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For refcode 1996ApJ...458...18G:
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1996ApJ...458...18G SMALL GROUPS OF GALAXIES: A CLUE TO A CRITICAL UNIVERSE F. GOVERNATO, P. TOZZI, AND A. CAVALIERE II Universita di Roma, via della Ricerca Scientifica 1, I-00133 Roma, Italy Received 1995 January 12; accepted 1995 August 18 ABSTRACT We study the formation and the subsequent evolution of galaxy groups with mass of order 2 x 10^13^ M_sun_ in a critical universe and show the importance of secondary infall for their dynamical evolution. From N-body simulations we find that merging is effective in building at least one massive central remnant in a few crossing times soon after the collapse of the central region of the group. Merging is only slightly accelerated if galaxies have massive halos, because the mass initially associated with the individual galaxies is soon tidally stripped. Stripping is particularly effective for infalling galaxies, which thus easily avoid merging with the central remnants. We find that, as a rule, merging is effectively terminated, and the "merging runaway" ends when infall becomes dominant. We look for compact groups in our ensemble of simulations and compare their statistical properties with Hickson's compact groups (HCGs). We then discuss in terms of the Press & Schechter formalism the statistics of such groups and their evolution in different cosmological scenarios. Our results show that compact group formation is an ongoing and frequent process in a critical universe. In particular, our model reconciles the apparent contradiction between the observed absence of young-looking merger remnants and the high rate of strong galaxy interactions expected in compact groups. In open universes, instead, earlier formation of groups and suppression of secondary infall makes it more unlikely that compact groups survive as such until the present time. We conclude that the existence of dense and dynamically young groups of galaxies like HCGs concurs with other dynamical evidence pointing toward a high-density or a flat universe. Subject headings: cosmology: theory galaxies: clusters: general - galaxies: interactions
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