3,224 research outputs found
Probing the Mass Distribution with IRAS Galaxies
We present the results of three independent analyses in which we show that
IRAS galaxies faithfully trace the underlying distribution the mass in the
local universe. In the first analysis we compare the mass and the galaxy
density fields and show that they are consistent within 60 Mpc. In the
second one we show that the tidal velocity field of the Mark III catalog is
consistent with the tidal velocity field predicted from the distribution of
IRAS galaxies, hence indicating that IRAS galaxies trace the mass distribution
well beyond 60 Mpc. Finally, a third analysis aimed at determining the
mean biasing relation of IRAS galaxies showed no appreciable deviations from
the linear biasing prescription.Comment: 5 pages including 4 Figures + 1 LaTex macro. Contribution to
``Where's the Matter? Tracing Dark and Bright Matter with the New Generation
of Large Scale Surveys'', June 2001, Treyer & Tresse Eds, Frontier Grou
Testing the Least Action Principle in an Omega_0=1 Universe
The least action principle (LAP) is a dynamically rigorous method for
deriving the history of galaxy orbits. In particular it is an Omega_0 test,
predicting current epoch galaxy velocities as a function of position and of the
cosmological background. It is most usefully applied to in--falling structures,
such as the local group, where its application indicates that the preferred
cosmological model is Omega_0 = 0.1 and h=0.75 (h is the Hubble parameter in
units of 100 Km s^-1 Mpc^-1). The method assumes that all the mass acts as if
it were distributed as the visible galaxies. We test the reliability of the LAP
to Local Group-like systems extracted from Omega_0=1 N--body simulations. While
the orbits of the galaxies are qualitatively well reconstructed, the LAP
systematically underestimates the mass of the system. This failure is
attributed to the presence of extended halos weakly clustered around visible
galaxies which prevent a large fraction of the group mass from being detected
by the LAP technique. We conclude that the LAP method cannot rule out an
Omega_0=1 value on the Local Group scale. Better constraints on Omega_0 may be
obtained by applying this technique to in--falling systems, such as clusters,
containing objects with separations large compared to galaxy sizes.Comment: accepted by APJ, uuencoded-compressed-tarred PostScript file
including figures. SISSA Ref. 56/94/
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
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
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
The cosmological co-evolution of supermassive black holes, AGN and galaxies
We model the cosmological co-evolution of galaxies and their central
supermassive black holes (BHs) within a semi-analytical framework developed on
the outputs of the Millennium Simulation (Croton et al., 2006; De Lucia &
Blaizot, 2007). In this work, we analyze the model BH scaling relations,
fundamental plane and mass function, and compare them with the most recent
observational data. Furthermore, we extend the original code developed by
Croton et al. (2006) to follow the evolution of the BH mass accretion and its
conversion into radiation, and compare the derived AGN bolometric luminosity
function with the observed one. We find, for the most part, a very good
agreement between predicted and observed BH properties. Moreover, the model is
in good agreement with the observed AGN number density in 0<z<5, provided it is
assumed that the cold gas fraction accreted by BHs at high redshifts is larger
than at low redshifts (Marulli et al., 2008).Comment: Proceedings of "The Central Kiloparsec: Active Galactic Nuclei and
Their Hosts", Ierapetra, Crete, 4-6 June, 2008. To appear in Volume 79 of the
Memorie della Societa' Astronomica Italiana. 5 pages, 4 figure
Euclid:Impact of non-linear and baryonic feedback prescriptions on cosmological parameter estimation from weak lensing cosmic shear
Upcoming surveys will map the growth of large-scale structure with unprecented precision, improving our understanding of the dark sector of the Universe. Unfortunately, much of the cosmological information is encoded on small scales, where the clustering of dark matter and the effects of astrophysical feedback processes are not fully understood. This can bias the estimates of cosmological parameters, which we study here for a joint analysis of mock Euclid cosmic shear and Planck cosmic microwave background data. We use different implementations for the modelling of the signal on small scales and find that they result in significantly different predictions. Moreover, the different non-linear corrections lead to biased parameter estimates, especially when the analysis is extended into the highly non-linear regime, with the Hubble constant, H0, and the clustering amplitude, Ï8, affected the most. Improvements in the modelling of non-linear scales will therefore be needed if we are to resolve the current tension with more and better data. For a given prescription for the non-linear power spectrum, using different corrections for baryon physics does not significantly impact the precision of Euclid, but neglecting these correction does lead to large biases in the cosmological parameters. In order to extract precise and unbiased constraints on cosmological parameters from Euclid cosmic shear data, it is therefore essential to improve the accuracy of the recipes that account for non-linear structure formation, as well as the modelling of the impact of astrophysical processes that redistribute the baryons. This paper is published on behalf of the Euclid Consortium
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
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