Evolution of HI from z = 5 to the Present

Abstract

Studies of damped Lyα systems provide us with a good measure of the evolution of the HI column density distribution function and the contribution to the comoving mass density in neutral gas out to redshifts of z = 5. The column density distribution function at high redshift steepens for the highest column density HI absorbers, though the contribution to the comoving mass density of neutral gas remains flat from 2 < z < 5. Results from studies at z < 2 are finding substantial numbers of damped absorbers identified from Mg II absorption, compared to previous blind surveys. These results indicate that the contribution to the comoving mass density in neutral gas may be constant from z ∼ 0 to z ∼ 5. Details of recent work in the redshift range z < 2 work is covered elsewhere in this volume (see D. Nestor). We review here recent results for the redshift range 2 < z < 5.