HI-shielding of ${\rm H_2}$ in UV-irradiated protogalaxies: suppression of the photodissociation rate

Abstract

We study the impact of neutral hydrogen absorption on ${\rm H_2}$ photodissociation in protogalactic haloes exposed to soft-UV radiation. Lyman-series absorption can significantly deplete dissociating photons as line overlap with the ${\rm H_2}$ Lyman-Werner bands occurs for neutral column densities exceeding $10^{22}$ ${\rm cm^{-2}}$, but this effect has not been previously included in studies of protogalactic haloes. We use high-resolution three-dimensional hydrodynamic simulations to investigate this "HI-shielding" in three metal-free atomic cooling haloes collapsing at redshift $z \sim 10-20$. We use CLOUDY modeling to update a previous fitting formula for HI-shielding which is a better model for shielding of non-ground state ${\rm H_2}$ rovibrational populations and implement the new fit in our simulations. We find that the inclusion of HI-shielding increases the "critical flux" for suppression of ${\rm H_2}$ cooling in these haloes by $\sim 60-100$ per cent. The larger critical flux has implications in particular for the predicted numbers of candidate haloes in which "direct collapse" could seed massive ($\sim 10^5$ ${\rm M_\odot}$) black holes at $z \sim 15$.