nature of XBONGs is still subject of debate.
Good-quality photometric near-infrared data (ISAAC/VLT) of 4 low-redshift XBONGs
from the HELLAS2XMMsurvey have been used to search for the presence of the putative
nucleus, applying the surface-brightness decomposition technique. In two out of the
four sources, the presence of a nuclear weak component hosted by a bright galaxy has
been revealed. The results indicate that moderate amounts of gas and dust, covering a
large solid angle (possibly 4p) at the nuclear source, may explain the lack of optical
emission lines. A weak nucleus not able to produce suffcient UV photons may provide
an alternative or additional explanation. On the basis of an admittedly small sample, we
conclude that XBONGs constitute a mixed bag rather than a new source population. When
the presence of a nucleus is revealed, it turns out to be mildly absorbed and hosted by a
bright galaxy.
| Identifer | oai:union.ndltd.org:unibo.it/oai:amsdottorato.cib.unibo.it:351 |
| Date | 11 April 2007 |
| Creators | Civano, Francesca Maria <1979> |
| Contributors | Palumbo, Giorgio |
| Publisher | Alma Mater Studiorum - Università di Bologna |
| Source Sets | Università di Bologna |
| Language | English |
| Detected Language | English |
| Type | Doctoral Thesis, PeerReviewed |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
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