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It has recently been shown that the abundance of cold neutral gas may followa similar evolution as the star formation history. This is physicallymotivated, since stars form out of this component of the neutral gas and if thecase, would resolve the longstanding issue that there is a clear disparitybetween the total abundance of neutral gas and star forming activity over thehistory of the Universe. Radio-band 21-cm absorption traces the cold gas andcomparison with the Lyman-alpha absorption, which traces all of the gas,provides a measure of the cold gas fraction or the spin temperature. The recentstudy has shown that the spin temperature (degenerate with the ratio of theabsorber/emitter extent) appears to be anti-correlated with the star formationdensity, undergoing a similar steep evolution as the star formation rate overredshifts of 0 < z < 3, whereas the total neutral hydrogen exhibits littleevolution. Above z > 3, where the SFR shows a steep decline with redshift,there is insufficient 21-cm data to determine whether the spin temperaturecontinues to follow the SFR. Knowing this is paramount in ascertaining whetherthe 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 thelargest contemporary sample of reliable damped Lyman-alpha absorption systemsand conclude that, while today's largest radio interferometers can reach therequired sensitivity at z < 3.5, the Square Kilometre Array is required toprobe to higher redshifts.

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