Involved people at OAB: Bardelli, Zamorani, Zucca.
Another long term project in which Observatory staff members are involved is the multiwavelength study of the Shapley Concentration, the richest supercluster in the nearby Universe. This study is devoted to investigate the effect of the environment and of the merging phenomena on the physics of clusters and on their galaxy population. In particular, the central part of superclusters are ideal laboratories where to study dynamical processes, given the higher peculiar velocities induced by the density excess.
A redshift sample of 
galaxies has been obtained in the central
part of the Shapley Concentration, both in clusters and in the inter-cluster
field, allowing a detailed analysis of the properties of this supercluster.
On large scales, the geometry, extension, mass and overdensity of the structure
were determined, allowing a comparison with the theoretical predictions of
different galaxy formation scenarios: the existence of the
Shapley Concentration is consistent only with 
CDM or open CDM models
(Bardelli et al. 2000, MNRAS 312, 540).
Dynamical properties of single clusters in this region were derived (Bardelli et al. 1998, MNRAS 300, 589), as well as their luminosity functions and masses (Bardelli et al. 1998, MNRAS 296, 599): the general conclusion is that the core of the Shapley Concentration is dynamically active, presenting clusters in late stage of merging, as confirmed also by the presence of hot gas filaments between clusters (Bardelli et al. 1996, A&A 305, 435) and of an excess of blue galaxies in the expected position of the shock front.
Moreover, a large field survey at radio wavelengths (with the ATCA and VLA telescopes) has been performed, in order to study the effect of merging on the radio emission of galaxies. The result is that merging ``switched off" radio sources, causing a lack of radio galaxies (Venturi, Bardelli et al. 2000, MNRAS 314, 594). In order to investigate the link between radio and X-ray properties in merging clusters, ROSAT and SAX maps were analyzed (Ettori, Bardelli et al. 2000, MNRAS in press) and the physical parameters (temperature, gas distribution and metallicity) of the intercluster medium were derived. The X-ray analysis will be improved by further SAX and XMM observations time already allocated).
Finally, the effect of cluster merging on the galaxy population has been extensively investigated by studying the behavior of the morphology, the [OII] equivalent width and the star formation rate as a function of local density (Bardelli, Zucca & Baldi 2000, MNRAS submitted; Baldi, Bardelli & Zucca, in preparation).