Molecular hydrogen formation on dust grains in the high-redshift universe

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 - influences the formation rate of H ₂. The H₂ formation rate is found to be suppressed above gas kinetic temperatures of a few hundred K and for dust temperatures above 500 K and below 10 K. We highlight the differences between our treatment of the H₂ formation process and other descriptions in the literature. We also study the relative importance of H₂ formation on dust grains with respect to molecular hydrogen formation in the gas phase, through the H^- route. The ratio of the formation rates of these two routes depends to a large part on the dust abundance, on the electron abundance, and on the relative strength of the far-ultraviolet (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 H₂ due to the presence of dust grains can occur at redshifts z ≥ 3. This effect occurs for a dust-to-gas mass ratio as small as 10^-3 of the galactic value.