Evolution of the cold gas fraction and the star formation history: Prospects with current and future radio facilities

TL;DR

This paper investigates whether the cold neutral gas fraction follows the star formation history across cosmic time, using 21-cm radio observations and assessing the capabilities of current and future radio telescopes.

Contribution

It analyzes the evolution of the cold gas fraction and evaluates the potential of upcoming radio facilities like the SKA to study this at high redshifts.

Findings

Current radio telescopes can probe cold gas at z < 3.5.

The SKA is necessary to explore higher redshifts.

Cold gas fraction appears to follow star formation rate evolution.

Abstract

It has recently been shown that the abundance of cold neutral gas may follow a similar evolution as the star formation history. This is physically motivated, since stars form out of this component of the neutral gas and if the case, would resolve the longstanding issue that there is a clear disparity between the total abundance of neutral gas and star forming activity over the history of the Universe. Radio-band 21-cm absorption traces the cold gas and comparison with the Lyman-alpha absorption, which traces all of the gas, provides a measure of the cold gas fraction or the spin temperature. The recent study has shown that the spin temperature (degenerate with the ratio of the absorber/emitter extent) appears to be anti-correlated with the star formation density, undergoing a similar steep evolution as the star formation rate over redshifts of 0 < z < 3, whereas the total neutral hydrogen exhibits little evolution. Above z > 3, where the SFR shows a steep decline with redshift, there is insufficient 21-cm data to determine whether the spin temperature continues to follow the SFR. Knowing this is paramount in ascertaining whether the cold neutral gas does trace the star formation over the Universe's history. We explore the feasibility of resolving this with 21-cm observations of the largest contemporary sample of reliable damped Lyman-alpha absorption systems and conclude that, while today's largest radio interferometers can reach the required sensitivity at z < 3.5, the Square Kilometre Array is required to probe to higher redshifts.