TIDALLY TRIGGERED GALAXY FORMATION .1. EVOLUTION OF THE GALAXY LUMINOSITY FUNCTION

Motivated by accumulating evidence that large-scale galactic star formation is initiated and sustained by tidal interactions, we develop a phenomenological model for the galaxy luminosity function, commencing from a galaxy mass function that is predicted by a hierarchical model of structure formation such as the cold dark matter dominated cosmology. The epoch of luminous galaxy formation and the galactic star-formation rate are determined by the environment. Gas cooling and star-formation feedback are incorporated, and we are able to satisfactorily reproduce the present epoch luminosity function of bright galaxies and the distribution of galaxy colors. Biasing, via the preferential formation of luminous galaxies in denser regions associated with groups or clusters, is a natural outcome of our tidally triggered star-formation model. A large frequency is inferred of "failed" galaxies, prematurely stripped by supernova-driven winds, that populate groups and clusters in the form of low surface brightness gas-poor dwarfs, and of "retarded" galaxies, below the threshold for effective star formation, in the field, detectable as gas-rich, extremely low surface brightness objects. Predictions are presented for the evolution with redshift of the distribution of characteristic star formation time-scales, galaxy ages, and colors. We also make estimates of galaxy number counts, and suggest that dwarf galaxies undergoing bursts of star formation at z approximately 1 may dominate the counts at the faintest magnitudes.