2,394 research outputs found
The large scale clustering of radio sources
The observed two-point angular correlation function, w(theta), of mJy radio
sources exhibits the puzzling feature of a power-law behaviour up to very large
(almost 10 degrees) angular scales which cannot be accounted for in the
standard hierarchical clustering scenario for any realistic redshift
distribution of such sources. After having discarded the possibility that the
signal can be explained by a high density local source population, we find no
alternatives to assuming that - at variance with all the other extragalactic
populations studied so far, and in particular with optically selected quasars -
radio sources responsible for the large-scale clustering signal were
increasingly less clustered with increasing look-back time, up to at least z=1.
The data are accurately accounted for in terms of a bias function which
decreases with increasing redshift, mirroring the evolution with cosmic time of
the characteristic halo mass, M_{star}, entering the non linear regime. In the
framework of the `concordance cosmology', the effective halo mass controlling
the bias parameter is found to decrease from about 10^{15} M_{sun}/h at z=0 to
the value appropriate for optically selected quasars, 10^{13} M_{sun}/h, at
z=1.5. This suggests that, in the redshift range probed by the data, the
clustering evolution of radio sources is ruled by the growth of large-scale
structure, and that they are associated with the densest environments
virializing at any cosmic epoch. The data provide only loose constraints on
radio source clustering at z>1 so we cannot rule out the possibility that at
these redshifts the clustering evolution of radio sources enters a different
regime, perhaps similar to that found for optically selected quasars. The
dependence of w(theta) on cosmological parameters is also discussed.Comment: 9 pages, 8 figures. Accepted for publication on MNRA
A dark matter interpretation for the ARCADE excess?
The ARCADE 2 Collaboration has recently measured an isotropic radio emission
which is significantly brighter than the expected contributions from known
extra-galactic sources. The simplest explanation of such excess involves a
"new" population of unresolved sources which become the most numerous at very
low (observationally unreached) brightness. We investigate this scenario in
terms of synchrotron radiation induced by WIMP annihilations or decays in
extragalactic halos. Intriguingly, for light-mass WIMPs with thermal
annihilation cross-section, and fairly conservative clustering assumptions, the
level of expected radio emission matches the ARCADE observations.Comment: 5 pages, 3 figures. v2: one benchmark model added, comments and
references expanded, to appear in PR
Average fractional polarization of extragalactic sources at Planck frequencies
Recent detailed simulations have shown that an insufficiently accurate
characterization of the contamination of unresolved polarized extragalactic
sources can seriously bias measurements of the primordial cosmic microwave
background (CMB) power spectrum if the tensor-to-scalar ratio as
predicted by models currently of special interest (e.g., Starobinsky's
and Higgs inflation). This has motivated a reanalysis of the median
polarization fraction of extragalactic sources (radio-loud AGNs and dusty
galaxies) using data from the \textit{Planck} polarization maps. Our approach,
exploiting the intensity distribution analysis, mitigates or overcomes the most
delicate aspects of earlier analyses based on stacking techniques. By means of
simulations, we have shown that the residual noise bias on the median
polarization fraction, , of extragalactic sources is
generally \simlt 0.1\%. For radio sources, we have found , with no significant dependence on either frequency or flux
density, in good agreement with the earlier estimate and with high-sensitivity
measurements in the frequency range 5--40\,GHz. No polarization signal is
detected in the case of dusty galaxies, implying 90\% confidence upper limits
of \Pi_{\rm dusty}\simlt 2.2\% at 353\,GHz and of \simlt 3.9\% at 217\,GHz.
The contamination of CMB polarization maps by unresolved point sources is
discussed.Comment: 10 pages, 3 figures, 7 tables; revised version. In press on Astronomy
and Astrophysic
A reassessment of the evidence of the Integrated Sachs-Wolfe effect through the WMAP-NVSS correlation
We reassess the estimate of the cross-correlation of the spatial distribution
of the NRAO VLA Sky Survey (NVSS) radio sources with that of Cosmic Microwave
Background (CMB) anisotropies from the Wilkinson Microwave Anisotropy Probe
(WMAP). This re-analysis is motivated by the fact that most previous studies
adopted a redshift distribution of NVSS sources inconsistent with recent data.
We find that the constraints on the bias-weighted redshift distribution,
b(z)xN(z), of NVSS sources, set by the observed angular correlation function,
w(theta), strongly mitigate the effect of the choice of N(z). If such
constraints are met, even highly discrepant redshift distributions yield
NVSS-WMAP cross-correlation functions consistent with each other within
statistical errors. The models favoured by recent data imply a bias factor,
b(z), decreasing with increasing z, rather than constant, as assumed by most
previous analyses. As a consequence, the function b(z)xN(z) has more weight at
z<1, i.e. in the redshift range yielding the maximum contribution to the ISW in
a standard LambdaCDM cosmology. On the whole, the NVSS turns out to be better
suited for ISW studies than generally believed, even in the absence of an
observational determination of the redshift distribution. The NVSS-WMAP
cross-correlation function is found to be fully consistent with the prediction
of the standard LambdaCDM cosmology.Comment: 6 pages, 2 figures, submitted to MNRA
Angular Correlations of the X-Ray Background and Clustering of Extragalactic X-Ray Sources
The information content of the autocorrelation function (ACF) of intensity
fluctuations of the X-ray background (XRB) is analyzed. The tight upper limits
set by ROSAT deep survey data on the ACF at arcmin scales imply strong
constraints on clustering properties of X-ray sources at cosmological distances
and on their contribution to the soft XRB. If quasars have a clustering radius
r_0=12-20 Mpc (H_0=50), and their two point correlation function, is constant
in comoving coordinates as indicated by optical data, they cannot make up more
40-50% of the soft XRB (the maximum contribution may reach 80% in the case of
stable clustering, epsilon=0). Active Star-forming (ASF) galaxies clustered
like normal galaxies, with r_0=10-12 Mpc can yield up to 20% or up to 40% of
the soft XRB for epsilon=-1.2 or epsilon=0, respectively. The ACF on degree
scales essentially reflects the clustering properties of local sources and is
proportional to their volume emissivity. The upper limits on scales of a few
degrees imply that hard X-ray selected AGNs have r_0<25 Mpc if epsilon=0 or
r_0<20 Mpc if epsilon=-1.2. No significant constraints are set on clustering of
ASF galaxies, due to their low local volume emissivity. The possible signal on
scales >6 deg, if real, may be due to AGNs with r_0=20 Mpc; the contribution
from clusters of galaxies with r_0~50 Mpc is a factor 2 lower.Comment: ApJ, in press (20 July 1993); 28 pages, TeX, ASTRPD-93-2-0
Polarization Properties of Extragalactic Radio Sources and Their Contribution to Microwave Polarization Fluctuations
We investigate the statistical properties of the polarized emission of
extragalactic radio sources and estimate their contribution to the power
spectrum of polarization fluctuations in the microwave region. The basic
ingredients of our analysis are the NVSS polarization data, the multifrequency
study of polarization properties of the B3-VLA sample (Mack et al. 2002) which
has allowed us to quantify Faraday depolarization effects, and the 15 GHz
survey by Taylor et al. (2001), which has provided strong constraints on the
high-frequency spectral indices of sources. The polarization degree of both
steep- and flat-spectrum at 1.4 GHz is found to be anti-correlated with the
flux density. The median polarization degree at 1.4 GHz of both steep- and
flat-spectrum sources brighter than mJy is . The data by Mack et al. (2002) indicate a substantial mean Faraday
depolarization at 1.4 GHz for steep spectrum sources, while the depolarization
is undetermined for most flat/inverted-spectrum sources. Exploiting this
complex of information we have estimated the power spectrum of polarization
fluctuations due to extragalactic radio sources at microwave frequencies. We
confirm that extragalactic sources are expected to be the main contaminant of
Cosmic Microwave Background (CMB) polarization maps on small angular scales. At
frequencies GHz the amplitude of their power spectrum is expected to be
comparable to that of the -mode of the CMB. At higher frequencies, however,
the CMB dominates.Comment: 10 pages, A&A in pres
A Physical Model for Co-evolution of QSOs and of their Spheroidal Hosts
At variance with most semi-analytic models, in the Anti-hierarchical Baryon
Collapse scenario (Granato et al. 2001, 2004) the main driver of the galaxy
formation and evolution is not the merging sequence but are baryon processes.
This approach emphasizes, still in the framework of the hierarchical clustering
paradigm for dark matter halos, feedback processes from supernova explosions
and from active nuclei, that tie together star formation in spheroidal galaxies
and the growth of black holes at their centers. We review some recent results
showing the remarkably successful predictive power of this scenario, which
allows us to account for the evolution with cosmic time of a broad variety of
properties of galaxies and active nuclei, which proved to be very challenging
for competing models.Comment: Invited talk at the Specola Vaticana Workshop on "AGN and Galaxy
Evolution", Castel Gandolfo, 3-6 October 2005, 10 pages, 2 figure
From First Galaxies to QSOs: feeding the baby monsters
We present a physical model for the coevolution of massive spheroidal
galaxies and active nuclei at their centers. Supernova heating is increasingly
effective in slowing down the star formation and in driving gas outflows in
smaller and smaller dark matter halos. Thus the more massive protogalaxies
virializing at early times are the sites of faster star formation. The
correspondingly higher radiation drag causes a faster angular momentum loss by
the gas and induces a larger accretion rate onto the central black hole. In
turn, the kinetic energy of the outflows powered by the active nuclei can
unbind the residual gas in a time shorter for larger halos. The model accounts
for a broad variety of dynamical, photometric and metallicity properties of
early-type galaxies, for the M_BH -- \sigma relation and for the local
supermassive black-hole mass function.Comment: 6 pages, contributed paper to Proceedings of the Conference on
"Growing Black Holes" held in Garching, Germany, on June 21-25, 2004, edited
by A. Merloni, S. Nayakshin and R. Sunyaev, Springer-Verlag series of "ESO
Astrophysics Symposia
Astrophysical and Cosmological Information from Large-scale sub-mm Surveys of Extragalactic Sources
We present a quantitative analysis of the astrophysical and cosmological
information that can be extracted from the many important wide-area, shallow
surveys that will be carried out in the next few years. Our calculations
combine the predictions of the physical model by Granato et al. (2004) for the
formation and evolution of spheroidal galaxies with up-to-date phenomenological
models for the evolution of starburst and normal late-type galaxies and of
radio sources. We compute the expected number counts and the redshift
distributions of these source populations separately and then focus on
proto-spheroidal galaxies. For the latter objects we predict the counts and
redshift distributions of strongly lensed sources at 250, 350, 500, and 850
micron, the angular correlation function of sources detected in the surveys
considered, the angular power spectra due to clustering of sources below the
detection limit in Herschel and Planck surveys. An optimal survey for selecting
strongly lensed proto-spheroidal galaxies is described, and it is shown how
they can be easily distinguished from the other source populations. We also
discuss the detectability of the imprints of the 1-halo and 2-halo regimes on
angular correlation functions and clustering power spectra, as well as the
constraints on cosmological parameters that can be obtained from the
determinations of these quantities. The novel data relevant to derive the first
sub-millimeter estimates of the local luminosity functions of starburst and
late-type galaxies, and the constraints on the properties of rare source
populations, such as blazars, are also briefly described.Comment: 16 pages, 10 figures. Accepted for publication on MNRA
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