209 research outputs found
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
High resolution simulations of the reionization of an isolated Milky Way - M31 galaxy pair
We present the results of a set of numerical simulations aimed at studying
reionization at galactic scale. We use a high resolution simulation of the
formation of the Milky Way-M31 system to simulate the reionization of the local
group. The reionization calculation was performed with the post-processing
radiative transfer code ATON and the underlying cosmological simulation was
performed as part of the CLUES project. We vary the source models to bracket
the range of source properties used in the literature. We investigate the
structure and propagation of the galatic ionization fronts by a visual
examination of our reionization maps. Within the progenitors we find that
reionization is patchy, and proceeds locally inside out. The process becomes
patchier with decreasing source photon output. It is generally dominated by one
major HII region and 1-4 additional isolated smaller bubbles, which eventually
overlap. Higher emissivity results in faster and earlier local reionization. In
all models, the reionization of the Milky Way and M31 are similar in duration,
i.e. between 203 Myr and 22 Myr depending on the source model, placing their
zreion between 8.4 and 13.7. In all models except the most extreme, the MW and
M31 progenitors reionize internally, ignoring each other, despite being
relatively close to each other even during the epoch of reionization. Only in
the case of strong supernova feedback suppressing star formation in haloes less
massive than 10^9 M_sun, and using our highest emissivity, we find that the MW
is reionized by M31.Comment: Accepted for publication in ApJ. 14 pages, 4 figures, 1 tabl
A Dynamical Classification of the Cosmic Web
A dynamical classification of the cosmic web is proposed. The large scale
environment is classified into four web types: voids, sheets, filaments and
knots. The classification is based on the evaluation of the deformation tensor,
i.e. the Hessian of the gravitational potential, on a grid. The classification
is based on counting the number of eigenvalues above a certain threshold,
lambda_th at each grid point, where the case of zero, one, two or three such
eigenvalues corresponds to void, sheet, filament or a knot grid point. The
collection of neighboring grid points, friends-of-friends, of the same web
attribute constitutes voids, sheets, filaments and knots as web objects.
A simple dynamical consideration suggests that lambda_th should be
approximately unity, upon an appropriate scaling of the deformation tensor. The
algorithm has been applied and tested against a suite of (dark matter only)
cosmological N-body simulations. In particular, the dependence of the volume
and mass filling fractions on lambda_th and on the resolution has been
calculated for the four web types. Also, the percolation properties of voids
and filaments have been studied.
Our main findings are: (a) Already at lambda_th = 0.1 the resulting web
classification reproduces the visual impression of the cosmic web. (b) Between
0.2 < lambda_th < 0.4, a system of percolated voids coexists with a net of
interconected filaments. This suggests a reasonable choice for lambda_th as the
parameter that defines the cosmic web. (c) The dynamical nature of the
suggested classification provides a robust framework for incorporating
environmental information into galaxy formation models, and in particular the
semi-analytical ones.Comment: 11 pages, 6 figures, submitted to MNRA
Properties of voids in the Local Volume
Current explanation of the overabundance of dark matter subhalos in the Local
Group (LG) indicates that there maybe a limit on mass of a halo, which can host
a galaxy. This idea can be tested using voids in the distribution of galaxies:
at some level small voids should not contain any (even dwarf) galaxies. We use
observational samples complete to M_B=-12 with distances less than 8 Mpc to
construct the void function (VF): the distribution of sizes of voids empty of
any galaxies. There are ~ 30 voids with sizes ranging from 1 to 5 Mpc. We also
study the distribution of dark matter halos in very high resolution simulations
of the LCDM model. The theoretical VF matches the observations remarkably well
only if we use halos with circular velocities larger than 45 +/- 10 km/s. This
agrees with the Local Group predictions. Small voids look quite similar to heir
giant cousins: the density has a minimum at the center of a void and it
increases as we get closer to the border. Thus, both the Local Group data and
the nearby voids indicate that isolated halos below 45 +/- 10 km/s must not
host galaxies and that small (few Mpc) voids are truly dark.Comment: 5 pages 1 figure. To appear in proceedings of the conference
"Galaxies in the Local Volume", Sydney, 8 to 13 July 200
Vast planes of satellites in a high resolution simulation of the Local Group: comparison to Andromeda
We search for vast planes of satellites (VPoS) in a high resolution
simulation of the Local Group performed by the CLUES project, which improves
significantly the resolution of former similar studies. We use a simple method
for detecting planar configurations of satellites, and validate it on the known
plane of M31. We implement a range of prescriptions for modelling the satellite
populations, roughly reproducing the variety of recipes used in the literature,
and investigate the occurence and properties of planar structures in these
populations. The structure of the simulated satellite systems is strongly
non-random and contains planes of satellites, predominantly co-rotating, with,
in some cases, sizes comparable to the plane observed in M31 by Ibata et al..
However the latter is slightly richer in satellites, slightly thinner and has
stronger co-rotation, which makes it stand out as overall more exceptional than
the simulated planes, when compared to a random population. Although the
simulated planes we find are generally dominated by one real structure, forming
its backbone, they are also partly fortuitous and are thus not kinematically
coherent structures as a whole. Provided that the simulated and observed planes
of satellites are indeed of the same nature, our results suggest that the VPoS
of M31 is not a coherent disc and that one third to one half of its satellites
must have large proper motions perpendicular to the plane
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