3,427 research outputs found
Cluster versus POTENT Density and Velocity Fields: Cluster Biasing and Omega
The density and velocity fields as extracted from the Abell/ACO clusters are
compared to the corresponding fields recovered by the POTENT method from the
Mark~III peculiar velocities of galaxies. In order to minimize non-linear
effects and to deal with ill-sampled regions we smooth both fields using a
Gaussian window with radii ranging between 12 - 20\hmpc. The density and
velocity fields within 70\hmpc exhibit similarities, qualitatively consistent
with gravitational instability theory and a linear biasing relation between
clusters and mass. The random and systematic errors are evaluated with the help
of mock catalogs. Quantitative comparisons within a volume containing
independent samples yield
\betac\equiv\Omega^{0.6}/b_c=0.22\pm0.08, where is the cluster biasing
parameter at 15\hmpc. If , as indicated by the cluster
correlation function, our result is consistent with .Comment: 18 pages, latex, 2 ps figures 6 gif figures. Accepted for
pubblications in MNRA
Anisotropy probe of galactic and extra-galactic Dark Matter annihilations
We study the flux and the angular power spectrum of gamma-rays produced by
Dark Matter (DM) annihilations in the Milky Way (MW) and in extra-galactic
halos. The annihilation signal receives contributions from: a) the smooth MW
halo, b) resolved and unresolved substructures in the MW, c) external DM halos
at all redshifts, including d) their substructures. Adopting a self-consistent
description of local and extra-galactic substructures, we show that the
annihilation flux from substructures in the MW dominates over all the other
components for angles larger than O(1) degrees from the Galactic Center, unless
an extreme prescription is adopted for the substructures concentration. We also
compute the angular power spectrum of gamma-ray anisotropies and find that, for
an optimistic choice of the particle physics parameters, an interesting
signature of DM annihilations could soon be discovered by the Fermi LAT
satellite at low multipoles, l<100, where the dominant contribution comes from
MW substructures with mass M>10^4 solar masses. For the substructures models we
have adopted, we find that the contribution of extra-galactic annihilations is
instead negligible at all scales.Comment: 14 pages, 7 figure
Modeling the QSO luminosity and spatial clustering at low redshifts
We investigate the ability of hierarchical models of QSO formation and
evolution to match the observed luminosity, number counts and spatial
clustering of quasars at redshift z<2. These models assume that the QSO
emission is triggered by galaxy mergers, that the mass of the central black
hole correlates with halo properties and that quasars shine at their Eddington
luminosity except, perhaps, during the very early stages of evolution. We find
that models based on simple analytic approximations successfully reproduce the
observed B-band QSO luminosity function at all redshifts, provided that some
mechanisms is advocated to quench mass accretion within haloes larger than
about 1e13 Msun that host bright quasars. These models also match the observed
strength of QSO clustering at z~0.8. At larger redshifts, however, they
underpredict the QSO biasing which, instead, is correctly reproduced by
semi-analytic models in which the halo merger history and associated BHs are
followed by Monte Carlo realizations of the merger hierarchy. We show that the
disagreement between the luminosity function predicted by semi-analytic models
and observations can be ascribed to the use of B-band data, which are a biased
tracer of the quasar population, due to obscuration.Comment: 13 pages, 9 figures. Accepted by MNRA
Reconstructing Positions \& Peculiar Velocities of Galaxy Clusters within 25000 km/sec: The Bulk Velocity
Using a dynamical 3-D reconstruction procedure we estimate the peculiar
velocities of Abell/ACO galaxy clusters from their measured redshift
within 25000 km/sec. The reconstruction algorithm relies on the linear
gravitational instability hypothesis, assumes linear biasing and requires an
input value of the cluster -parameter (), which we estimated in Branchini \& Plionis (1995)
to be . The resulting cluster velocity field is dominated
by a large scale streaming motion along the Perseus Pisces--Great Attractor
base-line directed towards the Shapley concentration, in qualitative agreement
with the galaxy velocity field on smaller scales. Fitting the predicted cluster
peculiar velocities to a dipole term, in the local group frame and within a
distance of km/sec, we recover extremely well both the local group
velocity and direction, in disagreement with the Lauer \& Postman (1994)
observation. However, we find a probability that their observed
velocity field could be a realization of our corresponding one, if the latter
is convolved with their large distance dependent errors. Our predicted cluster
bulk velocity amplitude agrees well with that deduced by the POTENT and the da
Costa et al. (1995) analyses of observed galaxy motions at
km/sec; it decreases thereafter while at the Lauer \& Postman limiting depth
( km/sec) its amplitude is km/sec, in comfortable
agreement with most cosmological models.Comment: 8 pages, uuencoded compressed tarred postscript file uncluding text
and 3 figures. Accepted in ApJ Letter
A numerical study of the effects of primordial non-Gaussianities on weak lensing statistics
While usually cosmological initial conditions are assumed to be Gaussian,
inflationary theories can predict a certain amount of primordial
non-Gaussianity which can have an impact on the statistical properties of the
lensing observables. In order to evaluate this effect, we build a large set of
realistic maps of different lensing quantities starting from light-cones
extracted from large dark-matter only N-body simulations with initial
conditions corresponding to different levels of primordial local
non-Gaussianity strength . Considering various statistical
quantities (PDF, power spectrum, shear in aperture, skewness and bispectrum) we
find that the effect produced by the presence of primordial non-Gaussianity is
relatively small, being of the order of few per cent for values of compatible with the present CMB constraints and reaching at most 10-15
per cent for the most extreme cases with . We also discuss
the degeneracy of this effect with the uncertainties due to the power spectrum
normalization and matter density parameter , finding
that an error in the determination of () of about 3
(10) per cent gives differences comparable with non-Gaussian models having
. These results suggest that the possible presence of an
amount of primordial non-Gaussianity corresponding to is not
hampering a robust determination of the main cosmological parameters in present
and future weak lensing surveys, while a positive detection of deviations from
the Gaussian hypothesis is possible only breaking the degeneracy with other
cosmological parameters and using data from deep surveys covering a large
fraction of the sky.Comment: accepted by MNRA
Constraints on a scale-dependent bias from galaxy clustering
We forecast the future constraints on scale-dependent parametrizations of
galaxy bias and their impact on the estimate of cosmological parameters from
the power spectrum of galaxies measured in a spectroscopic redshift survey. For
the latter we assume a wide survey at relatively large redshifts, similar to
the planned Euclid survey, as baseline for future experiments. To assess the
impact of the bias we perform a Fisher matrix analysis and we adopt two
different parametrizations of scale-dependent bias. The fiducial models for
galaxy bias are calibrated using a mock catalogs of H emitting galaxies
mimicking the expected properties of the objects that will be targeted by the
Euclid survey.
In our analysis we have obtained two main results. First of all, allowing for
a scale-dependent bias does not significantly increase the errors on the other
cosmological parameters apart from the rms amplitude of density fluctuations,
, and the growth index , whose uncertainties increase by a
factor up to two, depending on the bias model adopted. Second, we find that the
accuracy in the linear bias parameter can be estimated to within 1-2\%
at various redshifts regardless of the fiducial model. The non-linear bias
parameters have significantly large errors that depend on the model adopted.
Despite of this, in the more realistic scenarios departures from the simple
linear bias prescription can be detected with a significance at
each redshift explored.
Finally, we use the Fisher Matrix formalism to assess the impact of assuming
an incorrect bias model and found that the systematic errors induced on the
cosmological parameters are similar or even larger than the statistical ones.Comment: new section added; conclusions unchanged; accepted for publication in
PR
Reconstructing Positions and Peculiar Velocities of Galaxy Clusters within 20000 km/sec. I: The Cluster 3D Dipole
Starting from the observed distribution of galaxy clusters in redshift space
we use a two--step procedure to recover their distances and peculiar
velocities. After statistically correcting for the unobserved cluster
distribution in the zone of avoidance () and also for a
smooth absorption at higher 's, we use a dynamical iterative algorithm to
recover the real--space cluster positions by minimizing the redshift space
distortions. The whole procedure assumes that clusters trace the mass, that
peculiar velocities are caused by gravity and that linear perturbation theory
applies. The amplitude of the cluster dipole measured in the 3D space turns out
to be less than that measured in redshift space. In both cases the
dipole direction is aligned with the Cosmic Microwave Background dipole within
, taking into account the Virgocentric infall component of the
Local Group motion. Observational errors, limitations in the reconstruction
procedure and the intrinsic cosmological variance, which is the dominant source
of uncertainty, render a stringent determination of the parameter whose
central value turns out to be while its total uncertainty
is . This implies that for a cluster-mass bias parameter of ,
a flat Universe is not excluded, contrary to previous cluster-dipole -space
analysis.Comment: uuencoded-compressed-tarred PostScript file containing 17 pages +
tables + 8 Figures
Detecting X-ray filaments in the low redshift Universe with XEUS and Constellation-X
We propose a possible way to detect baryons at low redshifts from the
analysis of X-ray absorption spectra of bright AGN pairs. A simple
semi-analytical model to simulate the spectra is presented. We model the
diffuse warm-hot intergalactic medium (WHIM) component, responsible for the
X-ray absorption, using inputs from high-resolution hydro-dynamical simulations
and analytical prescriptions. We show that the number of OVII absorbers per
unit redshift with column density larger than cm -
corresponding to an equivalent width of 1 km/s - which will be possibly
detectable by {\it XEUS}, is \magcir 30 per unit redshift. {\it
Constellation-X} will detect OVII absorptions per unit redshift with
an equivalent width of 10 km/s. Our results show that, in a CDM
Universe, the characteristic size of these absorbers at is
Mpc. The filamentary structure of WHIM can be probed by finding
coincident absorption lines in the spectra of background AGN pairs. We estimate
that at least 20 AGN pairs at separation \mincir 20 arcmin are needed to
detect this filamentary structure at a 3 level. Assuming observations
of distant sources using {\it XEUS} for exposure times of 500 ksec, we find
that the minimum source flux to probe the filamentary structure is erg cm s, in the 0.1-2.4 keV energy band. Thus,
most pairs of these extragalactic X-ray bright sources have already been
identified in the {\it ROSAT} All-Sky Survey. Re-observation of these objects
by future missions could be a powerful way to search for baryons in the low
redshift Universe.Comment: 18 pages, 10 Figures. Two figures added, Sections 2 and 3 expanded.
More optimistic results for Constellation-X. Accepted by MNRA
Density Contrast-Peculiar Velocity Relation in the Newtonian Gauge
In general relativistic framework of the large scale structure formation
theory in the universe, we investigate the relation between density contrast
and peculiar velocity in the Newtonian gauge. According to the gauge-invariant
property of the energy-momentum tensor in the Newtonian gauge, we consider the
perturbation of velocity in the energy-momentum tensor behaves as the Newtonian
peculiar velocity. It is shown that in the relativistic framework, the relation
between peculiar velocity and density contrast has an extra correction term
with respect to the Newtonian Peebles formula which in small scales, can be
ignorable . The relativistic correction of peculiar velocity for the structures
with the extension of few hundred mega parsec is about few percent which is
smaller than the accuracy of the recent observations for measuring peculiar
velocity. The peculiar velocity in the general relativistic framework also
changes the contribution of Doppler effect on the anisotropy of CMB.Comment: 9 pages, 1 figure, accepted in Int. J. Mod. Phys
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