2019MNRAS.484.3911C

There is a well-known disparity between the evolution of the star formation rate density, ψ_*, and the abundance of neutral hydrogen (H I), the raw material for star formation. Recently, however, we have shown that ψ_* may be correlated with the fraction of cool atomic gas, as traced through the 21-cm absorption of H I. This is expected since star formation requires cold (T ∼ 10 K) gas and so this could address the issue of why the star formation rate density does not trace the bulk atomic gas. The data are, however, limited to redshifts of z <= 2, where both ψ_* and the cold gas fraction exhibit a similar steep climb from this day (z = 0), and so it is unknown whether the cold gas fraction follows the same decline as ψ_* at higher redshift. In order to address this, we have used unpublished archival observations of 21-cm absorption in high redshift damped Lyman α absorption systems to increase the sample at z >= 2. The data suggest that the cold gas fraction does exhibit a decrease, although this is significantly steeper than ψ_* at z ∼ 3. This is, however,degeneratewith the extents of the absorbing galaxy and the background continuum emission and upon removing these, via canonical evolution models, we find the mean spin temperature of the gas to be <T_spin> ≃ 3000 K, compared to the ≃2000 K expected from the ψ_*T_spin ≃ 100 M_☉ yr^–1 Mpc^–3 K fit at z <= 2. These temperatures are consistent with the observed high neutral hydrogen column densities, which require T <= 4000 K in order for the gas not to be highly ionized.