83 research outputs found

### Passage of radiation through wormholes

We investigate numerically the process of the passage of a radiation pulse
through a wormhole and the subsequent evolution of the wormhole that is caused
by the gravitational action of this pulse. The initial static wormhole is
modeled by the spherically symmetrical Armendariz-Picon solution with zero
mass. The radiation pulses are modeled by spherically symmetrical shells of
self-gravitating massless scalar fields. We demonstrate that the compact signal
propagates through the wormhole and investigate the dynamics of the fields in
this process for both cases: collapse of the wormhole into the black hole and
for the expanding wormhole.Comment: 18 Pages, 13 figures, minor typos corrected, updated reference

### Physics of the interior of a black hole with an exotic scalar matter

We use a numerical code to consider the nonlinear processes arising when a
Reissner-Nordstrom black hole is irradiated by an exotic scalar field (modelled
as a free massless scalar field with an opposite sign for its energy-momentum
tensor). These processes are quite different from the processes arising in the
case of the same black hole being irradiated by a pulse of a normal scalar
field. In our case, we did not observe the creation of a spacelike strong
singularity in the T-region of the space-time. We investigate the antifocusing
effects in the gravity field of the exotic scalar field with the negative
energy density and the evolution of the mass function. We demonstrate the
process of vanishing of the black hole when it is irradiated by a strong pulse
of an exotic scalar field.Comment: 16 pages, 16 figures. Text has been rewritten and restructured,
Penrose diagrams have been added, appendix with convergence tests has been
added. Co-author has been added. Conclusions are unchange

### Constraining Cluster Physics with the Shape of X-ray Clusters: Comparison of Local X-ray Clusters versus LCDM Clusters

Simulations of cluster formation have demonstrated that condensation of
baryons into central galaxies during cluster formation can drive the shape of
the gas distribution in galaxy clusters significantly rounder, even at radii as
large as half of the virial radius. However, such simulations generally predict
stellar fractions within cluster virial radii that are ~2 to 3 times larger
than the stellar masses deduced from observations. In this work we compare
ellipticity profiles of clusters simulated with and without baryonic cooling to
the cluster ellipticity profiles derived from Chandra and ROSAT observations in
an effort to constrain the fraction of gas that cools and condenses into the
central galaxies within clusters. We find that the observed ellipticity
profiles are fairly constant with radius, with an average ellipticity of 0.18
+/- 0.05. The observed ellipticity profiles are in good agreement with the
predictions of non-radiative simulations. On the other hand, the ellipticity
profiles of the clusters in simulations that include radiative cooling, star
formation, and supernova feedback (but no AGN feedback) deviate significantly
from the observed ellipticity profiles at all radii. The simulations with
cooling overpredict (underpredict) ellipticity in the inner (outer) regions of
galaxy clusters. By comparing the simulations with and without cooling, we show
that the cooling of gas via cooling flows in the central regions of simulated
clusters causes the gas distribution to be more oblate in the central regions,
but makes the outer gas distribution more spherical. We find that late-time gas
cooling and star formation are responsible for the significantly oblate gas
distributions in cluster cores, but the gas shapes outside of cluster cores are
set primarily by baryon dissipation at high redshift z > 2.Comment: 10 pages, 6 figures, matching the published version in ApJ. Corrected
missing reference in the arxiv versio

### Formation of Galaxy Clusters

In this review, we describe our current understanding of cluster formation:
from the general picture of collapse from initial density fluctuations in an
expanding Universe to detailed simulations of cluster formation including the
effects of galaxy formation. We outline both the areas in which highly accurate
predictions of theoretical models can be obtained and areas where predictions
are uncertain due to uncertain physics of galaxy formation and feedback. The
former includes the description of the structural properties of the dark matter
halos hosting cluster, their mass function and clustering properties. Their
study provides a foundation for cosmological applications of clusters and for
testing the fundamental assumptions of the standard model of structure
formation. The latter includes the description of the total gas and stellar
fractions, the thermodynamical and non-thermal processes in the intracluster
plasma. Their study serves as a testing ground for galaxy formation models and
plasma physics. In this context, we identify a suitable radial range where the
observed thermal properties of the intra-cluster plasma exhibit the most
regular behavior and thus can be used to define robust observational proxies
for the total cluster mass. We put particular emphasis on examining assumptions
and limitations of the widely used self-similar model of clusters. Finally, we
discuss the formation of clusters in non-standard cosmological models, such as
non-Gaussian models for the initial density field and models with modified
gravity, along with prospects for testing these alternative scenarios with
large cluster surveys in the near future.Comment: 66 pages, 17 figures, review to be published in 2012 Annual Reviews
of Astronomy & Astrophysic

### Collapse Barriers and Halo Abundance: Testing the Excursion Set Ansatz

Our heuristic understanding of the abundance of dark matter halos centers
around the concept of a density threshold, or "barrier", for gravitational
collapse. If one adopts the ansatz that regions of the linearly evolved density
field smoothed on mass scale M with an overdensity that exceeds the barrier
will undergo gravitational collapse into halos of mass M, the corresponding
abundance of such halos can be estimated simply as a fraction of the mass
density satisfying the collapse criterion divided by the mass M. The key
ingredient of this ansatz is therefore the functional form of the collapse
barrier as a function of mass M or, equivalently, of the variance sigma^2(M).
Several such barriers based on the spherical, Zel'dovich, and ellipsoidal
collapse models have been extensively discussed. Using large scale cosmological
simulations, we show that the relation between the linear overdensity and the
mass variance for regions that collapse to form halos by the present epoch
resembles expectations from dynamical models of ellipsoidal collapse. However,
we also show that using such a collapse barrier with the excursion set ansatz
predicts a halo mass function inconsistent with that measured directly in
cosmological simulations. This inconsistency demonstrates a failure of the
excursion set ansatz as a physical model for halo collapse. We discuss
implications of our results for understanding the collapse epoch for halos as a
function of mass, and avenues for improving consistency between analytical
models for the collapse epoch and the results of cosmological simulations.Comment: Version accepted by ApJ, scheduled for May 2009, v696. High-res
version available at
http://kicp.uchicago.edu/~brant/astro-ph/excursion_set_ansatz/robertson_excursion_set_ansatz.pd

### Distorted 5-dimensional vacuum black hole

In this paper we study how the distortion generated by a static and neutral
distribution of external matter affects a 5-dimensional
Schwarzschild-Tangherlini black hole. A solution representing a particular
class of such distorted black holes admits an RxU(1)xU(1) isometry group. We
show that there exists a certain duality transformation between the black hole
horizon and a stretched singularity surfaces. The space-time near the distorted
black hole singularity has the same topology and Kasner exponents as those of a
5-dimensional Schwarzschild-Tangherlini black hole. We calculate the maximal
proper time of free fall of a test particle from the distorted black hole
horizon to its singularity and find that, depending on the distortion, it can
be less, equal to, or greater than that of a Schwarzschild-Tangherlini black
hole of the same horizon area. This implies that due to the distortion, the
singularity of a Schwarzschild-Tangherlini black hole can come close to its
horizon. A relation between the Kretschmann scalar calculated on the horizon of
a 5-dimensional static, asymmetric, distorted black hole and the trace of the
square of the Ricci tensor of the horizon surface is derived.Comment: 20 pages, 9 figure

### The Anisotropic Distribution of Galactic Satellites

We present a study of the spatial distribution of subhalos in galactic dark
matter halos using dissipationless cosmological simulations of the concordance
LCDM model. We find that subhalos are distributed anisotropically and are
preferentially located along the major axes of the triaxial mass distributions
of their hosts. The Kolmogorov-Smirnov probability for drawing our simulated
subhalo sample from an isotropic distribution is P_KS \simeq 1.5 \times
10^{-4}. An isotropic distribution of subhalos is thus not the correct null
hypothesis for testing the CDM paradigm. The nearly planar distribution of
observed Milky Way (MW) satellites is marginally consistent (probability \simeq
0.02) with being drawn randomly from the subhalo distribution in our
simulations. Furthermore, if we select the subhalos likely to be luminous, we
find a distribution that is consistent with the observed MW satellites. In
fact, we show that subsamples of the subhalo population with a
centrally-concentrated radial distribution, similar to that of the MW dwarfs,
typically exhibit a comparable degree of planarity. We explore the origin of
the observed subhalo anisotropy and conclude that it is likely due to (1)
preferential accretion of subhalos along filaments, often closely aligned with
the major axis of the host halo, and (2) evolution of satellite orbits within
the prolate, triaxial potentials typical of CDM halos. Agreement between
predictions and observations requires the major axis of the outer dark matter
halo of the Milky Way to be nearly perpendicular to the disk. We discuss
possible observational tests of such disk-halo alignment with current large
galaxy surveys.Comment: 14 pages (including appendix), 9 figures. Accepted for Publication in
ApJ. Minor changes to reflect referee's comment

### Fundamental differences between SPH and grid methods

We have carried out a hydrodynamical code comparison study of interacting
multiphase fluids. The two commonly used techniques of grid and smoothed
particle hydrodynamics (SPH) show striking differences in their ability to
model processes that are fundamentally important across many areas of
astrophysics. Whilst Eulerian grid based methods are able to resolve and treat
important dynamical instabilities, such as Kelvin-Helmholtz or Rayleigh-Taylor,
these processes are poorly or not at all resolved by existing SPH techniques.
We show that the reason for this is that SPH, at least in its standard
implementation, introduces spurious pressure forces on particles in regions
where there are steep density gradients. This results in a boundary gap of the
size of the SPH smoothing kernel over which information is not transferred.Comment: 15 pages, 13 figures, to be submitted to MNRAS. For high-resolution
figures, please see http://www-theorie.physik.unizh.ch/~agertz

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