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 $|\Psi_{E}({\bf r})|^{2}$ at coinciding points ${\bf r}$ as a function of the energy separation $\omega=|E-E'|$ 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