The ultraviolet Lyman and Werner absorption lines of H2 have been searched for in a number of high redshift quasar spectra, and detected unambiguously in at least 3 systems at redshifts z∼2. The lack of detectable H2 in most absorbers results from the strong selection in quasar studies against lines-of-sight with significant dust extinction. At high redshift, the ultraviolet radiation field is inferred to be higher than that observed in the local solar neighborhood, suggesting that vigorous star-formation is underway in these galaxies.

Introduction

Recent observations of the high redshift Universe, interpreted in the context of a new generation of computer simulated model Universes, are providing a clear picture of how large galaxies like the Milky Way formed. A number of different observations suggest that large galaxies were assembled from what appear at z = 2 – 3 to be several star-forming proto-galactic fragments (PGF's), widely distributed in space (Windhorst et al. 1994, Pascarelle et al. 1996ab, 1998; Steidel et al. 1996ab, Bechtold et al. 1998). Computer simulations suggest that initially small clumps of material collapsed at the intersection of sheets and filaments in the intergalactic medium, and began forming stars, and that eventually these clumps merged to form large galaxies (Haehnelt, Steinmetz & Rauch 1998, Steinmetz 1998 and references therein). Searches for the galaxies associated with damped Ly-α quasar absorbers show that at z ∼ 2 they are the same population of objects seen in the Hubble Deep Field faint galaxies and the Lyman dropout galaxies (Steidel et al. 1996ab; Bechtold et al. 1998).