35 research outputs found
Kurtosis and Large--Scale Structure
We discuss the non--linear growth of the excess kurtosis parameter of the
smoothed density fluctuation field ,
S_4\equiv[\lan\delta^{\,4}\ran-3\lan\delta^{\,2}\ran^2]/
\lan\delta^{\,2}\ran^3 in an Einstein--de Sitter universe. We assume Gaussian
primordial density fluctuations with scale--free power spectrum and analyze the dependence of on primordial spectral index ,
after smoothing with a Gaussian filter. As already known for the skewness ratio
, the kurtosis parameter is a {\it decreasing function} of , both in
exact perturbative theory and in the Zel'dovich approximation. The parameter
provides a powerful statistics to test different cosmological scenarios.Comment: 11 pages in Latex (plus 1 figure), SISSA 127/93/
Velocity Differences as a Probe of Non--Gaussian Density Fields
We examine the multi--point velocity field for non--Gaussian models as a
probe of non--Gaussian behavior. The two--point velocity correlation is not a
useful indicator of a non--Gaussian density field, since it depends only on the
power spectrum, even for non--Gaussian models. However, we show that the
distribution of velocity differences \bfv_1 - \bfv_2, where \bfv_1 and
\bfv_2 are measured at the points \bfr_1 and \bfr_2, respectively, is a
good probe of non--Gaussian behavior, in that p(\bfv_1 - \bfv_2) tends to be
non--Gaussian whenever the density field is non--Gaussian. As an example, we
examine the behavior of p(\bfv_1 - \bfv_2) for non--Gaussian seed models, in
which the density field is the convolution of a distribution of points with a
set of density profiles. We apply these results to the global texture model.Comment: 18 pages, LATEX style, SISSA-37-94-A, OSU-TA-4-9
Intrinsic and Extrinsic Galaxy Alignment
We show with analytic models that the assumption of uncorrelated intrinsic
ellipticities of target sources that is usually made in searches for weak
gravitational lensing due to large-scale mass inhomogeneities (``field
lensing'') is unwarranted. If the orientation of the galaxy image is determined
either by the angular momentum or the shape of the halo in which it forms, then
the image should be aligned preferentially with the component of the tidal
gravitational field perpendicular to the line of sight. Long-range correlations
in the tidal field will thus lead to long-range ellipticity-ellipticity
correlations that mimic the shear correlations due to weak gravitational
lensing. We calculate the ellipticity-ellipticity correlation expected if halo
shapes determine the observed galaxy shape, and we discuss uncertainties (which
are still considerable) in the predicted amplitude of this correlation. The
ellipticity-ellipticity correlation induced by angular momenta should be
smaller. We consider several methods for discriminating between the
weak-lensing (extrinsic) and intrinsic correlations, including the use of
redshift information. An ellipticity--tidal-field correlation also implies the
existence of an alignment of images of galaxies near clusters. Although the
intrinsic alignment may complicate the interpretation of field-lensing results,
it is inherently interesting as it may shed light on galaxy formation as well
as on structure formation.Comment: 7 pages, submitted to MNRA
Correlations of Cosmic Tidal Fields
We study correlations amongst tidal fields originated by the large-scale
distribution of matter in the Universe. The two-point tidal correlation is
described as a rank-4 tensor, whose elements can be written in terms of four
fundamental scalar functions ranging, with respect the spatial separation, from
purely transversal to purely longitudinal correlations. Tidal fields, both on
galaxy and cluster scales, reveal to be correlated over distances larger than
the mass-density correlation lenght, though traceless tidal fields show
anti-correlation between diagonal terms along orthogonal directions. The
cross-correlation between mass and tidal field is also analyzed. These results
are relevant for galaxy formation and the interpretation of large-scale weak
lensing phenomena.Comment: 6 pgs, 4 figs, using amssym.sty and mn.sty - MNRAS (in press
On cold diluted plasmas hit by short laser pulses
Adapting a plane hydrodynamical model we briefly revisit the study of the
impact of a very short and intense laser pulse onto a diluted plasma, the
formation of a plasma wave, its wave-breaking, the occurrence of the slingshot
effect.Comment: Latex file, 7 pages, 3 figures, now with higher resolution. To appear
in NIMA. Proceedings of Proceedings of the "3rd European Advanced Accelerator
Concepts (EAAC) Workshop", September 2017, Isola d'Elb
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
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
On the spatial distribution of dark matter halos
We study the spatial distribution of dark matter halos in the Universe in
terms of their number density contrast, related to the underlying dark matter
fluctuation via a non-local and non-linear bias random field. The description
of the matter dynamics is simplified by adopting the `truncated' Zel'dovich
approximation to obtain both analytical results and simulated maps. The halo
number density field in our maps and its probability distribution reproduce
with excellent accuracy those of halos in a high-resolution N-body simulation
with the same initial conditions. Our non-linear and non-local bias
prescription matches the N-body halo distribution better than any Eulerian
linear and local bias.Comment: 4 pages, LaTeX (uses emulateapj; included psfig.tex), 3 figures, 1
table. Shortened version, matching the size requirements of ApJ Letters.
Accepted for publicatio
Towards a working model for the abundance variations within Globular Clusters stars
A popular self--enrichment scenario for the formation of globular clusters
assumes that the abundance anomalies shown by the stars in many clusters are
due to a second stage of star formation occurring from the matter lost by the
winds of massive asymptotic giant branch (AGB) stars. Until today, the
modellizations of the AGB evolution by several different groups failed, for
different reasons, to account for the patterns of chemical anomalies. Here we
show that our own modelling can provide a consistent picture if we constrain
the three main parameters which regulate AGB evolution: 1) adopting a high
efficiency convection model; 2) adopting rates of mass loss with a high
dependence on the stellar luminosity; 3) assuming a very small overshooting
below the formal convective regions during the thermal pulse (TP) phase. The
first assumption is needed to obtain an efficient oxygen depletion in the AGB
envelopes, and the second one is needed to lose the whole stellar envelope
within few thermal pulses, so that the sum of CNO elements does not increase
too much, consistently with the observations. The third assumption is needed to
fully understand the sodium production. We also show that the Mg - Al
anticorrelation is explained adopting the higher limit of the NACRE rates for
proton captures by Mg25 and Mg26, and the models are consistent with the
recently discovered F-Al correlation. Problems remain to fully explain the
observed Mg isotopes ratios