13 research outputs found
Comoving Space Density and Obscured Fraction of High-Redshift Active Galactic Nuclei in the Subaru/{\it XMM-Newton} Deep Survey
We study the comoving space density of X-ray-selected luminous active
galactic nuclei (AGNs) and the obscured AGN fraction at high redshifts () in the Subaru/{\it XMM-Newton} Deep Survey (SXDS) field. From an X-ray
source catalog with high completeness of optical identification thanks to deep
optical images, we select a sample of 30 AGNs at with intrinsic
(de-absorbed and rest-frame 2--10 keV) luminosities of
erg s detected in the 0.5--2 keV band, consisting of 20 and 10 objects
with spectroscopic and photometric redshifts, respectively. Utilizing the
method, we confirm that the comoving space density of luminous
AGNs decreases with redshift above . When combined with the {\it
Chandra}-COSMOS result of Civano et al.\ (2011), the density decline of AGNs
with erg s is well represented by a power law
of . We also determine the fraction of X-ray obscured
AGNs with cm in the Compton-thin population to be
0.54, by carefully taking into account observational biases
including the effects of photon statistics for each source. This result is
consistent with an independent determination of the type-2 AGN fraction based
on optical properties, for which the fraction is found to be 0.590.09.
Comparing our result with that obtained in the local Universe, we conclude that
the obscured fraction of luminous AGNs increases significantly from to
by a factor of 2.51.1.Comment: 12 pages, 12 figures, 1 table. Accepted for publication in Ap
The Chandra COSMOS Survey: III. Optical and Infrared Identification of X-ray Point Sources
The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra program that
has imaged the central 0.9 deg^2 of the COSMOS field down to limiting depths of
1.9 10^-16 erg cm^-2 s-1 in the 0.5-2 keV band, 7.3 10^-16 erg cm^-2 s^-1 in
the 2-10 keV band, and 5.7 10^-16 erg cm^-2 s-1 in the 0.5-10 keV band. In this
paper we report the i, K and 3.6micron identifications of the 1761 X-ray point
sources. We use the likelihood ratio technique to derive the association of
optical/infrared counterparts for 97% of the X-ray sources. For most of the
remaining 3%, the presence of multiple counterparts or the faintness of the
possible counterpart prevented a unique association. For only 10 X-ray sources
we were not able to associate a counterpart, mostly due to the presence of a
very bright field source close by. Only 2 sources are truly empty fields.
Making use of the large number of X-ray sources, we update the "classic locus"
of AGN and define a new locus containing 90% of the AGN in the survey with full
band luminosity >10^42 erg/s. We present the linear fit between the total i
band magnitude and the X-ray flux in the soft and hard band, drawn over 2
orders of magnitude in X-ray flux, obtained using the combined C-COSMOS and
XMM-COSMOS samples. We focus on the X-ray to optical flux ratio (X/O) and we
test its known correlation with redshift and luminosity, and a recently
introduced anti-correlation with the concentration index (C). We find a strong
anti-correlation (though the dispersion is of the order of 0.5 dex) between C
and X/O, computed in the hard band, and that 90% of the obscured AGN in the
sample with morphological information live in galaxies with regular morphology
(bulgy and disky/spiral), suggesting that secular processes govern a
significant fraction of the BH growth at X-ray luminosities of 10^43- 10^44.5
erg/s.Comment: 21 pages, 17 figures, 4 tables; accepted for publication in ApJS. The
catalog is available at the urls listed in the pape
Black hole accretion and host galaxies of obscured quasars in XMM-COSMOS
We explore the connection between black hole growth at the center of obscured
quasars selected from the XMM-COSMOS survey and the physical properties of
their host galaxies. We study a bolometric regime ( 8 x 10^45 erg/s)
where several theoretical models invoke major galaxy mergers as the main
fueling channel for black hole accretion. We confirm that obscured quasars
mainly reside in massive galaxies (Mstar>10^10 Msun) and that the fraction of
galaxies hosting such powerful quasars monotonically increases with the stellar
mass. We stress the limitation of the use of rest-frame color-magnitude
diagrams as a diagnostic tool for studying galaxy evolution and inferring the
influence that AGN activity can have on such a process. We instead use the
correlation between star-formation rate and stellar mass found for star-forming
galaxies to discuss the physical properties of the hosts. We find that at z ~1,
~62% of Type-2 QSOs hosts are actively forming stars and that their rates are
comparable to those measured for normal star-forming galaxies. The fraction of
star-forming hosts increases with redshift: ~71% at z ~2, and 100% at z ~3. We
also find that the the evolution from z ~1 to z ~3 of the specific
star-formation rate of the Type-2 QSO hosts is in excellent agreement with that
measured for star-forming galaxies. From the morphological analysis, we
conclude that most of the objects are bulge-dominated galaxies, and that only a
few of them exhibit signs of recent mergers or disks. Finally, bulge-dominated
galaxies tend to host Type-2 QSOs with low Eddington ratios (lambda<0.1), while
disk-dominated or merging galaxies have at their centers BHs accreting at high
Eddington ratios (lambda > 0.1).Comment: Accepted by A&A. 20 pages, 16 figures, 2 tables. A version with
higher resolution figures and SED fits of Appendix A is available at
http://www.eso.org/~vmainier/QSO2/qso2.pd
Structural controls on granitoid-hosted gold mineralization and paleostress history of the Edikan gold deposits, Kumasi Basin, southwestern Ghana
The >9 Moz total aggregate gold endowment at the Edikan mine, Kumasi Basin, Ghana, is contained within a cluster of orogenic gold deposits located along the Akropong fault zone. The granitoid-hosted orebodies at Edikan (e.g., AG2, AG3, Fobinso, Esuajah), essentially an interconnected mesh of gold-bearing quartz veins, formed during deformation event D-3Edk, which postdates the penetrative regional D-2Edk deformation. The gold-bearing quartz veins developed in, and adjacent to, N-S- and NW-SE-trending, low-angle thrust faults that crosscut lithological contacts and earlier formed, steeply dipping D-2Edk faults. Our paleostress analysis shows that the D-3Edk deformation, during which the mineralized fault system developed, was characterized by a WNW-ESE "hybrid" compression that evolved to a strike-slip regime. This progressive deformation is best described with the following stress regimes: WNW-ESE transpression-pure compression (T1) associated with low-angle thrusting, subsequent transpression-strike-slip (T2), and later strike-slip-transtension (T3) associated with steeply dipping strike-slip faulting. The bulk of the granitoid-hosted gold mineralization at Edikan is associated with two principal sets of gold-bearing quartz veins, including low-angle fault-fill veins controlled by thrusts and shallow dipping oblique-extension veins that developed during T1. The activation of the reverse and sinistral strike-slip faults led to the development of restraining jogs characterized by abundant shallow and steeply dipping gold-quartz veins with moderately NE-plunging ore shoots. The geometry of the mineralized fault-fracture meshes is consistent with fault-valve behavior in a horizontal compressive stress regime under sustained conditions of supralithostatic fluid pressures at low differential stress