# The evolution of cold neutral gas and the star formation history

## TL;DR

This paper investigates how the fraction of cold neutral gas in the universe evolves at high redshift and its relation to star formation history, using 21-cm absorption data to extend previous studies beyond redshift 2.

## Contribution

It provides new insights into the evolution of cold neutral gas at z > 2 using archival 21-cm absorption data, revealing a steeper decline than star formation rate density.

## Key findings

- Cold gas fraction decreases steeply at z ~ 3
- Mean spin temperature is around 3000 K at high redshift
- Temperatures are consistent with high neutral hydrogen column densities

## Abstract

There is a well known disparity between the evolution the star formation rate density, {\psi}*, and the abundance of neutral hydrogen (HI), the raw material for star formation. Recently, however, we have shown that {\psi}* may be correlated with the fraction of cool atomic gas, as traced through the 21-cm absorption of HI. 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 {\psi}* and the cold gas fraction exhibit a similar steep climb from the present day (z = 0), and so it is unknown whether the cold gas fraction follows the same decline as {\psi}* at higher redshift. In order to address this, we have used unpublished archival observations of 21-cm absorption in high redshift damped Lyman-{\alpha} absorption systems to increase the sample at z 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 {\psi}* at z ~ 3. This is, however, degenerate with 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> ~ 3000 K, compared to the ~2000 K expected from the 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 ionised.