On the origin of early type galaxies and the evolution of the interaction rate in the field

Using cosmological N-body simulations of critical (SCDM) and open (Ω=0.3, OCDM) cold dark matter models we have identified dark matter halos which are associated with the progenitors of present day bright early–type galaxies. By following their merging history, we show how early–type galaxies that formed within massive halos at redshift ' 3 are now preferentially residing in clusters and groups. On the other hand, those that formed through major merging events between redshift 1 and the present have not yet been accreted into larger, virialized structures. This result is in agreement with analytical predictions in hierarchical clustering models. CDM models are able to explain both the ancient and uniform population of ellipticals that dominates in clusters together with the more recent and heterogeneous population of field ellipticals. Predictions for the comoving number density of bright early–type galaxies are given, and are shown to be consistent with the observed luminosity function. We predict that the number density of interacting bright binary galaxies, from which the field population of ellipticals may have originated, is proportional to (1+z) and (1+z) in SCDM and OCDM respectively. This result is consistent with previous analytical estimates and is discussed together with recent observational constraints. Subject headings: galaxies: formation, ellipticals, cosmology: large scale structure 2Present address: Physics Department, Univ. of Durham, Science Labs, South Road, Durham, UK