Molecules at High Redshift: The Evolution of the Cool Phase of Protogalactic Disks

Abstract

We study the formation of molecular hydrogen, after the epoch of reionization, in the context of canonical galaxy formation theory due to hierarchical clustering. There is an initial epoch of H2 production in the gas phase through the H- route that ends at a redshift of order unity. We assume that the fundamental units in the gas phase of protogalaxies during this epoch are similar to diffuse clouds found in our own Galaxy, and we restrict our attention to protogalactic disks, although some of our analysis applies to multiphase halo gas. Giant molecular clouds are not formed until lower redshifts. Star formation in the protogalactic disks can become self-regulated. The process responsible for the feedback is the heating of the gas by the internal stellar radiation field that can dominate the background radiation field at various epochs. If the gas is heated to above 2000--3000 K, the hydrogen molecules are collisionally dissociated, and we assume that in their absence the star formation process is strongly suppressed because of insufficient cooling. As we demonstrate by the analysis of phase diagrams, the H2-induced cool phase disappears. A priori, the cool phase with molecular hydrogen cooling can only achieve temperatures >=300 K. Consequently, it is possible to define a maximum star formation rate during this epoch. Plausible estimates give a rate of