Since the appearance of the widely used model proposed by Press and Schechter (1974), much theoretical work was devoted to the study of the mass function of cosmic structures. In fact, the mass function can provide constraints on the cosmological model of the large scale structure of the Universe. Groups of galaxies may represent an example of cosmic structures quite suitable for this analysis. In fact they have been observed by many authors and many catalogues of data are now available. Moreover, the estimation of the mass of each group is not too uncertain since many redshifts of group members are available in new catalogues. The first estimate of cosmological parameters using groups of galaxies is due to Gott and Turner (1977) who used their group catalogue (Turner and Gott 1976) to estimate the index n of the power spectrum of primordial density fluctuations. Now a much richer and more accurate amount of data is available so that the estimation of the group mass function and the comparison with theoretical models can be performed on more solid observational grounds. I consider five different group catalogues available in the literature: Geller and Huchra (1983); Tully (1987b ); Vennik (1984); Ramella, Geller and Huchra (1989) and Maia, da Costa and Latham (1989). The features of each catalogue are analysed in detail and particular attention is paid to the effect of observational biases. Several mass estimators proposed in the literature are considered and the dependence of the derived mass function on the estimator used is tested. In the literature it is commonly assumed that groups have reached a stationary dynamical equilibrium so that the conditions required by the virial theorem hold. This assumption is tested for the groups of each catalogue using a general method whose main features are described. Finally the set of all five group catalogues is tested in order to determine whether they give homogeneous distributions not only for the mass but also for all the main physical parameters of galaxy groups. The main results I obtained can be outlined in the following points: • In each catalogue all mass estimators yield nearly the same mass distribution, so that all considered mass estimators seem to be homogeneous. • In each catalogue the groups are likely to be in a phase characterized by strong dynamical evolution and only a small fraction of the observed groups ( f'V 103) have reached the virial equilibrium; hence, I have tried to correct the mass of each group in order to account for the non-virialized dynamical state. • The set of five catalogues analysed is not homogeneous not only with respect to the mass distribution but also with respect to the distributions of all main physical parameters of groups. • The group catalogue property mainly responsible for the detected inhomogeneity seems to be the group identification algorithm, although other features of catalogues may play a non-negligible role. • The analysis to groups within 20 M pc from us is also considered in order to reduce the observational bias that seems to affect all the catalogues; the main results, concerning the inhomogeneity of catalogues, the stability against various mass estimators and the dynamical state of groups, do not change; hence, it seems that the presence of observational bias does not significantly affect the results obtained. These results suggest the need of a new definition for groups of galaxies and consequently the introduction of a new identification algorithm that could possibly overcome the inhomogeneity shown by present catalogues. A useful tool to test new identification algorithms can be provided by numerical simulations of galaxy clustering. The mass distribution functions that I have obtained from the various catalogues can be profitably compared with theoretical predictions (e.g. Press and Shechter, 197 4) in order to extract valuable constraints on the cosmological models of the formation and development of the large scale structure of the universe.
|Titolo:||The Mass Distribution Function of Groups of Galaxies|
|Data di pubblicazione:||4-dic-1990|
|Appare nelle tipologie:||8.1 PhD thesis|