  Unlike the neutral gas density, which remains largely constant over redshifts
of 0 < z < 5, the star formation density exhibits a strong redshift dependence,
increasing from the present day before peaking at a redshift of z ~ 2.5. Thus,
there is a stark contrast between the star formation rate and the abundance of
raw material available to fuel it. However, using the ratio of the strength of
the HI 21-cm absorption to the total neutral gas column density to quantify the
spin temperature of the gas, it has recently been shown that its reciprocal may
trace the star formation density. This would be expected on the grounds that
the cloud of gas must be sufficiently cool to collapse under its own gravity.
This, however, relies on very limited data and so here we explore the potential
of applying the above method to absorbers for which individual column densities
are not available (primarily MgII absorption systems). By using the mean value
as a proxy to the column density of the gas at a given redshift, we do, again,
find that 1/T (degenerate with the absorber-emitter size ratio) traces the SF
density. If confirmed by higher redshift data, this could offer a powerful tool
for future surveys for cool gas throughout the Universe with the Square
Kilometre Array.

Astrophysics of Galaxies;Cosmology and Nongalactic Astrophysics