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In this Thesis different observational and theoretical aspects related to the dynamics of galaxy clusters have been addressed, with particular attention to the characterization of substructures inside them. The Thesis has been organized into four major parts. In the first part, the astrophysical context is presented, with a description of the main properties of galaxy clusters and focus on the observational signatures of clusters mergers and subclustering. The use of optical data is emphasized, but radio and X-ray properties are discussed as well. In the second part, the WINGS project, a wide-field, multiwavelength imaging and spectroscopic survey of galaxies in 77 nearby clusters, is introduced. In particular, the measurements of redshifts for galaxies in 47 nearby clusters belonging to the WINGS sample are presented. More than four thousand galaxy redshift determinations, out of the 6000 presented, resulted to be completely new, increasing significantly the statistics. Literature data are then used to complete the WINGS-spe catalog, in order to increase the number of redshift measurements available for the analyses presented in the following chapters. In the third part of the Thesis, the complete data-set of redshifts is analysed. The procedure used to select cluster members is outlined and the global redshift distributions and the velocity diagrams for the whole WINGS sample are given. The procedure for the virial analysis is described and, the density, velocity dispersion and mean velocity radial profiles are analysed. NFW models turn out to be an adequate representation of the spatial distribution of galaxies in clusters in 86% of the clusters. Clusters with strong evidence of substructures, are generally those for which the NFW fit to the observed profiles is worse. The last part of the Thesis is devoted to the topic of substructure analysis inside the WINGS clusters. First, dfferent methods of analysis are considered (DEDICA, Dressler and Shectman, and Friends-of-Friends), then the maps of substructures inside clusters, obtained with each method, are presented and the results obtained by the different approaches are compared. A high level of substructuring (higher than generally found in previous investigations) in nearby galaxy clusters is found, indicative of a non-relaxed dynamical condition. It thus appears that, even at current epochs, most galaxy clusters are not fully virialized, in the process of formation by accretion of small groups.

233 pages