33 research outputs found
Obscured and unobscured AGN populations in a hard-X-ray selected sample of the XMDS survey
Our goal is to probe the populations of obscured and unobscured AGN
investigating their optical-IR and X-ray properties as a function of X-ray
flux, luminosity and redshift within a hard X-ray selected sample of 136 X-ray
sources in the XMM Medium Deep Survey (XMDS) with wide multiwavelength
coverage. The XMDS area is covered with optical photometry from the VVDS and
CFHTLS surveys and infrared Spitzer data. Based on the X-ray luminosity and
X-ray to optical ratio, 132 sources are likely AGN, of which 122 have
unambiguous optical - IR identification. The observed optical and IR spectral
energy distributions of sources are fitted with AGN/galaxy templates in order
to classify them and compute photometric redshifts. 70% of the AGN are fitted
by a type 2 AGN or a star forming galaxy template and are grouped together in a
single class of ``optically obscured'' AGN. They have ``red'' optical colors
and generally show significant X-ray absorption from X-ray spectra or hardness
ratios (N cm). Sources with SEDs typical of type 1 AGN
have ``blue'' optical colors and exhibit X-ray absorption in about 30% of
cases. We performed a stacking analysis for obscured and type 1 AGN. The
stacked X-ray spectrum of obscured AGN is flatter than that of type 1 AGN and
has an average spectral slope of Gamma = 1.6. The subsample of objects fitted
by a galaxy template has an even harder stacked spectrum, with Gamma = 1.2 -
1.3. The obscured fraction is larger at lower fluxes, lower redshifts and lower
luminosities. X-ray absorption is less common than ``optical'' obscuration and
its incidence is nearly constant with redshift and luminosity. This implies
that X-ray absorption is not necessarily related to optical obscuration.Comment: 33 pages, 21 figures, accepted for publication in A&
The XMM-LSS Survey: A well controlled X-ray cluster sample over the D1 CFHTLS area
We present the XMM-LSS cluster catalogue corresponding to the CFHTLS D1 area.
The list contains 13 spectroscopically confirmed, X-ray selected galaxy
clusters over 0.8 deg2 to a redshift of unity and so constitutes the highest
density sample of clusters to date. Cluster X-ray bolometric luminosities range
from 0.03 to 5x10^{44} erg/s. In this study, we describe our catalogue
construction procedure: from the detection of X-ray cluster candidates to the
compilation of a spectroscopically confirmed cluster sample with an explicit
selection function. The procedure further provides basic X-ray products such as
cluster temperature, flux and luminosity. We detected slightly more clusters
with a (0.5-2.0 keV) X-ray fluxes of >2x10^{-14} erg/s/cm^{-2} than we expected
based on expectations from deep ROSAT surveys. We also present the
Luminosity-Temperature relation for our 9 brightest objects possessing a
reliable temperature determination. The slope is in good agreement with the
local relation, yet compatible with a luminosity enhancement for the 0.15 < z<
0.35 objects having 1 < T < 2 keV, a population that the XMM-LSS is identifying
systematically for the first time. The present study permits the compilation of
cluster samples from XMM images whose selection biases are understood. This
allows, in addition to studies of large-scale structure, the systematic
investigation of cluster scaling law evolution, especially for low mass X-ray
groups which constitute the bulk of our observed cluster population. All
cluster ancillary data (images, profiles, spectra) are made available in
electronic form via the XMM-LSS cluster database.Comment: 12 pages 5 figures, MNRAS accepted. The paper with full resolution
cluster images is available at
http://vela.astro.ulg.ac.be/themes/spatial/xmm/LSS/rel_pub_e.htm
A multi-wavelength survey of AGN in the XMM-LSS field: I. Quasar selection via the KX technique
AIMS: We present a sample of candidate quasars selected using the
KX-technique. The data cover 0.68 deg^2 of the X-ray Multi-Mirror (XMM)
Large-Scale Structure (LSS) survey area where overlapping multi-wavelength
imaging data permits an investigation of the physical nature of selected
sources. METHODS: The KX method identifies quasars on the basis of their
optical (R and z') to near-infrared (Ks) photometry and point-like morphology.
We combine these data with optical (u*,g'r',i',z') and mid-infrared (3.6-24
micron) wavebands to reconstruct the spectral energy distributions (SEDs) of
candidate quasars. RESULTS: Of 93 sources selected as candidate quasars by the
KX method, 25 are classified as quasars by the subsequent SED analysis.
Spectroscopic observations are available for 12/25 of these sources and confirm
the quasar hypothesis in each case. Even more, 90% of the SED-classified
quasars show X-ray emission, a property not shared by any of the false
candidates in the KX-selected sample. Applying a photometric redshift analysis
to the sources without spectroscopy indicates that the 25 sources classified as
quasars occupy the interval 0.7 < z < 2.5. The remaining 68/93 sources are
classified as stars and unresolved galaxies.Comment: 13 pages, 9 figures, A&A 494, p. 579-589. Replaced with published
version. Fig. 9 in first astro-ph submission has been update
The XMM-LSS survey: The XMDS/VVDS 4 sigma catalogue
We present a first catalogue of X-ray sources resulting from the central area
of the XMM-LSS (Large Scale Structure survey). We describe the reduction
procedures and the database tools we developed and used to derive a well
defined catalogue of X-ray sources. The present catalogue is limited to a
sub-sample of 286 sources detected at 4 sigma in the 1 deg^2 area covered by
the photometric VVDS (VIRMOS VLT Deep Survey), which allows us to provide
optical and radio identifications. We also discuss the X-ray properties of a
larger X-ray sample of 536 sources detected at > 4 sigma in the full 3 deg^2
area of the XMM Medium Deep Survey (XMDS) independently of the optical
identification. We also derive the logN-logS relationship for a sample of more
than one thousand sources that we discuss in the context of other surveys at
similar fluxes.Comment: 15+6 pages, 12 figures, accepted for publication in Astronomy &
Astrophysics The online catalogue announced in the paper will be accessible
in about 2 weeks due to technical reasons Fig. 2 replaced with a low
resolution on
Spectral Energy Distributions of Hard X-ray selected AGNs in the XMDS Survey
We present the spectral energy distributions (SEDs) of a hard X-ray selected
sample. The sample contains 136 sources with F(2-10 keV)>10^-14 erg/cm^2/s and
132 are AGNs. The sources are detected in a 1 square degree area of the
XMM-Newton-Medium Deep Survey where optical data from the VVDS, CFHTLS surveys,
and infrared data from the SWIRE survey are available. Based on a SED fitting
technique we derive photometric redshifts with sigma(1+z)=0.11 and 6% of
outliers and identify AGN signatures in 83% of the objects. This fraction is
higher than derived when a spectroscopic classification is available. The
remaining 17+9-6% of AGNs shows star-forming galaxy SEDs (SF class). The
sources with AGN signatures are divided in two classes, AGN1 (33+6-1%) and AGN2
(50+6-11). The AGN1 and AGN2 classes include sources whose SEDs are fitted by
type 1 and type 2 AGN templates, respectively. On average, AGN1s show soft
X-ray spectra, consistent with being unabsorbed, while AGN2s and SFs show hard
X-ray spectra, consistent with being absorbed. The analysis of the average SEDs
as a function of X-ray luminosity shows a reddening of the IR SEDs, consistent
with a decreasing contribution from the host galaxy at higher luminosities. The
AGNs in the SF classes are likely obscured in the mid-infrared, as suggested by
their low L(3-20micron)/Lcorr(0.5-10 keV) ratios. We confirm the previously
found correlation for AGNs between the radio luminosity and the X-ray and the
mid-infrared luminosities. The X-ray-radio correlation can be used to identify
heavily absorbed AGNs. However, the estimated radio fluxes for the missing AGN
population responsible for the bulk of the background at E>10 keV are too faint
to be detected even in the deepest current radio surveys.Comment: Accepted for publication in Ap
The XMM-LSS catalogue: X-ray sources and associated optical data. Version I
Following the presentation of the XMM-LSS X-ray source detection package by
Pacaud et al., we provide the source lists for the first 5.5 surveyed square
degrees. The catalogues pertain to the [0.5-2] and [2-10] keV bands and contain
in total 3385 point-like or extended sources above a detection likelihood of 15
in either band. The agreement with deep logN-logS is excellent. The main
parameters considered are position, countrate, source extent with associated
likelihood values. A set of additional quantities such as astrometric
corrections and fluxes are further calculated while errors on the position and
countrate are deduced from simulations. We describe the construction of the
band-merged catalogue allowing rapid sub-sample selection and easy
cross-correlation with external multi-wavelength catalogues. A small optical
CFHTLS multi-band subset of objects is associated wich each source along with
an X-ray/optical overlay. We make the full X-ray images available in FITS
format. The data are available at CDS and, in a more extended form, at the
Milan XMM-LSS database.Comment: 13 pages, 7 figures and 11 tables (fig. 1 and 6 are enclosed in
reduced resolution), MNRAS Latex, accepted by MNRA
The XMM Large Scale Structure Survey: Properties and Two-Point Angular Correlations of Point-like Sources
We analyze X-ray sources detected over 4.2 pseudo-contiguous sq. deg. in the
0.5-2 keV and 2-10 keV bands down to fluxes of 2x10^{-15} and 8x10^{-15}
erg/s/cm^2 respectively, as part of the XMM Large Scale Structure Survey. The
logN-logS in both bands shows a steep slope at bright fluxes, but agrees well
with other determinations below ~2x10^{-14} erg/s/cm^2. The detected sources
resolve close to 30 per cent of the X-ray background in the 2-10 keV band. We
study the two-point angular clustering of point sources using nearest
neighbours and correlation function statistics and find a weak, positive signal
for ~1130 sources in the 0.5-2 keV band, but no correlation for ~400 sources in
the 2-10 keV band below scales of 100 arcsec. A sub-sample of ~200 faint
sources with hard X-ray count ratios, that is likely to be dominated by
obscured AGN, does show a positive signal with the data allowing for a large
scaling of the angular correlation length, but only at the ~2 (3) sigma level,
based on re-sampling (Poisson) statistics. We discuss possible implications and
emphasize the importance of wider, complete surveys in order to fully
understand the large scale structure of the X-ray sky.Comment: A&A in press; High resolution version at
http://www-xray.ast.cam.ac.uk/~pg/publications.htm
The XMM large scale structure survey: optical vs. X-ray classifications of active galactic nuclei and the unified scheme
Our goal is to characterize AGN populations by comparing their X-ray and
optical classifications. We present a sample of 99 spectroscopically identified
X-ray point sources in the XMM-LSS survey which are significantly detected in
the [2-10] keV band, and with more than 80 counts. We performed an X-ray
spectral analysis for all of these 99 X-ray sources. Introducing the fourfold
point correlation coefficient, we find only a mild correlation between the
X-ray and the optical classifications, as up to 30% of the sources have
differing X-ray and optical classifications: on one hand, 10% of the type 1
sources present broad emission lines in their optical spectra and strong
absorption in the X-rays. These objects are highly luminous AGN lying at high
redshift and thus dilution effects are totally ruled out, their discrepant
nature being an intrinsic property. Their X-ray luminosities and redshifts
distributions are consistent with those of the unabsorbed X-ray sources with
broad emission lines. On the other hand, 25/32 are moderate luminosity AGN,
which are both unabsorbed in the X-rays and only present narrow emission lines
in their optical spectra. The majority of them have an optical spectrum which
is representative of the host galaxy. We finally infer that dilution of the AGN
by the host galaxy seems to account for their nature. 5/25 have been defined as
Seyfert 2. In conclusion, most of these 32 discrepant cases can be accounted
for by the standard AGN unified scheme, as its predictions are not met for only
12% of the 99 X-ray sources. ABRIDGEDComment: 25 pages, 19 figures, Accepted for publication in A&
The XMM-LSS survey: the Class 1 cluster sample over the initial 5 square degrees and its cosmological modelling
We present a sample of 29 galaxy clusters from the XMM-LSS survey over an
area of some 5deg2 out to a redshift of z=1.05. The sample clusters, which
represent about half of the X-ray clusters identified in the region, follow
well defined X-ray selection criteria and are all spectroscopically confirmed.
For all clusters, we provide X-ray luminosities and temperatures as well as
masses. The cluster distribution peaks around z=0.3 and T =1.5 keV, half of the
objects being groups with a temperature below 2 keV. Our L-T(z) relation points
toward self-similar evolution, but does not exclude other physically plausible
models. Assuming that cluster scaling laws follow self-similar evolution, our
number density estimates up to z=1 are compatible with the predictions of the
concordance cosmology and with the findings of previous ROSAT surveys. Our well
monitored selection function allowed us to demonstrate that the inclusion of
selection effects is essential for the correct determination of the evolution
of the L-T relation, which may explain the contradictory results from previous
studies. Extensive simulations show that extending the survey area to 10deg2
has the potential to exclude the non-evolution hypothesis, but that constraints
on more refined ICM models will probably be limited by the large intrinsic
dispersion of the L-T relation. We further demonstrate that increasing the
dispersion in the scaling laws increases the number of detectable clusters,
hence generating further degeneracy [in addition to sigma8, Omega_m, L(M,z) and
T(M,z)] in the cosmological interpretation of the cluster number counts. We
provide useful empirical formulae for the cluster mass-flux and mass-count-rate
relations as well as a comparison between the XMM-LSS mass sensitivity and that
of forthcoming SZ surveys.Comment: Accepted for publication by MNRAS. Full resolution images as well as
additional cluster data are available through a dedicated database at
http://l3sdb.in2p3.fr:8080/l3sdb