Modelling intergalactic low ionisation metal absorption line systems near the epoch of reionization

Abstract

We interpret observations of intergalactic low ionisation metal absorption systems at redshifts z $\gtrsim$5 in terms of pressure-confined clouds. We find clouds confined by the expected pressure of galactic haloes with masses $11<\log M_h/h^{-1}M_\odot<12$ provide a good description of the column density ratios between low ionisation metal absorbers. Some of the ratios, however, require extending conventional radiative transfer models of irradiated slabs to spherical (or cylindrical) clouds to allow for lines of sight passing outside the cores of the clouds. Moderate depletion of silicon onto dust grains is also indicated in some systems. The chemical abundances inferred span the range between solar and massive-star dominated stellar populations as may arise in starburst galaxies. The typical HI column densities matching the data correspond to Damped Lyman-$\alpha$ Absorbers (DLAs) or sub-DLAs, with sizes of 40 pc to 3 kpc, gas masses $3.5<\log M_c/M_\odot<8$ and metallicites $0.001-0.01Z_\odot$. Such systems continue to pose a challenge for galaxy-scale numerical simulations to reproduce.