Early Hierarchical Formation of Massive Galaxies Triggered by Interactions

To address the problem concerning the early formation of stars in massivegalaxies, we present the results of a semi-analytic model of galaxy formationwhich includes a physical description of starbursts triggered by galaxyinteractions. These originate from the destabilization of cold galactic gasoccurring in galaxy encounters, which in part feeds the accretion onto blackholes powering quasars, and in part drives circumnuclear starsbursts atredshifts $z\approx 2-4$, preferentially in massive objects. This speeds up theformation of stars in massive galaxies at high redshifts without altering it inlow mass galactic halos. Thus, at intermediate $z\approx 1.5-2$ we find that aconsiderable fraction of the stellar content of massive galaxies is already inplace, at variance with the predictions of previous hierarchical models. Theresulting high-$z$ star formation rate and B-band luminosity functions, and theluminosity and redshift distribution of galaxies in K-band at $z\lesssim 2$ areall in good agreement with the existing observations concerning the brightgalaxy population.Comment: 7 pages, accepted for publication in Ap