110 research outputs found
The XMM-Newton Needles in the Haystack Survey: the local X-ray luminosity function of 'normal' galaxies
In this paper we estimate the local (z<0.22) X-ray luminosity function of
`normal' galaxies derived from the XMM-Newton Needles in the Haystack Survey.
This is an on-going project that aims to identify X-ray selected `normal'
galaxies (i.e. non-AGN dominated) in the local Universe. We are using a total
of 70 XMM-Newton fields covering an area of 11 sq. degrees which overlap with
the Sloan Digital Sky Survey Data Release-2. `Normal' galaxies are selected on
the basis of their resolved optical light profile, their low X-ray--to--optical
flux ratio (log(f_x/f_o)<-2) and soft X-ray colours. We find a total of 28
candidate `normal' galaxies to the 0.5-8 keV band flux limit of ~2x10^{-15}
cgs. Optical spectra are available for most sources in our sample (82 per
cent). These provide additional evidence that our sources are bona-fide
'normal' galaxies with X-ray emission coming from diffuse hot gas emission
and/or X-ray binaries rather than a supermassive black hole. Sixteen of our
galaxies have narrow emission lines or a late-type Spectral Energy Distribution
(SED) while the remaining 12 present only absorption lines or an early-type
SED. Combining our XMM-Newton sample with 18 local (z<0.22) galaxies from the
Chandra Deep Field North and South surveys, we construct the local X-ray
luminosity function of `normal' galaxies. This can be represented with a
Schechter form with a break at L* ~ 3x10^{41} cgs and a slope of ~1.78+/-0.12.
Using this luminosity function and assuming pure luminosity evolution of the
form
(1+z)^{3.3} we estimate a contribution to the X-ray background from `normal'
galaxies of 10-20 per cent (0.5-8 keV). Finally, we derive, for the first time,
the luminosity functions for early and late type systems separately.Comment: To appear to MNRAS, 9 page
X-ray AGN in the XMM-LSS galaxy clusters: no evidence of AGN suppression
We present a study of the overdensity of X-ray selected AGN in 33 galaxy
clusters in the XMM-LSS field, up to redhift z=1.05. Previous studies have
shown that the presence of X-ray selected AGN in rich galaxy clusters is
suppressed. In the current study we investigate the occurrence of X-ray
selected AGN in low and moderate X-ray luminosity galaxy clusters. Due to the
wide contiguous XMM-LSS survey area we are able to extend the study to the
cluster outskirts. We therefore determine the projected overdensity of X-ray
point-like sources out to 6r_{500} radius. To provide robust statistical
results we also use a stacking analysis of the cluster projected overdensities.
We investigate whether the observed X-ray overdensities are to be expected by
estimating also the corresponding optical galaxy overdensities. We find a
positive X-ray projected overdensity at the first radial bin, which is however
of the same amplitude as that of optical galaxies. Therefore, no suppression of
X-ray AGN activity with respect to the field is found, implying that the
mechanisms responsible for the suppression are not so effective in lower
density environments. After a drop to roughly the background level between 2
and 3r_{500}, the X-ray overdensity exhibits a rise at larger radii,
significantly larger than the corresponding optical overdensity. Finally, using
redshift information of all optical counterparts, we derive the spatial
overdensity profile of the clusters. We find that the agreement between X-ray
and optical overdensities in the first radial bin is also suggested in the
3-dimensional analysis. However, we argue that the X-ray overdensity "bump" at
larger radial distance is probably a result of flux boosting by gravitational
lensing of background QSOs. For high redshift clusters an enhancement of X-ray
AGN activity in their outskirts is still possible.Comment: 16 pages. Accepted for publication in A&
The XMM spectral catalog of SDSS optically selected Seyfert 2 galaxies
We present an X-ray spectroscopic study of optically selected (SDSS) Seyfert
2 (Sy2) galaxies. The goal is to study the obscuration of Sy2 galaxies beyond
the local universe, using good quality X-ray spectra in combination with high
S/N optical spectra for their robust classification. We analyzed all available
XMM-Newton archival observations of narrow emission line galaxies that meet the
above criteria in the redshift range 0.05<z<0.35. We initially selected narrow
line AGN using the SDSS optical spectra and the BPT classification diagram. We
further modeled and removed the stellar continuum, and we analyzed the residual
emission line spectrum to exclude any possible intermediate-type Seyferts. Our
final catalog comprises 31 Sy2 galaxies with median redshift z~0.1. X-ray
spectroscopy is performed using the available X-ray spectra from the 3XMM and
the XMMFITCAT catalogs. Implementing various indicators of obscuration, we find
seven (~23%) Compton-thick AGN. The X-ray spectroscopic Compton-thick
classification agrees with other commonly used diagnostics, such as the X-ray
to mid-IR luminosity ratio and the X-ray to [OIII] luminosity ratio. Most
importantly, we find four (~13%) unobscured Sy2 galaxies, at odds with the
simplest unification model. Their accretion rates are significantly lower than
the rest of our Sy2 sample, in agreement with previous studies that predict the
absence of the broad line region below a certain Eddington ratio threshold.Comment: 12 pages, 6 figures, accepted for publication in A&
Mining for normal galaxies in the First XMM-Newton Serendipitous Source Catalog
This paper uses the 1st XMM Serendipitous Source Catalog compiled by the XMM
Science Center to identify low-z X-ray selected normal galaxy candidates. Our
sample covers a total area of ~6deg^2 to the 0.5-2keV limit 1e-15cgs. A total
of 23 sources are selected on the basis of low X-ray-to-optical flux ratio
logfx/fopt<-2, soft X-ray spectral properties and optical spectra, when
available, consistent with stellar than AGN processes. This sample is combined
with similarly selected systems from the Needles in the Haystack Survey
(Georgantopoulos et al. 2005) to provide a total of 46 z<0.2 X-ray detected
normal galaxies, the largest low-z sample yet available. This is first used to
constrain the normal galaxy logN-logS at bright fluxes (>1e-15cgs). We estimate
a slope of -1.46\pm0.13 for the cumulative number counts consistent with the
euclidean prediction. We further combine our sample with 23 local (z<0.2)
galaxies from the CDF North and South to construct the local X-ray luminosity
function of normal galaxies. A Schechter form provides a good fit to the data
with a break at logL*~41.02cgs and a slope of -1.76\pm0.10. Finally, for the
sample of 46 systems we explore the association between X-ray luminosity and
host galaxy properties, such as SFR and stellar mass. We find that the L_X of
the emission-line systems correlates with Ha luminosity and 1.4GHz radio power,
both providing an estimate of the current star-formation rate. In the case of
early type galaxies with absorption line optical spectra we use the K-band as
proxy to stellar mass and find a correlation of the form L_X~L_K^1.5. This is
flatter than the L_X-L_B relation for local ellipticals. This may be due to
either L_K providing a better proxy to galaxy mass or selection effects biasing
our sample against very luminous early-type galaxies.Comment: Accepted for publication in MNRA
Searching for highly obscured AGN in the XMM-Newton serendipitous source catalog
The majority of active galactic nuclei (AGN) are obscured by large amounts of
absorbing material that makes them invisible at many wavelengths. X-rays, given
their penetrating power, provide the most secure way for finding these AGN. The
XMM-Newton serendipitous source catalog is the largest catalog of X-ray sources
ever produced; it contains about half a million detections. These sources are
mostly AGN. We have derived X-ray spectral fits for very many 3XMM-DR4 sources
( 114 000 observations, corresponding to 77 000 unique
sources), which contain more than 50 source photons per detector. Here, we use
a subsample of 1000 AGN in the footprint of the SDSS area (covering
120 deg) with available spectroscopic redshifts. We searched for highly
obscured AGN by applying an automated selection technique based on X-ray
spectral analysis that is capable of efficiently selecting AGN. The selection
is based on the presence of either a) flat rest-frame spectra; b) flat observed
spectra; c) an absorption turnover, indicative of a high rest-frame column
density; or d) an Fe K line with an equivalent width > 500 eV. We found
81 highly obscured candidate sources. Subsequent detailed manual spectral fits
revealed that 28 of them are heavily absorbed by column densities higher than
10 cm. Of these 28 AGN, 15 are candidate Compton-thick AGN on the
basis of either a high column density, consistent within the 90% confidence
level with N 10 cm, or a large equivalent width
(>500 eV) of the Fe K line. Another six are associated with
near-Compton-thick AGN with column densities of 510
cm. A combination of selection criteria a) and c) for low-quality
spectra, and a) and d) for medium- to high-quality spectra, pinpoint highly
absorbed AGN with an efficiency of 80%.Comment: 18 pages, 10 figures, accepted for publication in A&
The XXL Survey: XII. Optical spectroscopy of X-ray-selected clusters and the frequency of AGN in superclusters
This article belongs to the first series of XXL publications. It presents
multifibre spectroscopic observations of three 0.55 sq.deg. fields in the XXL
Survey, which were selected on the basis of their high density of
X-ray-detected clusters. The observations were obtained with the
AutoFib2+WYFFOS (AF2) wide-field fibre spectrograph mounted on the 4.2m William
Herschel Telescope. The paper first describes the scientific rationale, the
preparation, the data reduction, and the results of the observations, and then
presents a study of active galactic nuclei (AGN) within three superclusters. We
obtained redshifts for 455 galaxies in total, 56 of which are counterparts of
X-ray point-like sources. We were able to determine the redshift of the merging
supercluster XLSSC-e, which consists of six individual clusters at z~0.43, and
we confirmed the redshift of supercluster XLSSC-d at z~0.3. More importantly,
we discovered a new supercluster, XLSSC-f, that comprises three galaxy clusters
also at z~0.3. We find a significant 2D overdensity of X-ray point-like sources
only around the supercluster XLSSC-f. This result is also supported by the
spatial (3D) analysis of XLSSC-f, where we find four AGN with compatible
spectroscopic redshifts and possibly one more with compatible photometric
redshift. In addition, we find two AGN (3D analysis) at the redshift of
XLSSC-e, but no AGN in XLSSC-d. Comparing these findings with the optical
galaxy overdensity we conclude that the total number of AGN in the area of the
three superclusters significantly exceeds the field expectations. The
difference in the AGN frequency between the three superclusters cannot be
explained by the present study because of small number statistics. Further
analysis of a larger number of superclusters within the 50 sq. deg. of the XXL
is needed before any conclusions on the effect of the supercluster environment
on AGN can be reached.Comment: 11 pages, published by A&
The 2-10 keV unabsorbed luminosity function of AGN from the XMM-Newton LSS, CDFS and COSMOS surveys
The XMM-LSS, XMM-COSMOS, and XMM-CDFS surveys are complementary in terms of
sky coverage and depth. Together, they form a clean sample with the least
possible variance in instrument effective areas and PSF. Therefore this is one
of the best samples available to determine the 2-10 keV luminosity function of
AGN and its evolution. The samples and the relevant corrections for
incompleteness are described. A total of 2887 AGN is used to build the LF in
the luminosity interval 10^42-10^46 erg/s, and in the redshift interval
0.001-4. A new method to correct for absorption by considering the probability
distribution for the column density conditioned on the hardness ratio is
presented. The binned luminosity function and its evolution is determined with
a variant of the Page-Carrera method, improved to include corrections for
absorption and to account for the full probability distribution of photometric
redshifts. Parametric models, namely a double power-law with LADE or LDDE
evolution, are explored using Bayesian inference. We introduce the
Watanabe-Akaike information criterion (WAIC) to compare the models and estimate
their predictive power. Our data are best described by the LADE model, as
hinted by the WAIC indicator. We also explore the 15-parameter extended LDDE
model recently proposed by Ueda et al., and find that this extension is not
supported by our data. The strength of our method is that it provides:
un-absorbed non-parametric estimates; credible intervals for luminosity
function parameters; model choice according to which one has more predictive
power for future data.Comment: In press on A&A. The revised version corrects typos and the LF
normalisations in tables 1,2,5 and figs.9-12, which were on an incorrect
scale. Online material available at
http://www.astro.lu.se/~piero/xlf/xlf-paper-tables2.tgz . The software is
available on the author's website
http://www.astro.lu.se/~piero/LFTools/index.html and on github:
https://github.com/piero-ranalli/LFTool
The XXL Survey VII: A supercluster of galaxies at z=0.43
The XXL Survey is the largest homogeneous and contiguous survey carried out
with XMM-Newton. Covering an area of 50 square degrees distributed over two
fields, it primarily investigates the large-scale structures of the Universe
using the distribution of galaxy clusters and active galactic nuclei as tracers
of the matter distribution. Given its depth and sky coverage, XXL is
particularly suited to systematically unveiling the clustering of X-ray
clusters and to identifying superstructures in a homogeneous X-ray sample down
to the typical mass scale of a local massive cluster. A friends-of-friends
algorithm in three-dimensional physical space was run to identify large-scale
structures. In this paper we report the discovery of the highest redshift
supercluster of galaxies found in the XXL Survey. We describe the X-ray
properties of the clusters members of the structure and the optical follow-up.
The newly discovered supercluster is composed of six clusters of galaxies at a
median redshift z around 0.43 and distributed across approximately 30 by 15 arc
minutes (10 by 5 Mpc on sky) on the sky. This structure is very compact with
all the clusters residing in one XMM pointing; for this reason this is the
first supercluster discovered with the XXL Survey. Spectroscopic follow-up with
WHT (William Herschel Telescope) and NTT (New Technology Telescope) confirmed a
median redshift of z = 0.43. An estimate of the X-ray mass and luminosity of
this supercluster and of its total gas mass put XLSSC-e at the average mass
range of superclusters; its appearance, with two members of equal size, is
quite unusual with respect to other superclusters and provides a unique view of
the formation process of a massive structure.Comment: A&A, accepted; special XXL issu
The Activity of the Neighbours of Seyfert Galaxies
We present a follow-up study on a series of papers concerning the role of
close interactions as a possible triggering mechanism of AGN activity. We have
already studied the close (<100kpc/h) and the large scale (<1 Mpc/h)
environment of a local sample of Sy1, Sy2 and bright IRAS galaxies (BIRG) and
their respective control samples. The results led us to the conclusion that a
close encounter appears capable of activating a sequence where an absorption
line galaxy (ALG) galaxy becomes first a starburst, then a Sy2 and finally a
Sy1. Here we investigate the activity of neighboring galaxies of different
types of AGN, since both galaxies of an interacting pair should be affected. To
this end we present the optical spectroscopy and X-ray imaging of 30
neighbouring galaxies around two local (z<0.034) samples of 10 Sy1 and 13 Sy2
galaxies. Based on the optical spectroscopy we find that more than 70% of all
neighbouring galaxies exhibit star forming and/or nuclear activity (namely
recent star formation and/or AGN), while an additional X-ray analysis showed
that this percentage might be significantly higher. Furthermore, we find a
statistically significant correlation, at a 99.9% level, between the value of
the neighbour's [OIII]/H\beta ratio and the activity type of the central active
galaxy, i.e. the neighbours of Sy2 galaxies are systematically more ionized
than the neighbours of Sy1s. This result, in combination with trends found
using the Equivalent Width of the H\alpha emission line and the stellar
population synthesis code STARLIGHT, indicate differences in the stellar mass,
metallicity and star formation history between the samples. Our results point
towards a link between close galaxy interactions and activity and also provide
more clues regarding the possible evolutionary sequence inferred by our
previous studies.Comment: Accepted for publication in A&A. 17 pages, 4 figures. arXiv admin
note: substantial text overlap with arXiv:0910.1355v1 (withdrawn
Local and Large scale Environment of Seyfert Galaxies
We present a three-dimensional study of the local (<100 h^-1} kpc) and the
large scale (<1 h^{-1} Mpc) environment of the two main types of Seyfert AGN
galaxies. For this purpose we use 48 Sy1 galaxies (with redshifts in the range
0.007<z<0.036) and 56 Sy2 galaxies (with 0.004<z<0.020), located at high
galactic latitudes, as well as two control samples of non-active galaxies
having the same morphological, redshift, and diameter size distributions as the
corresponding Seyfert samples. Using the Center for Astrophysics (CfA2) and
Southern Sky Redshift Survey (SSRS) galaxy catalogues (m_B~15.5) and our own
spectroscopic observations (m_B~18.5), we find that within a projected distance
of 100 h^-1 kpc and a radial velocity separation of dv<600 km/sec around each
of our AGNs, the fraction of Seyfert 2 galaxies with a close neighbor is
significantly higher than that of their control (especially within 75 h^{-1}
kpc) and Seyfert 1 galaxy samples, confirming a previous two-dimensional
analysis of Dultzin-Hacyan et al. We also find that the large-scale environment
around the two types of Seyfert galaxies does not vary with respect to their
control sample galaxies. However, in the Seyfert 2 and control galaxy samples
do differ significantly when compared to the corresponding Seyfert 1 samples.
Since the main difference between these samples is their morphological type
distribution, we argue that the large-scale environmental difference cannot be
attributed to differences in nuclear activity but rather to their different
type of host galaxies.Comment: accepted for publication in ApJ, Abstract size reduced (according to
new rules) and corrected reference
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