548 research outputs found
The Primordial Perturbation Spectrum and Large Scale Structure
Observational constraints on standard CDM spectra and perturbation spectra
with broken scale invariance are discussed.Comment: To appear in the proceedings of the 17th Texas Symposium, 4 pages
uuencoded, including figures
On the shape of dark matter halos from MultiDark Planck simulations
The halo shape plays a central role in determining important observational
properties of the haloes such as mass, concentration and lensing
cross-sections. The triaxiality of lensing galaxy clusters has a substantial
impact on the distribution of the largest Einstein radii, while weak lensing
techniques are sensitive to the intrinsic halo ellipticity. In this work, we
provide scaling relations for the shapes of dark matter haloes as a function of
mass (peak height) and redshift over more than four orders of magnitude in halo
masses, namely from to M. We have
analysed four dark matter only simulations from the MultiDark cosmological
simulation suite with more than 56 billion particles within boxes of 4.0, 2.5,
1.0 and 0.4 Gpc size assuming \textit{Planck} cosmology. The dark
matter haloes have been identified in the simulations using the {\sc rockstar}
halo finder, which also determines the axis ratios in terms of the
diagonalization of the inertia tensor. In order to infer the shape for a
hypothetical halo of a given mass at a given redshift, we provide fitting
functions to the minor-to-major and intermediate-to-major axis ratios as a
function of the peak height.Comment: Accepted for publication in MNRAS (14 pages, 13 figures). The
ROCKSTAR outputs used in this paper are available at
https://www.cosmosim.org/cms/simulations/data
Dark Matter decay and annihilation in the Local Universe: CLUES from Fermi
We present all-sky simulated Fermi maps of gamma-rays from dark matter decay
and annihilation in the Local Universe. The dark matter distribution is
obtained from a constrained cosmological simulation of the neighboring
large-scale structure provided by the CLUES project. The dark matter fields of
density and density squared are then taken as an input for the Fermi
observation simulation tool to predict the gamma-ray photon counts that Fermi
would detect in 5 years of all-sky survey for given dark matter models.
Signal-to-noise sky maps have also been obtained by adopting the current
Galactic and isotropic diffuse background models released by the Fermi
collaboration. We point out the possibility for Fermi to detect a dark matter
gamma-ray signal in local extragalactic structures. In particular, we conclude
here that Fermi observations of nearby clusters (e.g. Virgo and Coma) and
filaments are expected to give stronger constraints on decaying dark matter
compared to previous studies. As an example, we find a significant
signal-to-noise ratio in dark matter models with a decay rate fitting the
positron excess as measured by PAMELA. This is the first time that dark matter
filaments are shown to be promising targets for indirect detection of dark
matter. On the other hand, the prospects for detectability of annihilating dark
matter in local extragalactic structures are less optimistic even with extreme
cross-sections. We make the dark matter density and density squared maps
available online at http://www.clues-project.org/articles/darkmattermaps.htmlComment: 7 pages, 2 figures, 1 table. Matches version published in ApJ
Letters. High-resolution version of Figure 1 together with FITS files of the
dark matter density and density squared maps at:
http://www.clues-project.org/articles/darkmattermaps.html ; Version 3
includes 1-page ApJL Erratum: S/N values corrected, conclusions now point to
a better detectability of Virgo and Coma in gamma-ray
Large scale environmental bias of the QSO line of sight proximity effect
We analyse the proximity zone of the intergalactic matter around
high-redshift quasars in a cosmological environment. In a box of 64 h-1 Mpc
base length we employ dark matter only simulations. For estimating the hydrogen
temperature and density distribution we use the effective equation of state.
Hydrogen is assumed to be in photoionisation equilibrium with a model
background flux which is fit to recent observations of the mean optical depth
and transmission flux statistics. At redshifts z = 3, 4, and 4.8, we select
model quasar positions at the centre of the 20 most massive halos and 100 less
massive halos identified in the simulation. From each assumed quasar position
we cast 100 random lines of sight for two box length including the changes in
the ionisation fractions by the QSO flux field and derive mock Ly{\alpha}
spectra. The proximity effect describes the dependence of the mean normalised
optical depth {\xi} = {\tau}eff, QSO/{\tau}eff, Ly{\alpha} as a function of the
ratio of the ionisation rate by the QSO and the background field, {\omega} =
{\Gamma}QSO/{\Gamma}UVB, i.e. the profile {\xi} = (1 + {\omega}/a)-0.5, where a
strength parameter a is introduced. The strength parameter measures the
deviation from the theoretical background model and is used to quantify any
influence of the environmental density field. We reproduce an unbiased
measurement of the proximity effect which is not affected by the host halo
mass. The scatter between different lines of sight and different quasar host
positions increases with decreasing redshift. Around the host halos, we find
only a slight average overdensity in the proximity zone at comoving radii of 1
< rc < 10h-1 Mpc. However, a clear power-law correlation of the strength
parameter with the average overdensity in rc is found, showing an
overestimation of the ionising background in overdense regions and an
underestimation in underdense regions.Comment: Accepted by Monthly Notices of the Royal Astronomical Society. 15
pages, 12 figure
The Shape-Alignment relation in CDM Cosmic Structures
In this paper we study the supercluster - cluster morphological properties
using one of the largest ( SPH+N-body simulations of large
scale structure formation in a CDM model, based on the publicly
available code GADGET. We find that filamentary (prolate-like) shapes are the
dominant supercluster and cluster dark matter halo morphological feature, in
agreement with previous studies. However, the baryonic gas component of the
clusters is predominantly spherical. We investigate the alignment between
cluster halos (using either their DM or baryonic components) and their parent
supercluster major-axis orientation, finding that clusters show such a
preferential alignment. Combining the shape and the alignment statistics, we
also find that the amplitude of supercluster - cluster alignment increases
although weakly with supercluster filamentariness.Comment: Accepted for puplication in MNRAS, 10 pages, 15 figure
The structure of the ICM from High Resolution SPH simulations
We present results from a set of high (512^3 effective resolution), and
ultra-high (1024^3) SPH adiabatic cosmological simulations of cluster formation
aimed at studying the internal structure of the intracluster medium (ICM). We
derive a self-consistent analytical model of the structure of the intracluster
medium (ICM). We discuss the radial structure and scaling relations expected
from purely gravitational collapse, and show that the choice of a particular
halo model can have important consequences on the interpretation of
observational data. The validity of the approximations of hydrostatic
equilibrium and a polytropic equation of state are checked against results of
our simulations. The properties of the ICM are fully specified when a
'universal' profile is assumed for either the dark or the baryonic component.
We also show the first results from an unprecedented large-scale simulation of
500 Mpc/h and 2 times 512^3 gas and dark matter particles. This experiment will
make possible a detailed study of the large-scale distribution of clusters as a
function of their X-ray properties.Comment: 5 pages, 3 figures, to appear in the Proceedings of IAU Colloquium
195: "Outskirts of Galaxy Clusters: intense life in the suburbs", Torino
Italy, March 200
Bullet Clusters in the MareNostrum Universe
We estimate the expected distribution of displacements between the dark
matter and gas cores in simulated clusters. We use the MareNostrum Universe,
one of the largest non radiative, SPH Lambda CDM cosmological simulations. We
find that projected 2-D displacements between dark matter and gas, equal or
larger than the observed in the Bullet Cluster, are expected in 1% to 2% of the
clusters with masses larger than 10^{14} Msun. The 2-D displacement
distribution is roughly the same between redshifts 0<z<0.5 when multiplied by a
factor of (1+z)^{-1/2}. We conclude that the separations between dark matter
and gas as observed in the bullet cluster can be easily found in a Lambda CDM
universe. Furthermore we find that the displacement distribution is not very
sensitive to the normalization of the power spectrum. Upcoming surveys could
extend the measurements of these displacements between dark matter and gas into
large samples of hundreds of clusters, providing a potential test for Lambda
CDM.Comment: 20 pages, 4 figures. Accepted for publication in Ap
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