The Nature of High-Redshift Galaxies

Using semi-analytic models of galaxy formation, we investigate the properties of high redshift galaxies ($z\sim3$) and compare them with the observed population of Lyman-break galaxies (LBGs). In addition to the usual quiescent mode of star formation, we introduce a physical model for starbursts triggered by galaxy-galaxy interactions. We find that with the merger rate that arises naturally in the merging hierarchy in CDM-type models, a significant fraction of bright galaxies identified at high redshift ($z \ga 2$) may be low-mass, bursting satellite galaxies within massive dark matter halos. The abundance of bright galaxies as a function of redshift and the luminosity function at $z\sim3$ both appear to be in better agreement with the data when the starburst mode is included, especially when the effects of dust are considered. The objects that we identify as ``LBGs'' have star formation rates, half-light radii, I$-$K colours, and internal velocity dispersions that are in good agreement with the available data. We also investigate global quantities such as the star formation rate density and cold gas and metal content of the Universe as a function of redshift. Based on the combined constraints from these observations, we propose a scenario in which quiescent star formation at $z\ga2$ is relatively inefficient, and most of the observed LBGs are starbursts. We argue that this ``bursting satellite'' scenario can resolve the apparent paradox of LBGs that cluster like massive dark matter halos but have narrow line-widths