Modelling rich clusters of galaxies

We develop a self-consistent dynamical model for spherically-symmetric clusters of galaxies. The total mass profile and velocity dispersion profiles of galaxies are derived by taking both, galaxies and intracluster gas, in hydrostatic equilibrium, and by assuming the latter to follow a polytropic distributionT^–1. We use the strongest and better established correlations among observed properties of clusters to fix the values of the resulting free parameters, and so, to reduce the general freedom of the model.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

New polar ring galaxies in rich clusters of galaxies

Four polar ring galaxies discovered in rich clusters of galaxies are presented. Brief comments on their structural properties are given.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

Double galaxies in galactic clusters

We have identified 127 galactic pairs in six of Abell's rich galactic clusters. We have established positive correlations between certain parameters of the components of the pairs. We conclude that, despite the gravitational influence of other members of the cluster, a subsystem of double galaxies exists in rich clusters as a structural element of these structures.Translated fromAstrofizika, Vol. 37, No. 3, 1994.The author is grateful to L. V. Mirzoyan and A. R. Petrosyan for helpful remarks.     

Further investigations of clusters of galaxies

The number-density distributions of ten further clusters of galaxies were derived by counting galaxies on the red Polomar Sky Survey prints. For eight isolated clusters the radial number-density distributions and the radial cumulative galaxy distributions were calculated.     

Investigation of 10 clusters of galaxies

The number-density distribution of 10 clusters of galaxies was derived by counting galaxies on the red Palomar Sky Survey prints. For six isolated clusters the radial number-density distribution and the cumulative galaxy-distribution were calculated.     

Investigation of nine clusters of galaxies

The number-density distributions of further 9 clusters of galaxies were derived by counting galaxies on the red Palomar Sky Survey prints. For all the clusters the radial number density distribution and the radial cumulative galaxy distribution were calculated.     

Further investigations of clusters of galaxies

The number-density distribution in ten clusters of galaxies was derived by counting galaxies on the red Palomar Sky Survey prints. For seven isolated clusters the radial number-density distribution and the radial cumulative galaxy distribution were calculated.     

Supersonic motions of galaxies in clusters

We study motions of galaxies in galaxy clusters formed in the concordance Λ cold dark matter cosmology. We use high-resolution cosmological simulations that follow the dynamics of dark matter and gas and include various physical processes critical for galaxy formation: gas cooling, heating and star formation. Analysing the motions of galaxies and the properties of intracluster gas in a sample of eight simulated clusters at z = 0, we study the velocity dispersion profiles of the dark matter, gas and galaxies. We measure the mean velocity of galaxy motions and gas sound speed as a function of radius and calculate the average Mach number of galaxy motions. The simulations show that galaxies, on average, move supersonically with the average Mach number of ≈1.4, approximately independent of the cluster-centric radius. The supersonic motions of galaxies may potentially provide an important source of heating for the intracluster gas by driving weak shocks and via dynamical friction, although these heating processes appear to be inefficient in our simulations. We also find that galaxies move slightly faster than the dark matter particles. The magnitude of the velocity bias,   b _v ≈ 1.1  , is, however, smaller than the bias estimated for subhaloes in dissipationless simulations. Interestingly, we find velocity bias in the tangential component of the velocity dispersion, but not in the radial component. Finally, we find significant random bulk motions of gas. The typical gas velocities are of order ≈20–30 per cent of the gas sound speed. These random motions provide about 10 per cent of the total pressure support in our simulated clusters. The non-thermal pressure support, if neglected, will bias measurements of the total mass in the hydrostatic analyses of the X-ray cluster observations.     

Homogeneity of early-type galaxies across clusters

We studied the scatter across clusters of the color of the red sequence in are presentative and large sample of clusters (more than 200) detected on the Early Data Release of the Sloan Digital Sky Survey (EDR-SDSS) in the redshift range 0.06<z<0.34. We found an extreme degree of homogeneity in the color of the red sequence (the intrinsic scatter is about 0.02 to 0.03 mag)suggesting that either galaxies on the red sequence formed a long time ago (z>2) or else their star formation is universally delayed with preservation of a small spread in age formation. The latter possibility is ruled out by the mere existence of galaxies at high redshift. While the old age of ellipticals was already been claimed for a small heterogeneous collection of clusters, most of which are rich ones, we found that it holds for a ten to one hundred larger sample, representative of all clusters and groups detected on the EDR-SDSS. Hence, we claim the possible universality of the color of the galaxies on the red sequence. Furthermore, the sample includes a large number of very poor clusters (also called groups), not studied in previous works, for which the hierarchical and monolithic scenarios of elliptical formation predict different colors for the brightest ellipticals. The observed red sequence color does not depend on cluster/group richness at a level of 0.02 mag, while a 0.23 mag effect is expected according to the hierarchical prediction. Therefore, the stellar population of red sequence galaxies is similar in clusters and groups, in spite of different halo histories. Finally, since the observed rest-frame color of the red sequence does not depend on environment and redshift, it can be used as a distance indicator, with an error σ_z=0.018, a few times better than the precision achieved by other photometric redshift estimates and twice better than the precision of the Fundamental Plane for a single galaxy at the median redshift of the EDR-SDSS. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cooling flows in clusters of galaxies

Summary X-ray images and spectra of clusters of galaxies show strong evidence for cooling flows. In many clusters, the hot gas in the core is cooling at rates of 100Myr^–1 and greater. Few traces of the cooled gas have been observed, but it probably forms into low-mass stars (perhaps brown dwarf or even Jupiter-mass objects). X-ray surface-brightness profiles show that the cooling gas is highly inhomogeneous. Overdense gas cools rapidly to form cooled clumps distributed throughout the flow, with little of the gas ever reaching the cluster centre. Cooled and cooling clumps are disrupted because of their motion relative to the remainder of the gas, tending to produce small cooled fragments and, ultimately, low-mass stars. Large molecular clouds, which are the sites of massive star formation in our galaxy, do not occur in the outer parts of cooling flows. There is evidence of larger gas clumps and the formation of more massive stars in the central few kpc of some cooling flows. It is argued that cooling flows efficiently form dark matter. This has wider implications for the formation of dark matter in massive galaxies.     

Peculiar velocities of galaxies and clusters

We present a simple model for the shape of the distribution function of galaxy peculiar velocities. We show how both non-linear and linear theory terms combine to produce a distribution which has an approximately Gaussian core with exponential wings. The model is easily extended to study how the statistic depends on the type of particle used to trace the velocity field (dark matter particles, dark matter haloes, galaxies), and on the density of the environment in which the test particles are located. Comparisons with simulations suggest that our model is accurate. We also show that the evolution of the peculiar velocities depends on the local, rather than the global, density. Since clusters populate denser regions on average, using cluster velocities with the linear theory scaling may lead to an overestimate of the global value of Ω₀. Conversely, using linear theory with the global value of Ω₀ to scale cluster velocities from the initial to the present time results in an underestimate of their true velocities. In general, however, the directions of motions of haloes are rather well described by linear theory. Our results help to simplify models of redshift-space distortions considerably.     

Large-scale distribution of clusters of galaxies

The redshifts of 576 clusters of galaxies have been compiled from literature. This sample was used to study the large-scale distribution of clusters up to about z = 0.15. The distribution of the clusters reveals several chains and filaments 50 to 200 h⁻¹ Mpc in size. There are many empty regions and holes, too, some of them known previously.     

Suzaku observations of clusters of galaxies

We review results of Suzaku observations of the intracluster medium of clusters of galaxies whose O, Mg, Si, S and Fe abundances have been measured with good accuracy due to the good energy resolution and low background. Metal massto‐light ratios were derived and we will discuss the origin of the metals. We also review the results of the search for bulk motion and hard X‐ray emission. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Gravitational stability of dark energy in galaxies and clusters of galaxies

We analyze the behavior of the scalar field as dark energy of the Universe in a static world of galaxies and clusters of galaxies. We find the analytical solutions of evolution equations of the density and velocity perturbations of dark matter and dark energy, which interact only gravitationally, along with the perturbations of metric in a static world with background Minkowski metric. It was shown that quintessential and phantom dark energy in the static world of galaxies and clusters of galaxies is gravitationally stable and can only oscillate by the influence of self-gravity. In the gravitational field of dark matter perturbations, it is able to condense monotonically, but the amplitude of density and velocity perturbations on all scales remains small. It was also illustrated that the “accretion” of phantom dark energy in the region of dark matter overdensities causes formation of dark energy underdensities-the regions with negative amplitude of density perturbations of dark energy.     

Simulated ROSAT observations of clusters of galaxies

Detailed simulations have been made of the search for clusters of galaxies during the ROSAT all-sky x-ray survey (0.1–2 keV). The simulations use a conservative estimate of the cluster luminosity function to create an x-ray catalog specifying cluster position, redshift, flux, temperature and angular extent. This is used in combination with realistic models of the ROSAT scanning motion and of the background due to particles, cosmic x-rays and scattered solar x-rays, to build a simulated ROSAT data base. The data base has been subjected to the ROSAT source search algorithms and the results analyzed. Two cases were considered, a 2-degree wide great circle strip representing the general survey, and a region within 10 degrees of the ecliptic pole, where the exposure is unusually high. We present estimates of the minimum number of clusters to be detected, the fraction recognized as extended x-ray sources, and the minimum detectable energy flux from clusters during the survey.