224 research outputs found
X-ray spectral modelling of the AGN obscuring region in the CDFS: Bayesian model selection and catalogue
AGN are known to have complex X-ray spectra that depend on both the
properties of the accreting SMBH (e.g. mass, accretion rate) and the
distribution of obscuring material in its vicinity ("torus"). Often however,
simple and even unphysical models are adopted to represent the X-ray spectra of
AGN. In the case of blank field surveys in particular, this should have an
impact on e.g. the determination of the AGN luminosity function, the inferred
accretion history of the Universe and also on our understanding of the relation
between AGN and their host galaxies. We develop a Bayesian framework for model
comparison and parameter estimation of X-ray spectra. We take into account
uncertainties associated with X-ray data and photometric redshifts. We also
demonstrate how Bayesian model comparison can be used to select among ten
different physically motivated X-ray spectral models the one that provides a
better representation of the observations. Despite the use of low-count
spectra, our methodology is able to draw strong inferences on the geometry of
the torus. For a sample of 350 AGN in the 4 Ms Chandra Deep Field South field,
our analysis identifies four components needed to represent the diversity of
the observed X-ray spectra: (abridged). Simpler models are ruled out with
decisive evidence in favour of a geometrically extended structure with
significant Compton scattering. Regarding the geometry of the obscurer, there
is strong evidence against both a completely closed or entirely open toroidal
geometry, in favour of an intermediate case. The additional Compton reflection
required by data over that predicted by toroidal geometry models, may be a sign
of a density gradient in the torus or reflection off the accretion disk.
Finally, we release a catalogue with estimated parameters such as the accretion
luminosity in the 2-10 keV band and the column density, , of the
obscurer.Comment: 28 pages, 18 figures, catalogue available from
https://www.mpe.mpg.de/~jbuchner/agn_torus/analysis/cdfs4Ms_cat/, software
available from https://github.com/JohannesBuchner/BX
Observational constraints on the physics behind the evolution of AGN since z ~ 1
We explore the evolution with redshift of the rest-frame colours and space
densities of AGN hosts (relative to normal galaxies) to shed light on the
dominant mechanism that triggers accretion onto supermassive black holes as a
function of cosmic time. Data from serendipitous wide-area XMM surveys of the
SDSS footprint (XMM/SDSS, Needles in the Haystack survey) are combined with
Chandra deep observations in the AEGIS, GOODS-North and GOODS-South to compile
uniformly selected samples of moderate luminosity X-ray AGN [L_X(2-10keV) =
1e41-1e44erg/s] at redshifts 0.1, 0.3 and 0.8. It is found that the fraction of
AGN hosted by red versus blue galaxies does not change with redshift. Also, the
X-ray luminosity density associated with either red or blue AGN hosts remains
nearly constant since z=0.8. X-ray AGN represent a roughly fixed fraction of
the space density of galaxies of given optical luminosity at all redshifts
probed by our samples. In contrast the fraction of X-ray AGN among galaxies of
a given stellar mass decreases with decreasing redshift. These findings suggest
that the same process or combination of processes for fueling supermassive
black holes are in operation in the last 5 Gyrs of cosmic time. The data are
consistent with a picture in which the drop of the accretion power during that
period (1dex since z=0.8) is related to the decline of the space density of
available AGN hosts, as a result of the evolution of the specific
star-formation rate of the overall galaxy population. Scenarios which attribute
the evolution of moderate luminosity AGN since z \approx 1 to changes in the
suppermassive black hole accretion mode are not favored by our results.Comment: MNRAS accepted, 15 pages, 10 figure
Host galaxy colour gradients and accretion disc obscuration in AEGIS z~1 X-ray-selected active galactic nuclei
We describe the effect of AGN light on host galaxy optical and UV-optical
colours, as determined from X-ray-selected AGN host galaxies at z~1, and
compare the AGN host galaxy colours to those of a control sample matched to the
AGN sample in both redshift and stellar mass. We identify as X-ray-selected
AGNs 8.7 +4/-3 per cent of the red-sequence control galaxies, 9.8 +/-3 per cent
of the blue-cloud control galaxies, and 14.7 +4/-3 per cent of the green-valley
control galaxies. The nuclear colours of AGN hosts are generally bluer than
their outer colours, while the control galaxies exhibit redder nuclei. AGNs in
blue-cloud host galaxies experience less X-ray obscuration, while AGNs in
red-sequence hosts have more, which is the reverse of what is expected from
general considerations of the interstellar medium. Outer and integrated colours
of AGN hosts generally agree with the control galaxies, regardless of X-ray
obscuration, but the nuclear colours of unobscured AGNs are typically much
bluer, especially for X-ray luminous objects. Visible point sources are seen in
many of these, indicating that the nuclear colours have been contaminated by
AGN light and that obscuration of the X-ray radiation and visible light are
therefore highly correlated. Red AGN hosts are typically slightly bluer than
red-sequence control galaxies, which suggests that their stellar populations
are slightly younger. We compare these colour data to current models of AGN
formation. The unexpected trend of less X-ray obscuration in blue-cloud
galaxies and more in red-sequence galaxies is problematic for all AGN feedback
models, in which gas and dust is thought to be removed as star formation shuts
down. [See paper for full abstract.]Comment: Accepted to MNRAS. 19 pages, 14 figures, 1 table; table, four figures
(4, 6, 11, 13) revised to reflect corrected values for one of our objects;
results unchange
X-ray constraints on the fraction of obscured AGN at high accretion luminosities
The wide-area XMM-XXL X-ray survey is used to explore the fraction of
obscured AGN at high accretion luminosities, , and out to redshift . The sample covers an area
of about and provides constraints on the space density of
powerful AGN over a wide range of neutral hydrogen column densities extending
beyond the Compton-thick limit, . The fraction
of obscured Compton-thin () AGN is estimated
to be for luminosities
independent of redshift. For less luminous sources the fraction of obscured
Compton-thin AGN increases from at to at
. Studies that select AGN in the infrared via template fits to the
observed Spectral Energy Distribution of extragalactic sources estimate space
densities at high accretion luminosities consistent with the XMM-XXL
constraints. There is no evidence for a large population of AGN (e.g. heavily
obscured) identified in the infrared and missed at X-ray wavelengths. We
further explore the mid-infrared colours of XMM-XXL AGN as a function of
accretion luminosity, column density and redshift. The fraction of XMM-XXL
sources that lie within the mid-infrared colour wedges defined in the
literature to select AGN is primarily a function of redshift. This fraction
increases from about 20-30% at z=0.25 to about 50-70% at .Comment: MNRAS accepte
The role of AGN in the colour transformation of galaxies at redshifts z~1
We explore the role of AGN in establishing and/or maintaining the bimodal
colour distribution of galaxies by quenching their star-formation and hence,
causing their transition from the blue to the red cloud. Important tests for
this scenario include (i) the X-ray properties of galaxies in the transition
zone between the two clouds and (ii) the incidence of AGN in post-starbursts,
i.e. systems observed shortly after (<1Gyr) the termination of their
star-formation. We perform these tests by combining deep Chandra observations
with multiwavelength data from the AEGIS survey. Stacking the X-ray photons at
the positions of galaxies (0.4<z<0.9) not individually detected at X-ray
wavelengths suggests a population of obscured AGN among sources in the
transition zone and in the red cloud. Their mean X-ray and mid-IR properties
are consistent with moderately obscured low-luminosity AGN, Compton thick
sources or a mix of both. Morphologies show that major mergers are unlikely to
drive the evolution of this population but minor interactions may play a role.
The incidence of obscured AGN in the red cloud (both direct detections and
stacking results) suggests that BH accretion outlives the termination of the
star-formation. This is also supported by our finding that post-starburst
galaxies at z~0.8 and AGN are associated, in agreement with recent results at
low-z. A large fraction of post-starbursts and red cloud galaxies show evidence
for at least moderate levels of AGN obscuration. This implies that if AGN
outflows cause the colour transformation of galaxies, then some nuclear gas and
dust clouds either remain unaffected or relax to the central galaxy regions
after the quenching their star-formation.Comment: Accepted for publication in MNRA
The X-ray luminosity function of AGN at z~3
We combine Lyman-break colour selection with ultradeep (> 200 ks) Chandra
X-ray imaging over a survey area of ~0.35 deg^2 to select high redshift AGN.
Applying careful corrections for both the optical and X-ray selection
functions, the data allow us to make the most accurate determination to date of
the faint end of the X-ray luminosity function (XLF) at z~3. Our methodology
recovers a number density of X-ray sources at this redshift which is at least
as high as previous surveys, demonstrating that it is an effective way of
selecting high z AGN. Comparing to results at z=1, we find no evidence that the
faint slope of the XLF flattens at high z, but we do find significant (factor
~3.6) negative evolution of the space density of low luminosity AGN. Combining
with bright end data from very wide surveys we also see marginal evidence for
continued positive evolution of the characteristic break luminosity L*. Our
data therefore support models of luminosity-dependent density evolution between
z=1 and z=3. A sharp upturn in the the XLF is seen at the very lowest
luminosities (Lx < 10^42.5 erg s^-1), most likely due to the contribution of
pure X-ray starburst galaxies at very faint fluxes.Comment: 16 pages, 9 figures, accepted for publication in MNRA
Investigating evidence for different black hole accretion modes since redshift z~1
Chandra data in the COSMOS, AEGIS-XD and 4Ms CDFS are combined with
optical/near-IR photometry to determine the rest-frame U-V vs V-J colours of
X-ray AGN hosts at mean redshifts 0.40 and 0.85. This combination of colours
(UVJ) provides an efficient means of separating quiescent from star-forming,
including dust reddened, galaxies. Morphological information emphasises
differences between AGN split by their UVJ colours. AGN in quiescent galaxies
are dominated by spheroids, while star-forming hosts are split between bulges
and disks. The UVJ diagram of AGN hosts is then used to set limits on the
accretion density associated with evolved and star-forming systems. Most of the
black hole growth since z~1 is associated with star-forming hosts.
Nevertheless, ~15-20% of the X-ray luminosity density since z~1, is taking
place in the quiescent region of the UVJ diagram. For the z~0.40 subsample,
there is tentative evidence (2sigma significance), that AGN split by their UVJ
colours differ in Eddington ratio. AGN in star-forming hosts dominate at high
Eddington ratios, while AGN in quiescent hosts become increasingly important as
a fraction of the total population toward low Eddington ratios. At higher
redshift, z~0.8, such differences are significant at the 2sigma level only at
Eddington ratios >1e-3. These findings are consistent with scenarios in which
diverse accretion modes are responsible for the build-up of SMBHs at the
centres of galaxies. We compare our results with the GALFORM semi-analytic
model, which postulates two black hole fuelling modes, the first linked to
star-formation and the second occuring in passive galaxies. GALFORM predicts a
larger fraction of black hole growth in quiescent galaxies at z<1, compared to
the data. Relaxing the strong assumption of the model that passive AGN hosts
have zero star-formation rate could reconcile this disagreement.Comment: MNRAS accepte
Linking the X-ray and infrared properties of star-forming galaxies at z < 1.5
We present the most complete study to date of the X-ray emission from star formation in high-redshift (median z = 0.7; z −3 in both hard and soft X-ray bands. From the sources which are star formation dominated, only a small fraction are individually X-ray detected and for the bulk of the sample we calculate average X-ray luminosities through stacking. We find an average soft X-ray to infrared ratio of log ?L SX /L IR ? = −4.3 and an average hard X-ray to infrared ratio of log?L HX /L IR ?=−3.8.WereportthattheX-ray/IRcorrelationisapproximatelylinearthrough the entire range of L IR and z probed and, although broadly consistent with the local (z < 0.1) one, it does display some discrepancies. We suggest that these discrepancies are unlikely to be physical, i.e. due to an intrinsic change in the X-ray properties of star-forming galaxies with cosmic time, as there is no significant evidence for evolution of the L X /L IR ratio with redshift. Instead, they are possibly due to selection effects and remaining AGN contamination.
We also examine whether dust obscuration in the galaxy plays a role in attenuating X-rays from star formation, by investigating changes in the L X /L IR ratio as a function of the average dust temperature. We conclude that X-rays do not suffer any measurable attenuation in the host galaxy
AEGIS: The color-magnitude relation for X-ray selected AGN
We discuss the relationship between rest-frame color and optical luminosity
for X-ray sources in the range 0.6<z<1.4 selected from the Chandra survey of
the Extended Groth Strip (EGS). These objects are almost exclusively active
galactic nuclei (AGN). While there are a few luminous QSOs, most are relatively
weak or obscured AGN whose optical colors should be dominated by host galaxy
light. The vast majority of AGN hosts at z~1 are luminous and red, with very
few objects fainter than M_{B}=-20.5 or bluer than U-B=0.6. This places the AGN
in a distinct region of color-magnitude space, on the ``red sequence'' or at
the top of the ``blue cloud'', with many in between these two modes in galaxy
color. A key stage in the evolution of massive galaxies is when star formation
is quenched, resulting in a migration from the blue cloud to the red sequence.
Our results are consistent with scenarios in which AGN either cause or maintain
this quenching. The large numbers of red sequence AGN imply that strong,
ongoing star formation is not a necessary ingredient for AGN activity, as black
hole accretion appears often to persist after star formation has been
terminated.Comment: 5 pages, 2 figures, accepted for publication in AEGIS ApJ Letters
special editio
Two New Techniques for Evaluating Connectivity of Septic Fields to Great Lake Watersheds and Embayments
Pictometry Oblique Imagery was successfully used to map septic fields in Oak Orchard watershed. Analysis of the imagery proved to be efficient for finding leach fields, and between 66 to 81% of the septic fields previously mapped by the Genesee Orleans County Department of Health, were identified. The remainder were not identified because of canopy cover, or were either septic systems without leach fields, the septic field postdated the imagery, or were not visible. Consequently under ideal conditions (septic systems with leach fields and no canopy or shadows) the method should be able to identify over 80% of the systems. Imagery taken during the transition from dormant to growing season proved best for identifying leach fields. One example of a plume from a short circuited system was recorded. A total of 1277 septic fields were mapped in the watershed. Spatial distribution was heterogenous, with dense sites of septic fields concentrated along residential road corridors. Approximately 4.2% of the leach fields were located less than 100 feet of a tributary. This is below the minimum separation distance of a leach field to a waterbody that is required by the NYS Department of Health code. The average distance of a leach field to a tributary is 327 meters with 50% of the leach fields occurring within 240 meters of mapped tributaries. Maps of important septic field “hotspots” were developed for watershed stakeholders and include tributaries along Batavia-Elba Townline Rd., Marsh Creek, and tributaries near the intersections of Alleghany and Lockport Rds, Judge Rd and Knowlesville Rd., and Lockport and Albion Rd. Considerable numbers of septic fields occur along Lake Alice in the main stem of the river, however this stretch is probably diluted by water input from the Erie Canal at the Glendale Dam.
In a second set of experiments, a new DNA-based groundwater tracer was introduced to two septic systems to see if it could be used to trace individual septic systems. The tracer was not discovered in one site, however, a breakthrough curve was obtained in the second site 31 days after being introduced into the toilet. This tracer passed through at least 200 meters of groundwater flowpath and 1 km of stream. The results of these experiments suggest that frequent, systematic sampling and careful lab protocols to identify the signal to noise threshold of the procedure are critical to the success of the technique. In summary we suggest that Pictometry Oblique Imagery can be used to map septic fields in a watershed and that the DNA Tracer technique may be successful in some septic systems. Further research needs to be conducted to improve the success of the latter
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