34 research outputs found
Accretion History of AGN II: Constraints on AGN Spectral Parameters using the Cosmic X-ray Background
We constrain X-ray spectral shapes for the ensemble of AGN based on the shape
of the Cosmic X-ray Background (CXB). Specifically, we rule out regions of
X-ray spectral parameter space that do not reproduce the CXB in the energy
range 1-100 keV. The key X-ray spectral parameters are the photon index,
{\Gamma}; the cutoff energy, Ecutoff; and the reflection scaling factor, R.
Assuming each parameter follows a Gaussian distribution, we first explore the
parameter space using a Bayesian approach and a fixed X-ray luminosity function
(XLF). For {\sigma}_E = 36 keV and {\sigma}_R = 0.14, fixed at the observed
values from the Swift-BAT 70-month sample, we allow , and
to vary subject to reproducing the CXB. We report results for
{\sigma}_{\Gamma} = 0.1-0.5. In an alternative approach, we define the
parameter distributions, then forward model to fit the CXB by perturbing the
XLF using a neural network. This approach allows us to rule out parameter
combinations that cannot reproduce the CXB for any XLF. The marginalized
conditional probabilities for the four free parameters are: =
0.99^{+0.11}_{-0.26}, = 118^{+24}_{-23}, {\sigma}_{\Gamma} =
0.101^{+0.097}_{-0.001} and = 1.9^{+0.08}_{-0.09}. We provide an
interactive online tool for users to explore any combination of ,
{\sigma}_E, , {\sigma}_{\Gamma}, and {\sigma}_R including
different distributions for each absorption bin, subject to the integral CXB
constraint. The distributions observed in many AGN samples can be ruled out by
our analysis, meaning these samples can not be representative of the full AGN
population. The few samples that fall within the acceptable parameter space are
hard X-ray-selected, commensurate with their having fewer selection biases.Comment: Accepted by ApJ, 15 pages, 7 figures and 3 table
SDSS-IV eBOSS Spectroscopy of X-ray and WISE AGN in Stripe 82X: Overview of the Demographics of X-ray and Mid-Infrared Selected Active Galactic Nuclei
We report the results of a Sloan Digital Sky Survey-IV eBOSS program to
target X-ray sources and mid-infrared-selected WISE AGN candidates in a 36.8
deg region of Stripe 82. About half this survey (15.6 deg) covers the
largest contiguous portion of the Stripe 82 X-ray survey. This program
represents the largest spectroscopic survey of AGN candidates selected solely
by their WISE colors. We combine this sample with X-ray and WISE AGN in the
field identified via other sources of spectroscopy, producing a catalog of 4847
sources that is 82% complete to . Based on X-ray luminosities or WISE
colors, 4730 of these sources are AGN, with a median sample redshift of
. About 30% of the AGN are optically obscured (i.e., lack broad lines
in their optical spectra). BPT analysis, however, indicates that 50% of the
WISE AGN at have emission line ratios consistent with star-forming
galaxies, so whether they are buried AGN or star-forming galaxy contaminants is
currently unclear. We find that 61% of X-ray AGN are not selected as MIR AGN,
with 22% of X-ray AGN undetected by WISE. Most of these latter AGN have high
X-ray luminosities ( erg s), indicating that MIR
selection misses a sizable fraction of the highest luminosity AGN, as well as
lower luminosity sources where AGN heated dust is not dominating the MIR
emission. Conversely, 58% of WISE AGN are undetected by X-rays, though we
do not find that they are preferentially redder than the X-ray detected WISE
AGN.Comment: resubmitted to AAS Journals after addressing referee's comments. 21
pages, 17 figures, 5 Tables. Tables 4 & 5 summarize the demographics of the
sample. Figures 15 & 16 compare AGN populations from X-ray and MIR selection.
The catalog (in fits format) can be downloaded at
http://doi.org/10.5281/zenodo.257735
Accretion History of AGN III: Radiative Efficiency and AGN Contribution to Reionization
The cosmic history of supermassive black hole (SMBH) growth is important for
understanding galaxy evolution, reionization and the physics of accretion.
Recent NuSTAR, Swift-BAT and \textit{Chandra} hard X-ray surveys have provided
new constraints on the space density of heavily obscured Active Galactic Nuclei
(AGN). Using the new X-ray luminosity function derived from these data, we here
estimate the accretion efficiency of SMBHs and their contribution to
reionization. We calculate the total ionizing radiation from active galactic
nuclei (AGN) as a function of redshift, based on the X radiation and
distribution of obscuring column density, converted to UV wavelengths. Limiting
the luminosity function to unobscured AGN only, our results agree with current
UV luminosity functions of unobscured AGN. For realistic assumptions about the
escape fraction, the contribution of all AGN to cosmic reionization is
times lower than the galaxy contribution (23% at ). Our results also
offer an observationally constrained prescription that can be used in
simulations or models of galaxy evolution. To estimate the average efficiency
with which supermassive black holes convert mass to light, we compare the total
radiated energy, converted from X-ray light using a bolometric correction, to
the most recent local black hole mass density. The most likely value, , approaches the theoretical limit for a maximally rotating Kerr
black hole, , implying that on average growing supermassive black
holes are spinning rapidly.Comment: Accepted by ApJ, 13 pages, 5 figure
The Accretion History of AGNs. I. Supermassive Black Hole Population Synthesis Model
As matter accretes onto the central supermassive black holes in active galactic nuclei (AGNs), X-rays are emitted. We present a population synthesis model that accounts for the summed X-ray emission from growing black holes; modulo the efficiency of converting mass to X-rays, this is effectively a record of the accreted mass. We need this population synthesis model to reproduce observed constraints from X-ray surveys: the X-ray number counts, the observed fraction of Compton-thick AGNs [log (N H/cm−2) > 24], and the spectrum of the cosmic X-ray background (CXB), after accounting for selection biases. Over the past decade, X-ray surveys by XMM-Newton, Chandra, NuSTAR, and Swift-BAT have provided greatly improved observational constraints. We find that no existing X-ray luminosity function (XLF) consistently reproduces all these observations. We take the uncertainty in AGN spectra into account and use a neural network to compute an XLF that fits all observed constraints, including observed Compton-thick number counts and fractions. This new population synthesis model suggests that, intrinsically, 50% ± 9% (56% ± 7%) of all AGNs within z sime 0.1 (1.0) are Compton-thick
AGN Populations in Large Volume X-ray Surveys: Photometric Redshifts and Population Types found in the Stripe 82X Survey
Multi-wavelength surveys covering large sky volumes are necessary to obtain
an accurate census of rare objects such as high luminosity and/or high redshift
active galactic nuclei (AGN). Stripe 82X is a 31.3 deg X-ray survey with
and -Newton observations overlapping the legacy Sloan Digital
Sky Survey (SDSS) Stripe 82 field, which has a rich investment of
multi-wavelength coverage from the ultraviolet to the radio. The wide-area
nature of this survey presents new challenges for photometric redshifts for AGN
compared to previous work on narrow-deep fields because it probes different
populations of objects that need to be identified and represented in the
library of templates. Here we present an updated X-ray plus multi-wavelength
matched catalog, including counterparts, and estimated photometric
redshifts for 5961 (96% of a total of 6181) X-ray sources, which have a
normalized median absolute deviation, = 0.06 and an outlier
fraction, = 13.7%. The populations found in this survey, and the
template libraries used for photometric redshifts, provide important guiding
principles for upcoming large-area surveys such as and 3 (in
X-ray) and the Large Synoptic Survey Telescope (LSST; optical).Comment: Accepted for publication by The Astrophysical Journal (33 pages, 20
figures, 13 tables). Final catalog of counterparts and photo-z supplementing
the paper available here:
http://stripe82x.com/docs/stripe82x-photometric-redshifts-and-multiwavelength-data-catalog
The Chandra COSMOS Legacy Survey : Energy Spectrum of the Cosmic X-Ray Background and Constraints on Undetected Populations
Using Chandra observations in the 2.15 deg(2) COSMOS-legacy field, we present one of the most accurate measurements of the Cosmic X-ray Background (CXB) spectrum to date in the [0.3-7] keV energy band. The CXB has three distinct components: contributions from two Galactic collisional thermal plasmas at kT similar to 0.27 and 0.07 keV and an extragalactic power law with a photon spectral index Gamma = 1.45 +/- 0.02. The 1 keV normalization of the extragalactic component is 10.91 +/- 0.16 keV cm(-2) s(-1) sr(-1) keV(-1). Removing all X-ray-detected sources, the remaining unresolved CXB is best fit by a power law with normalization 4.18 +/- 0.26 keV cm(-2) s(-1) sr(-1) keV(-1) and photon spectral index Gamma = 1.57 +/- 0.10. Removing faint galaxies down to i(AB) similar to 27-28 leaves a hard spectrum with Gamma similar to 1.25 and a 1 keV normalization of similar to 1.37 keV cm(-2) s(-1) sr(-1) keV(-1). This means that similar to 91% of the observed CXB is resolved into detected X-ray sources and undetected galaxies. Unresolved sources that contribute similar to 8%-9% of the total CXB show marginal evidence of being harder and possibly more obscured than resolved sources. Another similar to 1% of the CXB can be attributed to still undetected star-forming galaxies and absorbed active galactic nuclei. According to these limits, we investigate a scenario where early black holes totally account for non-source CXB fraction and constrain some of their properties. In order to not exceed the remaining CXB and the z similar to 6 accreted mass density, such a population of black holes must grow in Compton-thick envelopes with N-H > 1.6 x 10(25) cm(-2) and form in extremely low-metallicity environments (Z(circle dot)) similar to 10(-3).Peer reviewe
The Accretion History of AGN: A Newly Defined Population of Cold Quasars
Quasars are the most luminous of active galactic nuclei (AGN), and are
perhaps responsible for quenching star formation in their hosts. The Stripe 82X
catalog covers 31.3 deg of the Stripe 82 field, of which the 15.6 deg
covered with XMM-Newton is also covered by Herschel/SPIRE. We have 2500 X-ray
detected sources with multi-wavelength counterparts, and 30% of these are
unobscured quasars, with erg/s and . We define a
new population of quasars which are unobscured, have X-ray luminosities in
excess of erg/s, have broad emission lines, and yet are also bright
in the far-infrared, with a 250m flux density of mJy. We
refer to these Herschel-detected, unobscured quasars as "Cold Quasars". A mere
4% (21) of the X-ray- and optically-selected unobscured quasars in Stripe 82X
are detected at 250m. These Cold Quasars lie at , have , and have star formation rates of
/yr. Cold Quasars are bluer in the mid-IR than the full
quasar population, and 72% of our Cold Quasars have WISE W3 11.5 [Vega],
while only 19% of the full quasar sample meets this criteria. Crucially, Cold
Quasars have on average as much star formation as the main
sequence of star forming galaxies at similar redshifts. Although dust-rich,
unobscured quasars have occasionally been noted in the literature before, we
argue that they should be considered as a separate class of quasars due to
their high star formation rates. This phase is likely short-lived, as the
central engine and immense star formation consume the gas reservoir. Cold
Quasars are type-1 blue quasars that reside in starburst galaxies.Comment: Accepted for publication in Ap