CHRISTOPHE ADAMI et al. .ps .pdf
We present the results of a comprehensive set of simulations designed to quantify the selection function of the Bright SHARC survey (Romer et al. 2000a) for distant clusters. The statistical signi cance of the simulations relied on the creation of thousands of arti cial clusters with redshifts and luminosities in the range 0:25 < z<0:95 and 0:5 < L_X < 10 10 44 erg s
Given that a galaxy system must be more massive than its constituent galaxies, the velocity dispersion of a virialized group or cluster of galaxies must be greater than some combination of velocity dispersions of its ellipticals and rotation velocities of its spiral members. Low velocity dispersion systems could either be chance alignments of galaxies within larger systems near cosmological turnaround or alternatively dense systems in the final stages of global coalescence. I compute the minimum velocity dispersion using a stricter criterion than the cosmological M ~ sv3 relation. I impose that the mass of the system, within any radius that is much larger than the typical inter-galaxy separation is greater than the sum of the masses of its constituent galaxies - if they were isolated - out to that same radius. Adopting the mass profiles that Navarro et al. obtained for structures in cosmological simulations, the minimum velocity dispersion of a group of reasonably massive galaxies obeys (sv)group2 \ge 0.5 åE (sv)E2 + 0.14 åD vrot2, where the sums are over ellipticals (E) and disks (D), and where vrot is the maximum deprojected rotational velocity of a disk galaxy. While internal kinematics data are currently too sparse to reach any statistically significant conclusions, scaling relations of galaxy luminosities with internal kinematics lead to a second formula for the minimum group velocity dispersion that turns out to be a good predictor, in a statistical sense, of the group velocity dispersion. Only 3 groups (HCG 38, 47 and 88) have statistically significant low velocity dispersions. Finally, analyzing the few compact groups with more secure velocity dispersions obtained through the inclusion of faint galaxies and the immediate group environment, leads to the conclusion that at least one-quarter of Hickson compact groups (including HCG 22, 42, and 88) have significantly low velocity dispersions, meaning that they are either caused by chance alignments of galaxies along the line of sight or alternatively in their final stages of coalescence.
SANDRO BARDELLI & ELENA ZUCCA
Rich superclusters are the ideal environment for the detection of cluster mergings, because the high peculiar velocities induced by the enhanced local density of the large-scale structure favour the cluster-cluster collisions, in the same way as seen in the simulations. The Shapley Concentration supercluster represents a unique laboratory where it is possible to follow cluster mergings and to test related astrophysical consequences, as the formation of shocks, radio halos, relics and wide angle tail radiosources, and the presence of galaxies with enhanced star formation. We present the results of an extensive multiwavelength survey of the central part of the Shapley Concentration, with the use of optical spectra, radio and X-ray data.
P. FLIN, J. KRYWULT
From Digitised Sky Survey we extracted regions containing 89 Abell clusters of galaxies and applied to the positions of galaxies the method based on the wavelet transform in a search for the presence of subclustering. This study has shown that substructures are present in, at least, 31 percent of cases.
CECILE FERRARI et al.
We present new results of a combined optical /X-Ray analysis of the galaxy cluster Abell 521. New data both in X-Ray (Chandra observation) , and in optical (multi-object spectroscopy) are well explained by the scenario of Arnaud et.al (2000) which suggests A521 has endeavored two phases of merging and is currently forming at the crossing of two filaments.
ELENA ZUCCA & SANDRO BARDELLI
We present the results of a redshift survey of both cluster and intercluster galaxies in the central part of the Shapley Concentration, the richest supercluster of clusters in the nearby Universe, consisting of ~ 2000 radial velocities. We estimate the total overdensity in galaxies of the supercluster and its mass and we discuss the cosmological implications of these results. Moreover, using a Principal Components Analysis technique, we study the influence of the cluster and supercluster dynamics on the galaxy spectral morphology.