1,826 research outputs found
Shapes of Gas, Gravitational Potential and Dark Matter in Lambda-CDM Clusters
We present analysis of the three-dimensional shape of intracluster gas in
clusters formed in cosmological simulations of the Lambda-CDM cosmology and
compare it to the shape of dark matter distribution and the shape of the
overall isopotential surfaces. We find that in simulations with radiative
cooling, star formation and stellar feedback (CSF), intracluster gas outside
the cluster core is more spherical compared to non-radiative (NR) simulations,
while in the core the gas in the CSF runs is more triaxial and has a distinctly
oblate shape. The latter reflects the ongoing cooling of gas, which settles
into a thick oblate ellipsoid as it loses thermal energy. The shape of the gas
in the inner regions of clusters can therefore be a useful diagnostic of gas
cooling. We find that gas traces the shape of the underlying potential rather
well outside the core, as expected in hydrostatic equilibrium. At smaller
radii, however, the gas and potential shapes differ significantly. In the CSF
runs, the difference reflects the fact that gas is partly rotationally
supported. Interestingly, we find that in NR simulations the difference between
gas and potential shape at small radii is due to random gas motions, which make
the gas distribution more spherical than the equipotential surfaces. Finally,
we use mock Chandra X-ray maps to show that the differences in shapes observed
in three-dimensional distribution of gas are discernible in the ellipticity of
X-ray isophotes. Contrasting the ellipticities measured in simulated clusters
against observations can therefore constrain the amount of cooling of the
intracluster medium and the presence of random gas motions in cluster cores.Comment: 11 pages, 8 figures, 3 tables, updated to match the version accepted
for publication in the Astrophysical Journa
Time-reversal symmetry breaking by ac field: Effect of commensurability in the frequency domain
It is shown that the variance of the linear dc conductance fluctuations in an
open quantum dot under a high-frequency ac pumping depends significantly on the
spectral content of the ac field. For a sufficiently strong ac field the dc
conductance fluctuations are much stronger for the periodic pumping than in the
case of the noise ac field of the same intensity. The reduction factor r in a
static magnetic field takes the universal value of 2 only for the white-noise
pumping. In general r may deviate from 2 thus signalling on the time-reversal
symmetry breaking by the ac field. For the bi-harmonic ac field of the form
A(t)=A_{0} [cos(\omega_{1} t)+cos(\omega_{2} t)] we predict the enchancement of
effects of T-symmetry breaking at commensurate frequencies
\omega_{2}/\omega_{1}=P/Q. In the high-temperature limit there is also the
parity effect: the enchancement is only present if either P or Q is even.Comment: 8 pages, 6 figures, submitted for "Electronic Correlations: from
meso- to nano-physics", edited by G. Montambaux and T. Martin, Rencontres de
Morion
Quantifying properties of ICM inhomogeneities
We present a new method to identify and characterize the structure of the
intracluster medium (ICM) in simulated galaxy clusters. The method uses the
median of gas properties, such as density and pressure, which we show to be
very robust to the presence of gas inhomogeneities. In particular, we show that
the radial profiles of median gas properties are smooth and do not exhibit
fluctuations at locations of massive clumps in contrast to mean and mode
properties. It is shown that distribution of gas properties in a given radial
shell can be well described by a log-normal PDF and a tail. The former
corresponds to a nearly hydrostatic bulk component, accounting for ~99% of the
volume, while the tail corresponds to high density inhomogeneities. We show
that this results in a simple and robust separation of the diffuse and clumpy
components of the ICM. The FWHM of the density distribution grows with radius
and varies from ~0.15 dex in cluster centre to ~0.5 dex at 2r_500 in relaxed
clusters. The small scatter in the width between relaxed clusters suggests that
the degree of inhomogeneity is a robust characteristic of the ICM. It broadly
agrees with the amplitude of density perturbations in the Coma cluster. We
discuss the origin of ICM density variations in spherical shells and show that
less than 20% of the width can be attributed to the triaxiality of the cluster
gravitational potential. As a link to X-ray observations of real clusters we
evaluated the ICM clumping factor with and without high density
inhomogeneities. We argue that these two cases represent upper and lower limits
on the departure of the observed X-ray emissivity from the median value. We
find that the typical value of the clumping factor in the bulk component of
relaxed clusters varies from ~1.1-1.2 at r_500 up to ~1.3-1.4 at r_200, in
broad agreement with recent observations.Comment: 16 pages, 12 figure, accepted to MNRA
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
Two-eigenfunction correlation in a multifractal metal and insulator
We consider the correlation of two single-particle probability densities
at coinciding points as a function of the
energy separation for disordered tight-binding lattice models
(the Anderson models) and certain random matrix ensembles. We focus on the
models in the parameter range where they are close but not exactly at the
Anderson localization transition. We show that even far away from the critical
point the eigenfunction correlation show the remnant of multifractality which
is characteristic of the critical states. By a combination of the numerical
results on the Anderson model and analytical and numerical results for the
relevant random matrix theories we were able to identify the Gaussian random
matrix ensembles that describe the multifractal features in the metal and
insulator phases. In particular those random matrix ensembles describe new
phenomena of eigenfunction correlation we discovered from simulations on the
Anderson model. These are the eigenfunction mutual avoiding at large energy
separations and the logarithmic enhancement of eigenfunction correlations at
small energy separations in the two-dimensional (2D) and the three-dimensional
(3D) Anderson insulator. For both phenomena a simple and general physical
picture is suggested.Comment: 16 pages, 18 figure
Peculiarities of neutron waveguides with thin Gd layer
Peculiarities of the formation of a neutron enhanced standing wave in the
structure with a thin highly absorbing layer of gadolinium are considered in
the article. An analogue of the poisoning effect well known in reactor physics
was found. The effect is stronger for the Nb/Gd/Nb system. Despite of this
effect, for a Nb/Gd bilayer and a Nb/Gd/Nb trilayer placed between Al2O3
substrate and Cu layer, it is shown theoretically and experimentally that one
order of magnitude enhancement of neutron density is possible in the vicinity
of the Gd layer. This enhancement makes it possible to study domain formation
in the Gd layer under transition of the Nb layer(s) into the superconducting
state (cryptoferromagnetic phase).Comment: 5 pages, 2 figure
Energy level statistics of a critical random matrix ensemble
We study level statistics of a critical random matrix ensemble of a power-law
banded complex Hermitean matrices. We compute numerically the level
compressibility via the level number variance and compare it with the
analytical formula for the exactly solvable model of Moshe, Neuberger and
Shapiro.Comment: 8 pages, 3 figure
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