A possible connection between the spin temperature of damped Lyman-alpha
  absorption systems and star formation history
  We present a comprehensive analysis of the spin temperature/covering factor
degeneracy, T/f, in damped Lyman-alpha absorption systems. By normalising the
upper limits and including these via a survival analysis, there is, as
previously claimed, an apparent increase in T/f with redshift at z > 1.
However, when we account for the geometry effects of an expanding Universe,
neglected by the previous studies, this increase in T/f at z > 1 is preceded by
a decrease at z < 1. Using high resolution radio images of the background
continuum sources, we can transform the T/f degeneracy to T/d^2, where d is the
projected linear size of the absorber. Again, there is no overall increase with
redshift, although a dip at z ~ 2 persists. Furthermore, we find d^2/T to
follow a similar variation with redshift as the star formation rate. This
suggests that, although the total hydrogen column density shows little relation
to the SFR, the fraction of the cold neutral medium may. Therefore, further
efforts to link the neutral gas with the star formation history should also
consider the cool component of the gas.
