from Blanchard et al.paper
from Blanchard et al.paper
This is probably an easy question to answer. In fact, there are several reasons why clusters of galaxies are regarded as useful probes of cosmology, including (i) since dynamical time-scale of clusters is comparable to the age of the universe, they should retain the cosmological initial condition fairly faithfully, (ii) clusters can be observed in various bands including optical, X-ray, radio, mm and submm bands, and in fact several on-going projects aim to make extensive surveys and detailed imaging/spectroscopic observations of clusters, (iii) to the first order, clusters are well approximated as a system of dark matter, gas and galaxies, and thus theoretically well-defined and relatively well-understood, at least compared with galaxies themselves, and (iv) on average one can observe a higher-$z$ universe with clusters than with galaxies. It is established that X-ray observations are particularly suited for the study of clusters since the X-ray emissivity is proportional to $n_e^2$ and thus less sensitive to the projection contamination which has been known to be a serious problem in their identifications with the optical data. Also the recent progress of interferometric mapping technique of clusters via the Sunyaev-Zel'dovich effect enables one to observe the high-redshift clusters without suffering from the cosmological dimming propto (1+z)^{-4}.
A. BLANCHARD, J. BARTLETT and R. SADAT .ps . pd
Clusters constitute one major source of information for cosmology. In the present paper we discuss their implication for the determination of the density parameter of Universe Omega_0. We concentrate on two global tests, which are expected to provide robust estimates, contrary to the local, possibly biased, classical dynamical argument. The first method uses the evolution of the abundance of clusters with redshift (Oukbir and Blanchard 1992), the second one make use of the baryon fraction in clusters (white et al. 1993). The former method needs reliable estimates of the local temperature distribution function as well as at high redshift (z geq 0.3). The latter can provide a direct estimate of Omega_0 provided that we have a reliable estimate of the baryon content of clusters and a good estimate of Omega_b from primordial nucleosynthesis. Although it has become fashionable to claim that these tests lead to low values for the density parameter of universe, our most recent analyses lead to rather convergent values Omega_0 sim 0.8-0.9, and are nicely consistent with what can be inferred from CMB data.
PETER SCHUECKER et al. .ps .pdf
The power spectrum is measured on scales from 15 to 800,h^{-1},{ Mpc} using the ROSAT-ESO Flux-Limited X-Ray (REFLEX) galaxy cluster catalogue. The REFLEX survey provides a sample of the 452 X-ray brightest southern clusters of galaxies with the nominal flux limit S=3.0times 10^{-12},{ erg},{ s}^{-1},{ cm}^{-2} for the ROSAT energy band (0.1-2.4),keV. The most important result is the detection of a broad maximum within the comoving wavenumber range 0.022le kle 0.030,h,{ Mpc}^{-1}. A semi-analytic description of the biased nonlinear power spectrum in redshift space gives the best agreement for low-density Cold Dark Matter models with or without a cosmological constant. A more detailed description of the results will be presented in Astronomy & Astrophysics.
S. BORGANI1 and P. ROSATI .ps .pdf
E. SALVADOR-SOLE and A. MANRIQUE .ps .pdf
We present a model of hierarchical galaxy formation in the spirit of the popular semianalytical models (SAMs) developed in the last decade by various groups. The new model is particularly handy, since fully analytic. It is based on a modified version of the Press-Schechter clustering model distinguishing between gentle accretion and major mergers which allows us to accurately derive the properties of newborn halos and their baryonic content from those of their progenitors just at the time of the merger, and then monitor the evolution of these properties during the accretion phase.
A.P. FAIRALL, and P.A. WOUDT .ps .pdf
It is generally accepted that clusters of galaxies mark the peaks of the mass density field. However, the mapping of nearby clusters, and their associated large-scale structures, is incomplete due to the obscuration of the Milky Way. Because of the preponderance of E/SO galaxies in clusters, optical searches have proved more successful than HI/IRAS surveys in detecting clusters in the ``Zone of Avoidance''. We report here on a number of clusters we have located in our survey of the southern Milky Way. They include ACO 3627 and other clusters believed to mark peaks in the``Great Attractor'' region, the most influential of the nearby large-scale structures.
We use the two-point correlation function in redshift space, xi(s), to study the clustering of the galaxies and groups of the Nearby Optical Galaxy (NOG) Sample, which is a nearly all-sky, complete, magnitude-limited sample of sim 7000 bright and nearby optical galaxies. The correlation function of galaxies is well-described by a power-law, xi(s)= (s/s_0)^{-gamma}, with gamma sim 1.5 and s_0sim 6.4; h^{-1} Mpc. We find evidence of morphological segregation between early- and late-type galaxies, with a gradual decreasing of the strength of clustering from the S0 to the late-type spirals, on intermediate scales. Furthermore, luminous galaxies (with M_Bleq -19.5 + 5 log h) are more clustered than dim galaxies. The groups show an excess of clustering with respect to galaxies. Groups with greater velocity dispersions, sizes, and masses are more clustered than those with lower values of these quantities.
E. HATZIMINAOGLOU , G. MATHEZ and A. MANRIQUE .ps .pdf
The growth of Super Massive Black Holes and the parallel development of activity in galactic nuclei are implemented in an analytic code of hierarchical clustering. The evolution of the luminosity function of quasars and AGN will be computed with special attention paid to the connection between quasars and Seyfert galaxies. One of the major interests of the model is the parallel study of quasar formation and evolution and the History of Star Formation.
ERIC TITTLEY AND MARK HENRIKSEN .ps .pdf
Gas residing in filaments in the large-scale structure of the universe may contribute a significant component to the total baryon budget of the universe as well as provide a supply of gas to nearby galaxy cluster halos. However, the gas will be difficult to observe if it has a low temperature or density. Filament gas may be easier to observe if the gas is viewed aligned along the line of sight, increasing the column density. Such alignments of filaments may have already been observed and mis-classified as interacting or merging systems of clusters of galaxies. Presented here is a candidate for a filamentary structure that is aligned along the line of sight. ASCA data reveals intercluster emission within this structure.
Most of the baryons in the present-day universe are thought to reside in intergalactic space at temperatures of $10^{6-7}$~K. X-ray emission from these baryons contributes a modest (10\%-20\%) fraction of the sim 1 ~keV background whose prominence depends on the amount of non-gravitational energy injected into intergalactic space by supernovae and AGNs. Because the virialized regions of groups and clusters cover a large percentage of the sky, observing the baryons belonging to individual intercluster filaments may prove quite difficult. However, information about the global properties of the intergalactic medium is contained in the surface-brightness distribution of the diffuse X-ray background.