87 research outputs found

### Gravitational instabilities and faster evolving density perturbations

The evolution of inhomogeneities in a spherical collapse model is studied by
expanding the Einstein equation in powers of inverse radial parameter. In the
linear regime, the density contrast is obtained for flat, closed and open
universes. In addition to the usual modes, an infinite number of new growing
modes are contained in the solutions for pressureless open and closed
universes. In the nonlinear regime, we obtain the leading growing modes in
closed forms for a flat universe and also, in the limits of small and large
times, for an open universe.Comment: latex, 17 pages; electronic address for programs correcte

### Decay amplitudes in two-dimensional QCD

Decay amplitudes for mesons in two-dimensional QCD are discussed. We show
that in spite of an infinite number of conserved charges, particle production
is not entirely suppressed. This phenomenon is explained in terms of quantum
corrections to the combined algebra of higher-conserved and spectrum-generating
currents. We predict the qualitative form of particle production probabilities
and verify that they are in agreement with numerical data. We also discuss
four-dimensional self-dual Yang-Mills theory in the light of our results.Comment: We discuss in more detail the background material and elaborate on
the consequences of the new results. New equations and text are added and the
figures are efficiently regenerated by a fortran program, latex file, 13
pages, 4 figures in encapsulated postscript files, uses epsf. minor changes,
version to appear in Physical Review

### Dark matter caustics and the enhancement of self-annihilation flux

Cold dark matter haloes are populated by caustics, which are yet to be
resolved in N-body simulations or observed in the Universe. Secondary infall
model provides a paradigm for the study of caustics in "typical" haloes
assuming that they have had no major mergers and have grown only by smooth
accretion. This is a particular characteristic of the smallest dark matter
haloes of about 10^{-5} Mo, which although "atypical" contain no substructures
and could have survived until now with no major mergers. Thus using this model
as the first guidline, we evaluate the neutralino self-annihilation flux for
these haloes. Our results show that caustics could leave a distinct sawteeth
signature on the differential and cumulative fluxes coming from the outer
regions of these haloes. The total annihilation signal from the regions away
from the centre can be boosted by about forty percents.Comment: To appear in JCAP, 4 pages, 3 figure

### A single-merger scenario for the formation of the giant stream and the warp of M31

We propose that the accretion of a dwarf spheroidal galaxy provides a common
origin for the giant southern stream and the warp of M31. We run about 40 full
N-body simulations with live M31, infalling galaxies with varying masses and
density profiles, and cosmologically-plausible initial orbital parameters.
Excellent agreement with a full range of observational data is obtained for a
model in which a dark-matter-rich dwarf spheroidal, whose trajectory lies on
the thin plane of corotating satellites of M31, is accreted from its turnaround
radius of about 200 kpcs into M31 at approximately 3 Gyrs ago. The satellite is
disrupted as it orbits in the potential well of the galaxy and forms the giant
stream and in return heats and warps the disk of M31. We show that our
cosmologically-motivated model is favoured by the kinematic data over the
phenomenological models in which the satellite starts its infall from a close
distance of M31. Our model predicts that the remnant of the disrupted satellite
resides in the region of the North-Eastern shelf of M31. The results here
suggest that the surviving satellites of M31 that orbit on the same thin plane,
as the disrupted satellite once did, could have all been accreted from an
intergalactic filament.Comment: 18 pages, 22 figures, 3 table

### Critical points of the cosmic velocity field and the uncertainties in the value of the Hubble constant

The existence of critical points for the peculiar velocity field is a natural
feature of the correlated vector field. These points appear at the junctions of
velocity domains with different orientations of their averaged velocity
vectors. Since peculiar velocities are the important cause of the scatter in
the Hubble expansion rate, we propose that a more precise determination of the
Hubble constant can be made by restricting analysis to a subsample of
observational data containing only the zones around the critical points of the
peculiar velocity field, associated with voids and saddle points. On
large-scales the critical points, where the first derivative of the
gravitational potential vanishes, can easily be identified using the density
field and classified by the behavior of the Hessian of the gravitational
potential. We use high-resolution N-body simulations to show that these regions
are stable in time and hence are excellent tracers of the initial conditions.
Furthermore, we show that the variance of the Hubble flow can be substantially
minimized by restricting observations to the subsample of such regions of
vanishing velocity instead of aiming at increasing the statistics by averaging
indiscriminately using the full data sets, as is the common approach.Comment: 17 pages, 6 figures and 2 tables, minor modifications after receiving
several useful comment

### The challenge of large and empty voids in the SDSS DR7 redshift survey

We present catalogues of voids for the SDSS DR7 redshift survey and for
Millennium I simulation mock data. We aim to compare the observations with
simulations based on a $\Lambda$CDM model and a semi-analytic galaxy formation
model. We use the void statistics as a test for these models. We assembled a
mock catalogue that closely resembles the SDSS DR7 catalogue and carried out a
parallel statistical analysis of the observed and simulated catalogue. We find
that in the observation and the simulation, voids tend to be equally spherical.
The total volume occupied by the voids and their total number are slightly
larger in the simulation than in the observation. We find that large voids are
less abundant in the simulation and the total luminosity of the galaxies
contained in a void with a given radius is higher on average than observed by
SDSS DR7 survey. We expect these discrepancies to be even more significant in
reality than found here since the present value of $\sigma_8$ given by WMAP7 is
lower than the value of 0.9 used in the Millennium I simulation. The reason why
the simulation fails to produce enough large and dark voids might be the
failure of certain semi-analytic galaxy formation models to reduce the
small-scale power of $\Lambda$CDM and to produce sufficient power on large
scales.Comment: Kolmogrov-Smirnov test added. version to appear in A&

### Action Model of Infall into the Virgo Cluster

The observed infall of galaxies into the Virgo Cluster puts strong
constraints on the mass of the cluster. A non-parametric fully non-linear
description of the infall can be made with orbit reconstructions based on
Numerical Action Methods. The mass of the cluster is determined to be $1.2
\times 10^{15} M_{\odot}$. The mass-to-light ratio for the cluster is found to
be seven times higher than the mean ratio found across the region within V=3000
km/s.Comment: 7 pages, 3 figures, version to appear in "Outskirt of Galaxy
Clusters", the proceedings of IAU Colloquim No 195, 200

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