SMALL-GROUPS OF GALAXIES - A TOOL TO INVESTIGATE THE DYNAMICS OF THE UNIVERSE

We simulate the formation and the subsequent evolution of groups of ga laxies of mass 210(13)M., taking into account the secondary infall exp ected in a critical universe. We then discuss in terms of the Press & Schechter (PS) formalism the statistics of such groups and their numbe r evolution in different cosmological scenarios. From the N-body simul ations we find that merging is effective in building at least one cent ral remnant in a few crossing times soon after the collapse of the cen tral overdense region of the group which, in current cosmologies, occu rs at z similar to 1-0.35. Merging is only slightly accelerated if gal axies have massive halos, because the mass initially associated to the individual galaxies is soon tidally stripped. This process is particu larly effective for infalling galaxies, which easily avoid merging wit h the central remnants. We find that, as a rule, merging stops when in fall becomes dominant, so ending the so called ''merging runaway''. We then look for compact groups in our ensemble of simulations, and comp are their statistical properties with Hickson's compact groups. Our re sults show that compact group formation is an ongoing and frequent pro cess in a critical universe, where the presence of secondary infall is a necessary ingredient. In particular, our model reconciles the appar ent 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, secondary infall is strong ly suppressed, making it unlikely that compact groups survive until th e present time. We conclude that the existence of dense and dynamicall y young groups of galaxies like HCGs constitutes strong evidence that we live in a high-density universe.