MOLECULES AT HIGH-REDSHIFT - THE EVOLUTION OF THE COOL PHASE OF PROTOGALACTIC DISKS

We study the formation of molecular hydrogen, after The epoch of reion ization, in the context of canonical galaxy formation theory due to hi erarchical clustering. There is an initial epoch of H-2 production in the gas phase through the H- route that ends at a redshift of order un ity. We assume that the fundamental units in the gas phase of protogal axies during this epoch are similar to diffuse clouds found in our own Galaxy, and we restrict our attention to protogalactic disks, althoug h some of our analysis applies to multiphase halo gas. Giant molecular clouds are not formed until lower redshifts. Star formation in the pr otogalactic disks can become self-regulated. The process responsible f or the feedback is the heating of the gas by the internal stellar radi ation field that can dominate the background radiation field at variou s epochs. If the gas is heated to above 2000-3000 K, the hydrogen mole cules are collisionally dissociated, and we assume that in their absen ce the star formation process is strongly suppressed because of insuff icient cooling. As we demonstrate by the analysis of phase diagrams, t he H-2-induced cool phase disappears. A priori, the cool phase with mo lecular hydrogen cooling can only achieve temperatures greater than or equal to 300 K. Consequently, it is possible to define a maximum star formation rate during this epoch. Plausible estimates give a rate of less than or similar to 0.2-2 M. yr(-1) for condensations correspondin g to 1 sigma and 2 sigma initial density fluctuations. For more massiv e structures, this limit is relaxed and in agreement with observations of high-redshift galaxies. Therefore, the production of metals and du st proceeds slowly in this phase. This moderate epoch is terminated by a phase transition to a cold, dense, and warm neutral/ionized medium once the metals and dust have increased to a level Z approximate to 0. 03-0.1 Z.. Then (1) atoms and molecules such as C, O, and CO become ab undant and cool the gas to below 300 K; (2) the dust abundance has bec ome sufficiently high to allow shielding of the molecular gas; and (3) molecular hydrogen formation can occur rapidly on grain surfaces. Thi s phase transition occurs at a redshift of approximately 1.5, with a f iducial range of 1.2 less than or equal to z less than or equal to 2, and initiates the rapid formation of molecular species, giant molecula r clouds, and stars. Consequently, the delayed initiation of the cold phase in the interstellar medium of protostellar disks at a metallicit y of Z less than or similar to 0.1 Z. is a plausible physical reason w hy the formation phase of the stellar disks of the bulk of the galaxie s occurs only at a redshift of order unity. The combination of feedbac k and a phase transition provides a natural resolution of the G-dwarf problem.