Molecular Hydrogen Formation on Dust Grains in the High Redshift Universe

Received ; accepted – 2 – ABSTRACT We study the formation of molecular hydrogen on dust grain surfaces and apply our results to the high redshift universe. We find that a range of physical parameters, in particular dust temperature and gas temperature, but not so much dust surface composition, influence the formation rate of H 2. The H 2 formation rate is found to be suppressed above gas kinetic temperatures of a few hundred K and for dust temperatures above 200-300 K and below 10 K. We highlight the differences between our treatment of the H 2 formation process and other descriptions in the literature. We also study the relative importance of H 2 formation on dust grains with respect to molecular hydrogen formation in the gas phase, through the H − route. The ratio of formation rates of these two routes depends to a large part on the dust abundance, on the electron abundance, and also on the relative strength of the FUV (extra-)galactic radiation field. We find that for a cosmological evolution of the star formation rate and dust density consistent with the Madau plot, a positive feedback effect on the abundance of H 2 due to the presence of dust grains can occur at redshifts z ≥ 3. This effect occurs for a dust-togas mass ratio as small as 10 −3 of the galactic value.