227 research outputs found
XZ: Deriving redshifts from X-ray spectra of obscured AGN
Context: Redshifts are fundamental for our understanding of extragalactic
X-ray sources. Ambiguous counterpart associations, expensive optical
spectroscopy and/or multimission multiwavelength coverage to resolve
degeneracies make estimation often difficult in practice.
Aims: We attempt to constrain redshifts of obscured Active Galactic Nuclei
(AGN) using only low-resolution X-ray spectra.
Methods: Our XZ method fits AGN X-ray spectra with a moderately complex
spectral model incorporating a corona, torus obscurer and warm mirror. Using
the Bayesian X-ray Astronomy (BXA) package, we constrain redshift, column
density, photon index and luminosity simultaneously. The redshift information
primarily comes from absorption edges in Compton-thin AGN, and from the Fe
K fluorescent line in heavily obscured AGN. A new generic background
fitting method allows us to extract more information from limited numbers of
source counts.
Results: We derive redshift constraints for 74/321 hard-band detected sources
in the Chandra deep field South. Comparing with spectroscopic redshifts, we
find an outlier fraction of 8%, indicating that our model assumptions are
valid. For three Chandra deep fields, we release our XZ redshift estimates.
Conclusions: The independent XZ estimate is easy to apply and effective for a
large fraction of obscured AGN in todays deep surveys without the need for any
additional data. Comparing to different redshift estimation methods, XZ can
resolve degeneracies in photometric redshifts, help to detect potential
association problems and confirm uncertain single-line spectroscopic redshifts.
With high spectral resolution and large collecting area, this technique will be
highly effective for Athena/WFI observations.Comment: 20 pages, 16 figures in paper, 14 in appendice
The PEP Survey: evidence for intense star-forming activity in the majority of radio-selected AGN at z>~1
In order to investigate the FIR properties of radio-active AGN, we have
considered three different fields where both radio and FIR observations are the
deepest to-date: GOODS-South, GOODS-North and the Lockman Hole. Out of a total
of 92 radio-selected AGN, ~64% are found to have a counterpart in Herschel
maps. The percentage is maximum in the GOODS-North (72%) and minimum (~50%) in
the Lockman Hole, where FIR observations are shallower. Our study shows that in
all cases FIR emission is associated to star-forming activity within the host
galaxy. Such an activity can even be extremely intense, with star-forming rates
as high as ~10^3-10^4 Msun/yr. AGN activity does not inhibit star formation in
the host galaxy, just as on-site star-formation does not seem to affect AGN
properties, at least those detected at radio wavelengths and for z>~1.
Furthermore, physical properties such as the mass and age distributions of the
galaxies hosting a radio-active AGN do not seem to be affected by the presence
of an ongoing star-forming event. Given the very high rate of FIR detections,
we stress that this refers to the majority of the sample: most radio-active AGN
are associated with intense episodes of star-formation. However, the two
processes proceed independently within the same galaxy, at all redshifts but in
the local universe, where powerful enough radio activity reaches the necessary
strength to switch off the on-site star formation. Our data also show that for
z>~1 the hosts of radio-selected star-forming galaxies and AGN are
indistinguishable from each other both in terms of mass and IR luminosity
distributions. The two populations only differentiate in the very local
universe, whereby the few AGN which are still FIR-active are found in galaxies
with much higher masses and luminosities.Comment: 20 pages, 22 figures, to appear in MNRA
Specific star formation rates to redshift 5 from the FORS Deep Field and the GOODS-S Field
We explore the build-up of stellar mass in galaxies over a wide redshift
range 0.4 < z < 5.0 by studying the evolution of the specific star formation
rate (SSFR), defined as the star formation rate per unit stellar mass, as a
function of stellar mass and age. Our work is based on a combined sample of ~
9000 galaxies from the FORS Deep Field and the GOODS-S field, providing high
statistical accuracy and relative insensitivity against cosmic variance. As at
lower redshifts, we find that lower-mass galaxies show higher SSFRs than higher
mass galaxies, although highly obscured galaxies remain undetected in our
sample. Furthermore, the highest mass galaxies contain the oldest stellar
populations at all redshifts, in principle agreement with the existence of
evolved, massive galaxies at 1 < z < 3. It is remarkable, however, that this
trend continues to very high redshifts of z ~ 4. We also show that with
increasing redshift the SSFR for massive galaxies increases by a factor of ~
10, reaching the era of their formation at z ~ 2 and beyond. These findings can
be interpreted as evidence for an early epoch of star formation in the most
massive galaxies, and ongoing star-formation activity in lower mass galaxies.Comment: Accepted for publication in ApJL; 4 pages, 2 color figures, uses
emulateapj.cl
Identifying and Repairing Catastrophic Errors in Galaxy Properties Using Dimensionality Reduction
Our understanding of galaxy evolution is derived from large surveys designed to maximize efficiency by only observing the minimum amount needed to infer properties for a typical galaxy. However, for a few percent of galaxies in every survey, these observations are insufficient and derived properties can be catastrophically wrong. Further, it is currently difficult or impossible to determine which objects have failed, so that these contaminate every study of galaxy properties. We develop a novel method to identify these objects by combining the astronomical codes that infer galaxy properties with the dimensionality reduction algorithm t-SNE, which groups similar objects to determine which inferred properties are out of place. This method provides an improvement for the COSMOS catalog, which already uses existing techniques for catastrophic error removal, and therefore should improve the quality of large catalogs and any studies that are sensitive to large redshift errors
Finding counterparts for All-sky X-ray surveys with Nway: a Bayesian algorithm for cross-matching multiple catalogues
We release the AllWISE counterparts and Gaia matches to 106,573 and 17,665
X-ray sources detected in the ROSAT 2RXS and XMMSL2 surveys with |b|>15. These
are the brightest X-ray sources in the sky, but their position uncertainties
and the sparse multi-wavelength coverage until now rendered the identification
of their counterparts a demanding task with uncertain results. New all-sky
multi-wavelength surveys of sufficient depth, like AllWISE and Gaia, and a new
Bayesian statistics based algorithm, NWAY, allow us, for the first time, to
provide reliable counterpart associations. NWAY extends previous distance and
sky density based association methods and, using one or more priors (e.g.,
colors, magnitudes), weights the probability that sources from two or more
catalogues are simultaneously associated on the basis of their observable
characteristics. Here, counterparts have been determined using a WISE
color-magnitude prior. A reference sample of 4524 XMM/Chandra and Swift X-ray
sources demonstrates a reliability of ~ 94.7% (2RXS) and 97.4% (XMMSL2).
Combining our results with Chandra-COSMOS data, we propose a new separation
between stars and AGN in the X-ray/WISE flux-magnitude plane, valid over six
orders of magnitude. We also release the NWAY code and its user manual. NWAY
was extensively tested with XMM-COSMOS data. Using two different sets of
priors, we find an agreement of 96% and 99% with published Likelihood Ratio
methods. Our results were achieved faster and without any follow-up visual
inspection. With the advent of deep and wide area surveys in X-rays (e.g.
SRG/eROSITA, Athena/WFI) and radio (ASKAP/EMU, LOFAR, APERTIF, etc.) NWAY will
provide a powerful and reliable counterpart identification tool.Comment: MNRAS, Paper accepted for publication. Updated catalogs are available
at www.mpe.mpg.de/XraySurveys/2RXS_XMMSL2 . NWAY available at
https://github.com/JohannesBuchner/nwa
A census of radio-selected AGNs on the COSMOS field and of their FIR properties
We use the new catalogue by Laigle et al. to provide a full census of VLA-COSMOS radio sources. We identify 90 per cent of such sources and sub-divide them into active galactic nuclei (AGNs) and star-forming galaxies on the basis of their radio luminosity. The AGN sample is complete with respect to radio selection at all z ≲ 3.5. Out of 704 AGNs, 272 have a counterpart in the Herschel maps. By exploiting the better statistics of the new sample, we confirm the results of Magliocchetti et al.: the probability for a radio-selected AGN to be detected at far-infrared (FIR) wavelengths is both a function of radio luminosity and redshift, whereby powerful sources are more likely FIR emitters at earlier epochs. Such an emission is due to star-forming processes within the host galaxy. FIR emitters and non-FIR emitters only differentiate in the z ≲ 1 universe. At higher redshifts, they are indistinguishable from each other, as there is no difference between FIR-emitting AGNs and star-forming galaxies. Lastly, we focus on radio AGNs which show AGN emission at other wavelengths. We find that mid-infrared (MIR) emission is mainly associated with ongoing star formation and with sources which are smaller, younger and more radio luminous than the average parent population. X-ray emitters instead preferentially appear in more massive and older galaxies. We can therefore envisage an evolutionary track whereby the first phase of a radio-active AGN and of its host galaxy is associated with MIR emission, while at later stages the source becomes only active at radio wavelengths and possibly also in the X-ray
A Far-infrared Characterization of 24 μm Selected Galaxies at 0 < z < 2.5 using Stacking at 70 μm and 160 μm in the COSMOS Field
We present a study of the average properties of luminous infrared galaxies detected directly at 24 μm in the COSMOS field using a median stacking analysis at 70 μm and 160 μm. Over 35,000 sources spanning 0 ≤ z ≤ 3 and 0.06 mJy ≤ S_(24) ≤ 3.0 mJy are stacked, divided into bins of both photometric redshift and 24 μm flux. We find no correlation of S_(70)/S_(24) flux density ratio with S_(24), but find that galaxies with higher S_(24) have a lower S_(160)/S_(24) flux density ratio. These observed ratios suggest that 24 μm selected galaxies have warmer spectral energy distributions (SEDs) at higher mid-IR fluxes, and therefore have a possible higher fraction of active galactic nuclei. Comparisons of the average S_(70)/S_(24) and S_(160)/S_(24) colors with various empirical templates and theoretical models show that the galaxies detected at 24 μm are consistent with "normal" star-forming galaxies and warm mid-IR galaxies such as Mrk 231, but inconsistent with heavily obscured galaxies such as Arp 220. We perform a χ^2 analysis to determine best-fit galactic model SEDs and total IR luminosities for each of our bins. We compare our results to previous methods of estimating L IR and find that previous methods show considerable agreement over the full redshift range, except for the brightest S_(24) sources, where they overpredict the bolometric IR luminosity at high redshift, most likely due to their warmer dust SED. We present a table that can be used as a more accurate and robust method for estimating bolometric infrared luminosity from 24 μm flux densities
Probing AGN Inner Structure with X-ray Obscured Type 1 AGN
Using the X-ray-selected active galactic nuclei (AGN) from the XMM-XXL north
survey and the SDSS Baryon Oscillation Spectroscopic Survey (BOSS)
spectroscopic follow-up of them, we compare the properties of X-ray unobscured
and obscured broad-line AGN (BLAGN1 and BLAGN2; below and above
cm), including their X-ray luminosity , black hole
mass, Eddington ratio , optical continuum and line
features. We find that BLAGN2 have systematically larger broad line widths and
hence apparently higher (lower) ()
than BLAGN1. We also find that the X-ray obscuration in BLAGN tends to coincide
with optical dust extinction, which is optically thinner than that in
narrow-line AGN (NLAGN) and likely partial-covering to the broad line region.
All the results can be explained in the framework of a multi-component, clumpy
torus model by interpreting BLAGN2 as an intermediate type between BLAGN1 and
NLAGN in terms of an intermediate inclination angle.Comment: 21 pages, 12 figures, published in MNRA
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