Cosmological Implications of the Uncertainty in H−Destruction Rate Coefficients

Abstract

In primordial gas, molecular hydrogen forms primarily through associative detachment of H- and H, thereby destroying the H-. The H- anion can also be destroyed by a number of other reactions, most notably by mutual neutralization with protons. However, neither the associative detachment nor the mutual neutralization rate coefficients are well determined: both may be uncertain by as much as an order of magnitude. This introduces a corresponding uncertainty into the H2 formation rate, which may have cosmological implications. Here, we examine the effect that these uncertainties have on the formation of H2 and the cooling of protogalactic gas in a variety of situations. We show that the effect is particularly large for protogalaxies forming in previously ionized regions, affecting our predictions of whether or not a given protogalaxy can cool and condense within a Hubble time, and altering the strength of the ultraviolet background that is required to prevent collapse.