2,487 research outputs found
The OPTX Project IV: How Reliable is [OIII] as a Measure of AGN Activity?
We compare optical and hard X-ray identifications of AGNs using a uniformly
selected (above a flux limit of f_2-8 keV = 3.5e-15 erg/cm2/s) and highly
optically spectroscopically complete ( > 80% for f_2-8 keV > 1e-14 erg/cm2/s
and > 60% below) 2-8 keV sample observed in three Chandra fields (CLANS,
CLASXS, and the CDF-N). We find that empirical emission-line ratio diagnostic
diagrams misidentify 20-50% of the X-ray selected AGNs that can be put on these
diagrams as star formers, depending on which division is used. We confirm that
there is a large (2 orders in magnitude) dispersion in the log ratio of the
[OIII]5007A to hard X-ray luminosities for the non-broad line AGNs, even after
applying reddening corrections to the [OIII] luminosities. We find that the
dispersion is similar for the broad-line AGNs, where there is not expected to
be much X-ray absorption from an obscuring torus around the AGN nor much
obscuration from the galaxy along the line-of-sight if the AGN is aligned with
the galaxy. We postulate that the X-ray selected AGNs that are misidentified by
the diagnostic diagrams have low [OIII] luminosities due to the complexity of
the structure of the narrow-line region, which causes many ionizing photons
from the AGN not to be absorbed. This would mean that the [OIII] luminosity can
only be used to predict the X-ray luminosity to within a factor of ~3 (one
sigma). Despite selection effects, we show that the shapes and normalizations
of the [OIII] and transformed hard X-ray luminosity functions show reasonable
agreement, suggesting that the [OIII] samples are not finding substantially
more AGNs at low redshifts than hard X-ray samples.Comment: Accepted for publication in the Astrophysical Journal. 11 pages, 10
figure
The Contribution of Active Galactic Nuclei to the Microjansky Radio Population
A X-ray background synthesis model is used to calculate the contribution of
Active Galactic Nuclei (AGNs) to the 1.4 GHz number counts between 100 nJy and
10 mJy. The number counts are broken down into contributions from radio-quiet
and radio-loud AGNs, obscured and unobscured AGNs, and for different ranges in
redshift and 2-10 keV X-ray luminosity, L_X. Compton-thick AGNs are included,
but only to the level required to fit the peak of the X-ray background. The
predicted radio counts show that the microJy AGN population will be dominated
by obscured, radio-quiet Seyfert galaxies with log L_X < 43, and spanning 0 < z
<~ 3. However, depending on the exact relationship between the radio and X-ray
luminosities in radio-quiet AGNs, additional radio flux due to star-formation
within AGN host galaxies may be necessary in order to match the observed AGN
counts at a flux density of ~50 microJy. The star-formation rates (SFR)
required are modest, only ~3 Msun per year, assuming a constant rate with z and
L_X. A more observationally and theoretically motivated relationship, where the
SFR \propto (1+z)^{1.76}(log L_X-40)^{3.5}, will also account for the observed
counts. The microJy AGN population will provide a very clean sample to trace
the accretion and galactic star-formation histories of Seyfert galaxies over a
significant fraction of cosmic time.Comment: 30 pages, 11 figures, accepted by the Ap
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 Presence of Weak Active Galactic Nuclei in High Redshift Star Forming Galaxies
We present [OIII 5007A] observations of the star forming galaxy HDF-BMZ1299
(z=1.598) using Keck Observatory's Adaptive Optics system with the
near-infrared integral field spectrograph OSIRIS. Using previous Halpha and
[NII] measurements of the same source, we are able for the first time to use
spatially resolved observations to place a high-redshift galaxy's substructure
on a traditional HII diagnostic diagram. We find that HDF-BMZ1299's spatially
concentrated nebular ratios in the central ~1.5 kiloparsec (0."2) are best
explained by the presence of an AGN: log([NII]/Halpha)=-0.22+/-0.05 and 2sigma
limit of log([OIII]/Hbeta)>0.26. The dominant energy source of this galaxy is
star formation, and integrating a single aperture across the galaxy yields
nebular ratios that are composite spectra from both AGN and HII regions. The
presence of an embedded AGN in HDF-BMZ1299 may suggest a potential
contamination in a fraction of other high-redshift star forming galaxies, and
we suggest that this may be a source of the "elevated" nebular ratios
previously seen in seeing-limited metallicity studies. HDF-BMZ1299's estimated
AGN luminosity is L_Halpha = 3.7e41 erg/s and L_[OIII] = 5.8e41 erg/s, making
it one of the lowest luminosity AGN discovered at this early epoch.Comment: 15 pages, 4 figures, ApJ Accepted, new version to be published
(updated text, figures, and table
Faint-end Quasar Luminosity Functions from Cosmological Hydrodynamic Simulations
We investigate the predictions for the faint-end quasar luminosity function
(QLF) and its evolution using fully cosmological hydrodynamic simulations which
self-consistently follow star formation, black hole growth and associated
feedback processes. We find remarkably good agreement between predicted and
observed faint end of the optical and X-ray QLFs (the bright end is not
accessible in our simulated volumes) at z < 2. At higher redshifts our
simulations tend to overestimate the QLF at the faintest luminosities. We show
that although the low (high) luminosity ranges of the faint-end QLF are
dominated by low (high) mass black holes, a wide range of black hole masses
still contributes to any given luminosity range. This is consistent with the
complex lightcurves of black holes resulting from the detailed hydrodynamics
followed in the simulations. Consistent with the results on the QLFs, we find
good agreement for the evolution of the comoving number density (in optical,
soft and hard X-ray bands) of AGN for luminosities above 10^43 erg/s. However,
the luminosity density evolution from the simulation appears to imply a peak at
higher redshift than constrained from hard X-ray data (but not in optical). Our
predicted excess at the faintest fluxes at z >= 2 does not lead to an
overestimate to the total X-ray background and its contribution is at most a
factor of two larger than the unresolved fraction of the 2-8 keV background.
Even though this could be explained by some yet undetected, perhaps heavily
obscured faint quasar population, we show that our predictions for the faint
sources at high redshifts (which are dominated by the low mass black holes) in
the simulations are likely affected by resolution effects.Comment: 12 pages, 9 figures; submitted and reviewed by MNRA
The deep look onto the hard X-ray sky: The Swift - INTEGRAL X-ray (SIX) survey
The super-massive black-holes in the centers of Active Galactic Nuclei (AGNs)
are surrounded by obscuring matter that can block the nuclear radiation.
Depending on the amount of blocked radiation, the flux from the AGN can be too
faint to be detected by currently flying hard X-ray (above 15 keV) missions. At
these energies only ~1% of the intensity of the Cosmic X-ray Background (CXB)
can be resolved into point-like sources that are AGNs. In this work we address
the question of the undetected sources contributing to the CXB with a very
sensitive and new hard X-ray survey: the SIX survey that is obtained with the
new approach of combining the Swift/BAT and INTEGRAL/IBIS X-ray observations.
We merge the observations of both missions. This enhances the exposure time and
reduces systematic uncertainties. As a result we obtain a new survey over a
wide sky area of 6200 deg^2 that is more sensitive than the surveys of
Swift/BAT or INTEGRAL/IBIS alone. Our sample comprises 113 sources: 86 AGNs
(Seyfert-like and blazars), 5 galaxies, 2 clusters of galaxies, 3 Galactic
sources, 3 previously detected unidentified X-ray sources, and 14 unidentified
sources. The scientific outcome from the study of the sample has been properly
addressed to study the evolution of AGNs at redshift below 0.4. We do not find
any evolution using the 1/V_max method. Our sample of faint sources are
suitable targets for the new generation hard X-ray telescopes with focusing
techniques.Comment: ApJS accepte
Quasars Are Not Light-Bulbs: Testing Models of Quasar Lifetimes with the Observed Eddington Ratio Distribution
We use the observed distribution of Eddington ratios as a function of
supermassive black hole (BH) mass to constrain models of AGN lifetimes and
lightcurves. Given the observed AGN luminosity function, a model for AGN
lifetimes (time above a given luminosity) translates directly to a predicted
Eddington ratio distribution. Models for self-regulated BH growth, in which
feedback produces a 'blowout' decay phase after some peak luminosity (shutting
down accretion) make specific predictions for the lifetimes distinct from those
expected if AGN are simply gas starved (without feedback) and very different
from simple phenomenological 'light bulb' models. Present observations of the
Eddington ratio distribution, spanning 5 decades in Eddington ratio, 3 in BH
mass, and redshifts z=0-1, agree with the predictions of self-regulated models,
and rule out 'light-bulb', pure exponential, and gas starvation models at high
significance. We compare the Eddington ratio distributions at fixed BH mass and
fixed luminosity (both are consistent, but the latter are much less
constraining). We present empirical fits to the lifetime distribution and show
how the Eddington ratio distributions place tight limits on AGN lifetimes at
various luminosities. We use this to constrain the shape of the typical AGN
lightcurve, and provide simple analytic fits. Given independent constraints on
episodic lifetimes, most local BHs must have gained their mass in no more than
a couple of bright episodes, in agreement with merger-driven fueling models.Comment: 21 pages, 13 figures, accepted to ApJ (revised to match accepted
version; modeling and tests of redshift evolution added
The XMM-Newton Serendipitous Survey. VI. The X-ray Luminosity Function
We present the X-ray luminosity function of AGN in three energy bands (Soft:
0.5-2 keV, Hard: 2-10 keV and Ultrahard: 4.5-7.5 keV). We have used the XMS
survey along with other highly complete flux-limited deeper and shallower
surveys for a total of 1009, 435 and 119 sources in the Soft, Hard and
Ultrahard bands, respectively. We have modeled the intrinsic absorption of the
Hard and Ultrahard sources (NH function) and computed the intrinsic X-ray
luminosity function in all bands using a Maximum Likelihood fit technique to an
analytical model. We find that the X-ray luminosity function (XLF) is best
described by a Luminosity-Dependent Density Evolution (LDDE) model. Our results
show a good overall agreement with previous results in the Hard band, although
with slightly weaker evolution. Our model in the Soft band present slight
discrepancies with other works in this band, the shape of our present day XLF
being significantly flatter. We find faster evolution in the AGN detected in
the Ultrahard band than those in the Hard band. The fraction of absorbed AGN in
the Hard and Ultrahard bands is dependent on the X-ray luminosity. We find
evidence of evolution of this fraction with redshift in the Hard band but not
in the Ultrahard band, possibly due to the low statistics. Our best-fit XLF
shows that the high-luminosity AGN are fully formed earlier than the less
luminous AGN. The latter sources account for the vast majority of the accretion
rate and mass density of the Universe, according to an anti-hierarchical black
hole growth scenario.Comment: 16 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
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
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