950 research outputs found
Generating Log-normal Mock Catalog of Galaxies in Redshift Space
We present a public code to generate a mock galaxy catalog in redshift space
assuming a log-normal probability density function (PDF) of galaxy and matter
density fields. We draw galaxies by Poisson-sampling the log-normal field, and
calculate the velocity field from the linearised continuity equation of matter
fields, assuming zero vorticity. This procedure yields a PDF of the pairwise
velocity fields that is qualitatively similar to that of N-body simulations. We
check fidelity of the catalog, showing that the measured two-point correlation
function and power spectrum in real space agree with the input precisely. We
find that a linear bias relation in the power spectrum does not guarantee a
linear bias relation in the density contrasts, leading to a cross-correlation
coefficient of matter and galaxies deviating from unity on small scales. We
also find that linearising the Jacobian of the real-to-redshift space mapping
provides a poor model for the two-point statistics in redshift space. That is,
non-linear redshift-space distortion is dominated by non-linearity in the
Jacobian. The power spectrum in redshift space shows a damping on small scales
that is qualitatively similar to that of the well-known Fingers-of-God (FoG)
effect due to random velocities, except that the log-normal mock does not
include random velocities. This damping is a consequence of non-linearity in
the Jacobian, and thus attributing the damping of the power spectrum solely to
FoG, as commonly done in the literature, is misleading.Comment: 38 pages, 16 figures, code publicly available as "lognormal_galaxies"
at http://wwwmpa.mpa-garching.mpg.de/~komatsu/codes.html Matches published
version : added figures and explanatory comment
Sparse component separation for accurate CMB map estimation
The Cosmological Microwave Background (CMB) is of premier importance for the
cosmologists to study the birth of our universe. Unfortunately, most CMB
experiments such as COBE, WMAP or Planck do not provide a direct measure of the
cosmological signal; CMB is mixed up with galactic foregrounds and point
sources. For the sake of scientific exploitation, measuring the CMB requires
extracting several different astrophysical components (CMB, Sunyaev-Zel'dovich
clusters, galactic dust) form multi-wavelength observations. Mathematically
speaking, the problem of disentangling the CMB map from the galactic
foregrounds amounts to a component or source separation problem. In the field
of CMB studies, a very large range of source separation methods have been
applied which all differ from each other in the way they model the data and the
criteria they rely on to separate components. Two main difficulties are i) the
instrument's beam varies across frequencies and ii) the emission laws of most
astrophysical components vary across pixels. This paper aims at introducing a
very accurate modeling of CMB data, based on sparsity, accounting for beams
variability across frequencies as well as spatial variations of the components'
spectral characteristics. Based on this new sparse modeling of the data, a
sparsity-based component separation method coined Local-Generalized
Morphological Component Analysis (L-GMCA) is described. Extensive numerical
experiments have been carried out with simulated Planck data. These experiments
show the high efficiency of the proposed component separation methods to
estimate a clean CMB map with a very low foreground contamination, which makes
L-GMCA of prime interest for CMB studies.Comment: submitted to A&
Higher order moments of the density field in a parameterized sequence of non-gaussian theories
We calculate the higher order moments in a sequence of models where the
initial density fluctuations are drawn from a chi^2_nu distribution with a
power-law power spectrum. For large values of nu the distribution is
approximately gaussian, and we reproduce the values known from perturbation
theory. As \nu is lowered the distribution becomes progressively more
non-gaussian, approximating models with rare, high-amplitude peaks. The limit
nu=1 is a realization of recently proposed isocurvature models for producing
early galaxy formation where the density perturbations are quadratic in a
gaussian field.Comment: 7 pages, 7 figures, to appear in MNRA
The mass density field in simulated non-Gaussian scenarios
In this work we study the properties of the mass density field in the
non-Gaussian world models simulated by Grossi et al. 2007. In particular we
focus on the one-point density probability distribution function of the mass
density field in non-Gausian models with quadratic non-linearities quantified
by the usual parameter f_NL. We find that the imprints of primordial
non-Gaussianity are well preserved in the negative tail of the probability
function during the evolution of the density perturbation. The effect is
already noticeable at redshifts as large as 4 and can be detected out to the
present epoch. At z=0 we find that the fraction of the volume occupied by
regions with underdensity delta < -0.9, typical of voids, is about 1.3 per cent
in the Gaussian case and increases to ~2.2 per cent if f_NL=-1000 while
decreases to ~0.5 per cent if f_NL=+1000. This result suggests that void-based
statistics may provide a powerful method to detect non-Gaussianity even at low
redshifts which is complementary to the measurements of the higher-order
moments of the probability distribution function like the skewness or the
kurtosis for which deviations from the Gaussian case are detected at the 25-50
per cent level.Comment: revised version, 9 Pages, 8 figures, MNRAS in pres
Non-Gaussianity from Inflation: Theory and Observations
This is a review of models of inflation and of their predictions for the
primordial non-Gaussianity in the density perturbations which are thought to be
at the origin of structures in the Universe. Non-Gaussianity emerges as a key
observable to discriminate among competing scenarios for the generation of
cosmological perturbations and is one of the primary targets of present and
future Cosmic Microwave Background satellite missions. We give a detailed
presentation of the state-of-the-art of the subject of non-Gaussianity, both
from the theoretical and the observational point of view, and provide all the
tools necessary to compute at second order in perturbation theory the level of
non-Gaussianity in any model of cosmological perturbations. We discuss the new
wave of models of inflation, which are firmly rooted in modern particle physics
theory and predict a significant amount of non-Gaussianity. The review is
addressed to both astrophysicists and particle physicists and contains useful
tables which summarize the theoretical and observational results regarding
non-Gaussianity.Comment: LaTeX file: 218 pages, 19 figures. Replaced to match the accepted
version in Physics Reports. A high-resolution version of Fig. 2 can be
downloaded from: http://www.pd.infn.it/~liguori/Non_Gaussianity
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