153 research outputs found
Contribution to the Three--Point Function of the Cosmic Microwave Background from the Rees--Sciama Effect
We compute the contribution to the three--point temperature correlation
function of the Cosmic Microwave Background coming from the non--linear
evolution of Gaussian initial perturbations, as described by the Rees--Sciama
(or integrated Sachs--Wolfe) effect. By expressing the collapsed three--point
function in terms of multipole amplitudes, we are able to calculate its
expectation value for any power spectrum and for any experimental setting on
large angular scales. We also give an analytical expression for the {\it rms}
collapsed three--point function arising from the cosmic variance of a Gaussian
fluctuation field. In the case of {\it COBE} DMR, we find that the predicted
signal is about three orders of magnitude below that expected from the cosmic
variance.Comment: uuencoded compressed postscript, 10 pages text, 2 pages figure
The Clustering of Lyman-break Galaxies
We calculate the statistical clustering of Lyman-break galaxies predicted in
a selection of currently-fashionable structure formation scenarios. These
models are all based on the cold dark matter model, but vary in the amount of
dark matter, the initial perturbation spectrum, the background cosmology and in
the presence or absence of a cosmological constant term. If Lyman-break
galaxies form as a result of hierarchical merging, the amplitude of clustering
depends quite sensitively on the minimum halo mass that can host such a galaxy.
Interpretation of the recent observations by Giavalisco et al. (1998) would
therefore be considerably clarified by a direct determination of the relevant
halo properties. For a typical halo mass around the
observations do not discriminate strongly between cosmological models, but if
the appropriate mass is larger, say (which seems
likely on theoretical grounds), then the data strongly favour models with a low
matter-density.Comment: 6 pages, Latex using MN style, 2 figures enclosed. Version accepted
for publication in MNRA
The galaxy velocity field and CDM models
It is generally accepted that some kind of non-baryonic dark matter accounts for most of the mass density of the universe. Considering such a component has become, in the last decade, a key ingredient in current theories of structure formation. In particular, the Cold Dark Matter (CDM) scenario has proven to be quite successful in explaining most of the observed properties of galaxies and of their large-scale distribution. The standard CDM model is characterized by a primordial Zel'dovich spectrum, of random-phase adiabatic perturbations in a universe with density parameter omega sub 0 = 1 and vanishing cosmological constant. This poster paper presents an analysis of observational data on peculiar motion of optical galaxies in comparison to the predictions of CDM models where the assumptions of the standard scenario: omega sub 0 = 1, n = 1, and bias parameter b = 1 are relaxed. In particular, CDM models with 0 less than n less than 1 and 0.4 less than omega sub 0 less than 1 are considered
Excursion set approach to the clustering of dark matter haloes in Lagrangian space
We present a stochastic approach to the spatial clustering of dark matter
haloes in Lagrangian space. Our formalism is based on a local formulation of
the `excursion set' approach by Bond et al., which automatically accounts for
the `cloud-in-cloud' problem in the identification of bound systems. Our method
allows to calculate correlation functions of haloes in Lagrangian space using
either a multi-dimensional Fokker-Planck equation with suitable boundary
conditions or an array of Langevin equations with spatially correlated random
forces. We compare the results of our method with theoretical predictions for
the halo auto-correlation function considered in the literature and find good
agreement with the results recently obtained within a treatment of halo
clustering in terms of `counting fields' by Catelan et al.. The possible effect
of spatial correlations on numerical simulations of halo merger trees is
finally discussed.Comment: LaTeX, 19 pages, 3 figures. Submitted to MNRA
Eulerian Perturbation Theory in Non-Flat Universes: Second-Order Approximation
The problem of solving perturbatively the equations describing the evolution
of self-gravitating collisionless matter in an expanding universe considerably
simplifies when directly formulated in terms of the gravitational and velocity
potentials: the problem can be solved {\it exactly}, rather than approximately,
even for cosmological models with arbitrary density parameter . The
Eulerian approach we present here allows to calculate the higher-order moments
of the initially Gaussian density and velocity fields: in particular, we
compute the gravitationally induced skewness of the density and
velocity-divergence fields for any value of , confirming the extremely
weak -dependence of the skewness previously obtained via Lagrangian
perturbation theory. Our results show that the separability assumption of
higher-order Eulerian perturbative solutions is restricted to the Einstein-de
Sitter case only.Comment: 17 pages, Latex (mn.sty), 1 figure, revised version (1 figure is
dropped; eq.46 is corrected and some consequent results are re-discussed), to
be published in Monthly Notices of the Royal Astronomical Societ
ASIC development for SiPM readout
The design of CMOS front-end electronics suitable for Silicon Photo-Multipliers (SiPM) is described in this paper, starting with the specification of an accurate electrical model of the detector and its experimental validation. A novel current-mode solution is proposed for the preamplifier and the discriminator, to cope with the large dynamic range and the extremely fast rise time of the detector signal. Experimental results achieved from front-end prototypes designed according to this current-mode approach demonstrate its effectiveness: dynamic range of the order of 50 pC and timing accuracy of the electronics alone of about 30 ps have been measured
Constraints on primordial non-Gaussianity from WMAP7 and Luminous Red Galaxies power spectrum and forecast for future surveys
We place new constraints on the primordial local non-Gaussianity parameter
f_NL using recent Cosmic Microwave Background anisotropy and galaxy clustering
data. We model the galaxy power spectrum according to the halo model,
accounting for a scale dependent bias correction proportional to f_NL/k^2. We
first constrain f_NL in a full 13 parameters analysis that includes 5
parameters of the halo model and 7 cosmological parameters. Using the WMAP7 CMB
data and the SDSS DR4 galaxy power spectrum, we find f_NL=171\pm+140 at 68%
C.L. and -69<f_NL<+492 at 95% C.L.. We discuss the degeneracies between f_NL
and other cosmological parameters. Including SN-Ia data and priors on H_0 from
Hubble Space Telescope observations we find a stronger bound: -35<f_NL<+479 at
95% C.L.. We also fit the more recent SDSS DR7 halo power spectrum data
finding, for a \Lambda-CDM+f_NL model, f_NL=-93\pm128 at 68% C.L. and
-327<f_{NL}<+177 at 95% C.L.. We finally forecast the constraints on f_NL from
future surveys as EUCLID and from CMB missions as Planck showing that their
combined analysis could detect f_NL\sim 5.Comment: 10 pages, 5 figures, 3 table
A new model for the full shape of the large-scale power spectrum
We present a new model for the full shape of large-scale the power spectrum
based on renormalized perturbation theory. To test the validity of this
prescription, we compare this model against power spectra measured in a suite
of 50 large volume, moderate resolution N-body simulations. Our results
indicate that this simple model provides an accurate description of the full
shape of the power spectrum taking into account the effects of non-linear
evolution, redshift-space distortions and halo bias for scales k < 0.15 h/Mpc,
making it a valuable tool for the analysis of forthcoming galaxy surveys. Even
though its application is restricted to large scales, this prescription can
provide tighter constraints on the dark energy equation of state parameter
w_{DE} than those obtained by modelling the baryonic acoustic oscillations
signal only, where the information of the broad-band shape of the power
spectrum is discarded. Our model is able to provide constraints comparable to
those obtained by applying a similar model to the full shape of the correlation
function, which is affected by different systematics. Hence, with accurate
modelling of the power spectrum, the same cosmological information can be
extracted from both statistics.Comment: Accepted for publication in MNRA
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