73,839 research outputs found
Advancement in the understanding of multifragmentation and phase transition for hot nuclei
Recent advancement on the knowledge of multifragmentation and phase
transition for hot nuclei is reported. It concerns i) the influence of radial
collective energy on fragment partitions and the derivation of general
properties of partitions in presence of such a collective energy, ii) a better
knowledge of freeze-out properties obtained by means of a simulation based on
all the available experimental information and iii) the quantitative study of
the bimodal behaviour of the heaviest fragment charge distribution for
fragmenting hot heavy quasi-projectiles which allows, for the first time, to
estimate the latent heat of the phase transition.Comment: 9 pages, Proceedings of IWM09, November 4-7, Catania (Italy
Properties of gas clumps and gas clumping factor in the intra cluster medium
The spatial distribution of gas matter inside galaxy clusters is not
completely smooth, but may host gas clumps associated with substructures. These
overdense gas substructures are generally a source of unresolved bias of X-ray
observations towards high density gas, but their bright luminosity peaks may be
resolved sources within the ICM, that deep X-ray exposures may be (already)
capable to detect. In this paper we aim at investigating both features, using a
set of high-resolution cosmological simulations with ENZO. First, we monitor
how the bias by unresolved gas clumping may yield incorrect estimates of global
cluster parameters and affects the measurements of baryon fractions by X-ray
observations. We find that based on X-ray observations of narrow radial strips,
it is difficult to recover the real baryon fraction to better than 10 - 20
percent uncertainty. Second, we investigated the possibility of observing
bright X-ray clumps in the nearby Universe (z<=0.3). We produced simple mock
X-ray observations for several instruments (XMM, Suzaku and ROSAT) and
extracted the statistics of potentially detectable bright clumps. Some of the
brightest clumps predicted by simulations may already have been already
detected in X- ray images with a large field of view. However, their small
projected size makes it difficult to prove their existence based on X-ray
morphology only. Preheating, AGN feedback and cosmic rays are found to have
little impact on the statistical properties of gas clumps.Comment: 17 pages, 11 figures. MNRAS accepte
Seen and unseen tidal caustics in the Andromeda galaxy
Indirect detection of high-energy particles from dark matter interactions is
a promising avenue for learning more about dark matter, but is hampered by the
frequent coincidence of high-energy astrophysical sources of such particles
with putative high-density regions of dark matter. We calculate the boost
factor and gamma-ray flux from dark matter associated with two shell-like
caustics of luminous tidal debris recently discovered around the Andromeda
galaxy, under the assumption that dark matter is its own supersymmetric
antiparticle. These shell features could be a good candidate for indirect
detection of dark matter via gamma rays because they are located far from the
primary confusion sources at the galaxy's center, and because the shapes of the
shells indicate that most of the mass has piled up near apocenter. Using a
numerical estimator specifically calibrated to estimate densities in N-body
representations with sharp features and a previously determined N-body model of
the shells, we find that the largest boost factors do occur in the shells but
are only a few percent. We also find that the gamma-ray flux is an order of
magnitude too low to be detected with Fermi for likely dark matter parameters,
and about 2 orders of magnitude less than the signal that would have come from
the dwarf galaxy that produces the shells in the N-body model. We further show
that the radial density profiles and relative radial spacing of the shells, in
either dark or luminous matter, is relatively insensitive to the details of the
potential of the host galaxy but depends in a predictable way on the velocity
dispersion of the progenitor galaxy.Comment: ApJ accepte
The zCOSMOS Survey. The dependence of clustering on luminosity and stellar mass at z=0.2-1
We study the dependence of galaxy clustering on luminosity and stellar mass
at redshifts z ~ [0.2-1] using the first zCOSMOS 10K sample.
We measure the redshift-space correlation functions xi(rp,pi) and its
projection wp(rp) for sub-samples covering different luminosity, mass and
redshift ranges. We quantify in detail the observational selection biases and
we check our covariance and error estimate techniques using ensembles of
semi-analytic mock catalogues. We finally compare our measurements to the
cosmological model predictions from the mock surveys.
At odds with other measurements, we find a weak dependence of galaxy
clustering on luminosity in all redshift bins explored. A mild dependence on
stellar mass is instead observed. At z~0.7, wp(rp) shows strong excess power on
large scales. We interpret this as produced by large-scale structure dominating
the survey volume and extending preferentially in direction perpendicular to
the line-of-sight. We do not see any significant evolution with redshift of the
amplitude of clustering for bright and/or massive galaxies.
The clustering measured in the zCOSMOS data at 0.5<z<1 for galaxies with
log(M/M_\odot)>=10 is only marginally consistent with predictions from the mock
surveys. On scales larger than ~2 h^-1 Mpc, the observed clustering amplitude
is compatible only with ~1% of the mocks. Thus, if the power spectrum of matter
is LCDM with standard normalization and the bias has no unnatural
scale-dependence, this result indicates that COSMOS has picked up a
particularly rare, ~2-3 sigma positive fluctuation in a volume of ~10^6 h^-1
Mpc^3. These findings underline the need for larger surveys of the z~1 Universe
to appropriately characterize the level of structure at this epoch.Comment: 18 pages, 21 figures, accepted for publication in Astronomy and
Astrophysic
Sparse sampling, galaxy bias, and voids
To study the impact of sparsity and galaxy bias on void statistics, we use a
single large-volume, high-resolution N-body simulation to compare voids in
multiple levels of subsampled dark matter, halo populations, and mock galaxies
from a Halo Occupation Distribution model tuned to different galaxy survey
densities. We focus our comparison on three key observational statistics:
number functions, ellipticity distributions, and radial density profiles. We
use the hierarchical tree structure of voids to interpret the impacts of
sampling density and galaxy bias, and theoretical and empirical functions to
describe the statistics in all our sample populations. We are able to make
simple adjustments to theoretical expectations to offer prescriptions for
translating from analytics to the void properties measured in realistic
observations. We find that sampling density has a much larger effect on void
sizes than galaxy bias. At lower tracer density, small voids disappear and the
remaining voids are larger, more spherical, and have slightly steeper profiles.
When a proper lower mass threshold is chosen, voids in halo distributions
largely mimic those found in galaxy populations, except for ellipticities,
where galaxy bias leads to higher values. We use the void density profile of
Hamaus et al. (2014) to show that voids follow a self-similar and universal
trend, allowing simple translations between voids studied in dark matter and
voids identified in galaxy surveys. We have added the mock void catalogs used
in this work to the Public Cosmic Void Catalog at http://www.cosmicvoids.net.Comment: 11 pages, 7 figures, MNRAS accepted. Minor changes from previous
version. Public catalog available at http://www.cosmicvoids.ne
Systematic comparison of force fields for microscopic simulations of NaCl in aqueous solutions: Diffusion, free energy of hydration and structural properties
In this paper we compare different force fields that are widely used
(Gromacs, Charmm-22/x-Plor, Charmm-27, Amber-1999, OPLS-AA) in biophysical
simulations containing aqueous NaCl. We show that the uncertainties of the
microscopic parameters of, in particular, sodium and, to a lesser extent,
chloride translate into large differences in the computed radial-distribution
functions. This uncertainty reflects the incomplete experimental knowledge of
the structural properties of ionic aqueous solutions at finite molarity.We
discuss possible implications on the computation of potential of mean force and
effective potentials.Comment: Revised and extended manuscrip
Transient cavities and the excess chemical potentials of hard-spheroid solutes in dipolar hard sphere solvents
Monte Carlo computer simulations are used to study transient cavities and the
solvation of hard-spheroid solutes in dipolar hard sphere solvents. The
probability distribution of spheroidal cavities in the solvent is shown to be
well described by a Gaussian function, and the variations of fit parameters
with cavity elongation and solvent properties are analyzed. The excess chemical
potentials of hard-spheroid solutes with aspect ratios in the range , and with volumes between one and twenty times that of a solvent
molecule, are presented. It is shown that for a given molecular volume and
solvent dipole moment (or temperature) a spherical solute has the lowest excess
chemical potential and hence the highest solubility, while a prolate solute
with aspect ratio should be more soluble than an oblate solute with aspect
ratio . For a given solute molecule, the excess chemical potential
increases with increasing temperature; this same trend is observed in the case
of hydrophobic solvation. To help interpret the simulation results, comparison
is made with a scaled-particle theory that requires prior knowledge of a
solute-solvent interfacial tension and the pure-solvent equation of state,
which parameters are obtained from simulation results for spherical solutes.
The theory shows excellent agreement with simulation results over the whole
range of solute elongations considered.Comment: 10 pages, 10 figure
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