453 research outputs found
The Phoenix survey: the pairing fraction of faint radio sources
The significance of tidal interactions in the evolution of the faint radio
population (sub-mJy) is studied using a deep and homogeneous radio survey (1.4
GHz), covering an area of 3.14 deg and complete to a flux density of 0.4
mJy. Optical photometric and spectroscopic data are also available for this
sample. A statistical approach is employed to identify candidate physical
associations between radio sources and optically selected `field' galaxies. We
find an excess of close pairs around optically identified faint radio sources,
albeit at a low significance level, implying that the pairing fraction of the
sub-mJy radio sources is similar to that of `field' galaxies (at the same
magnitude limit) but higher than that of local galaxies.Comment: 5 pages, 4 figures. Accepted for publication in MNRAS Letter
Searching for X-ray luminous 'normal' galaxies in 2dfGRS
We cross-correlated the Chandra XASSIST and XMM-Newton Serendipitous Source
Catalogues with the 2 degree Field Galaxy Redshift Survey (2dfGRS) database.
Our aim was to identify the most X-ray luminous (L_X > 10^42 erg s^-1) examples
of galaxies in the local Universe whose X-ray emission is dominated by stellar
processes rather than AGN activity ('normal' galaxies) as well as to test the
empirical criterion log(f_X/f_O) < -2 for separating AGN from NGs. With
XMM-Newton (Chandra) we covered an area of ~8.2 (~5.8)deg^2 down to a flux
limit of ~10^-15 (~1.6 x 10^-15) erg cm^-2 s^-1 and found 18 (20) 2dfGRS
galaxies. Using emission-line intensity ratios, we classified 6 2dfGRS spectra
as star-forming, H II nuclei, and 2 spectra as possible H II nuclei. The rest
of the objects are absorption-line galaxies and AGN, including 3 possible
LINERs. No luminous 'normal' galaxies have been found but out of 19 'normal'
galaxies in this sample 5 H II and 3 absorption-line galaxies have log(f_X/f_O)
> -2. We performed a similar search in two nearby-galaxy samples from the
literature. All 44 galaxies in the Zezas (2001) sample have log(f_X/f_O) < -2
and L_X < 10^42 erg s^-1. In the Fabbiano et al. (1992) sample, out of a total
of 170 'normal' galaxies, we found 16 galaxies with log(f_X/f_O) >-2, the
majority of which are massive ellipticals. Three of these have L_X > 10^42 erg
s^-1 .Comment: 8 pages, 2 figures. Accepted for publication in Astronomy &
Astrophysic
The radio spectra of reddened 2MASS QSOs: evidence for young radio jets
Multifrequency radio continuum observations (1.4-22 GHz) of a sample of
reddened QSOs are presented. We find a high incidence (13/16) of radio spectral
properties, such as low frequency turnovers, high frequency spectral breaks or
steep power-law slopes, similar to those observed in powerful compact steep
spectrum (CSS) and gigahertz-peaked spectrum (GPS) sources. The radio data are
consistent with relatively young radio jets with synchotron ages <1e6-1e7yr.
This calculation is limited by the lack of high resolution (milli-arcsec) radio
observations. For the one source in the sample that such data are available a
much younger radio age is determined, <2e3yr, similar to those of GPS/CSS
sources. These findings are consistent with claims that reddened QSOs are young
systems captured at the first stages of the growth of their supermassive black
holes. It also suggests that expanding radio lobes may be an important feedback
mode at the early stages of the evolution of AGN.Comment: 9 pages, to appear in MNRA
The Clustering of XMM-Newton Hard X-ray Sources
We present the clustering properties of hard (2-8 keV) X-ray selected sources
detected in a wide field (~2 deg^{2}) shallow [f(2-8 keV)~ 10^{-14} erg cm^{-2}
s^{-1}] and contiguous XMM-Newton survey. We perform an angular correlation
function analysis using a total of 171 sources to the above flux limit. We
detect a ~ 4\sigma correlation signal out to 300 arcsec with w(theta <
300^{''}) ~ 0.13 +- 0.03. Modeling the two point correlation function as a
power law of the usual form we find: theta_o=48.9^{+15.8}_{-24.5} arcsec and
gamma=2.2 +- 0.30. Fixing the correlation function slope to gamma=1.8 we obtain
theta_o=22.2^{+9.4}_{-8.6} arcsec. Using Limber's integral equation and a
variety of possible luminosity functions of the hard X-ray population, we find
a relatively large correlation length, ranging from r_o ~ 9 to 19 h^{-1} Mpc
(for gamma=1.8 and the concordance cosmological model), with this range
reflecting also different evolutionary models for the source luminosities and
clustering characteristics.Comment: In "Multiwavelength AGN Surveys" (Cozumel, December 8-12 2003), ed.
R. Maiolino and R. Mujica, Singapore: World Scientific, 200
Constraining the fraction of Compton-thick AGN in the Universe by modelling the diffuse X-ray background spectrum
This paper investigates what constraints can be placed on the fraction of
Compton-thick (CT) AGN in the Universe from the modeling of the spectrum of the
diffuse X-ray background (XRB). We present a model for the synthesis of the XRB
that uses as input a library of AGN X-ray spectra generated by the Monte Carlo
simulations described by Brightman & Nandra. This is essential to account for
the Compton scattering of X-ray photons in a dense medium and the impact of
that process on the spectra of obscured AGN. We identify a small number of
input parameters to the XRB synthesis code which encapsulate the minimum level
of uncertainty in reconstructing the XRB spectrum. These are the power-law
index and high energy cutoff of the intrinsic X-ray spectra of AGN, the level
of the reflection component in AGN spectra and the fraction of CT AGN in the
Universe. We then map the volume of the space allowed to these parameters by
current observations of the XRB spectrum in the range 3-100 keV. One of the
least constrained parameters is the fraction of CT AGN. Statistically
acceptable fits to the XRB spectrum at the 68% confidence level can be obtained
for CT fractions in the range 5-50%. This is because of degeneracies among
input parameters to the XRB synthesis code and uncertainties in the modeling of
AGN spectra (e.g. reflection). The most promising route for constraining the
fraction of CT AGN in the Universe is via the direct detection of those sources
in high energy (>10keV) surveys. It is shown that the observed fraction of CT
sources identified in the SWIFT/BAT survey, limits the intrinsic fraction of CT
AGN, at least at low redshift, to 10-20% (68% confidence level). We also make
predictions on the number density of CT sources that current and future X-ray
missions are expected to discover. Testing those predictions will constrain the
intrinsic fraction of CT AGN as a function of redshift.Comment: To appear in A&
Cold gas and star formation in a merging galaxy sequence
We explore the evolution of the cold gas and star-formation activity during
galaxy interactions, using a merging galaxy sequence comprising both pre- and
post-mergers. Data for this study come from the literature but supplemented by
new radio observations presented here. Firstly, we confirm that the
star-formation efficiency (SFE) increases close to nuclear coalescence. At
post-merger stages there is evidence that the SFE declines to values typical of
ellipticals. This trend can be attributed to M(H_2) depletion due to
interaction induced star-formation. However, there is significant scatter,
likely to arise from differences in the interaction details of individual
systems. Secondly, we find that the central molecular hydrogen surface density,
increases close to the final stages of the merging of the two nuclei. Such a
trend is also predicted by numerical simulations. Furthermore, there is
evidence for a decreasing fraction of cold gas mass from early interacting
systems to merger remnants, attributed to gas conversion into other forms. The
evolution of the total-radio to blue-band luminosity ratio, reflecting the
disk+nucleus star-formation activity, is also investigated. Although this ratio
is on average higher than that of isolated spirals, we find a marginal increase
along the merging sequence, attributed to the relative insensitivity of disk
star-formation to interactions. However, a similar result is also obtained for
the nuclear radio emission, although galaxy interactions are believed to
significantly affect the activity in the central galaxy regions. Finally, we
find that the FIR--radio flux ratio distribution of interacting galaxies is
consistent with star-formation being the main energising source.Comment: 18 pages, 17 figures, accepted for publication in MNRA
Keck Imaging of the Globular Cluster Systems in the Early--type Galaxies NGC 1052 and NGC 7332
The presence of two globular cluster subpopulations in early-type galaxies is
now the norm rather than the exception. Here we present two more examples for
which the host galaxy appears to have undergone a recent merger. Using
multi-colour Keck imaging of NGC 1052 and NGC 7332 we find evidence for a
bimodal globular cluster colour distribution in both galaxies, with roughly
equal numbers of blue and red globular clusters. The blue ones have similar
colours to those in the Milky Way halo and are thus probably very old and
metal-poor. If the red GC subpopulations are at least solar metallicity, then
stellar population models indicate young ages. We discuss the origin of
globular clusters within the framework of formation models. We conclude that
recent merger events in these two galaxies have had little effect on their
overall GC systems. We also derive globular cluster density profiles, global
specific frequencies and in the case of NGC 1052, radial colour gradients and
azimuthal distribution. In general these globular cluster properties are normal
for early-type galaxies.Comment: 11 pages, Latex, 15 figures, 2 tables, accepted by MNRA
XMM and Chandra measurements of the AGN intrinsic absorption: dependence on luminosity and redshift
We combine bright XMM data with the Chandra Deep Field South observations in
order to explore the behavior of the intrinsic AGN absorption, as a function of
redshift and luminosity.Our sample consists of 359 sources selected in the hard
2-8 keV band, spanning the flux range 6\times10^{-16}-$3\times10^{-13} erg s^-1
cm^-2 with a high rate of spectroscopic or photometric redshift completeness
(100 and 85 per cent respectively for the Chandra and XMM data. We derive the
column density values using X-ray spectral fits. We find that the fraction of
obscured AGN falls with increasing luminosity in agreement with previous
findings. The fraction of obscured AGN shows an apparent increase at high
redshifts (z>2). Simulations show that this effect can be most probably
attributed to the fact that at high redshifts the column densities are
overestimated.Comment: 14 pages, 9 figures, A&A accepte
Chandra and Spitzer observations of CDFS X-ray obscured QSOs
We present Chandra and Spitzer data of the 186, extragalactic, hard 2-10 keV X-ray selected sources, which lie in the central part of the Chandra Deep Field South (CDFS). For the vast majority of sources (99.5%) there is a spectroscopic or photometric redshift available. We classify 17 sources as X-ray obscured QSOs, according to strictly X-ray criteria, i.e. defined as having large hydrogen column densities (N_H>10^22 cgs) and luminosities (Lx>10^44 cgs). The surface density of X-ray obscured QSOs is ~210 sq. deg. We find 18 candidate Compton thick N_H>10^24 cgs sources, of which three have QSO luminosities (L_x>10^44 cgs). The X-ray obscured QSO comprise a mixed bag of objects, covering the redshift range z=1.3-4.3. Eight of these show narrow line optical spectra, two show no obscuration in their optical spectra presenting Broad Lines, while for the other seven there is only a photometric redshift available. About half of the X-ray obscured QSOs show high X-ray to optical flux ratios, X/O>1, and red colours, I-3.6>4. Combination of the X-ray with the mid-IR 8 or 24 micron flux can be used as an additional diagnostic to sift out the heavily obscured AGN. All X-ray selected QSOs present red mid-IR colours and can be easily separated among mid-IR sources, demonstrating that mid-IR selection provides a powerful tool for the detection of obscured QSOs
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