HI content, metallicities and spin temperatures of damped and sub-damped
  Lyman alpha systems in the redshift desert (0.6 < z < 1.7)
  The HI 21cm absorption optical depth and the N(HI) derived from Lya
absorption can be combined to yield the spin temperature (Ts) of DLAs. Although
Ts measurements exist for samples of DLAs with z <0.6 and z >1.7, the
intermediate redshift regime currently contains only 2 HI 21cm detections,
leading to a `redshift desert' that spans 4 Gyrs of cosmic time. To connect the
low and high z regimes, we present observations of the Lya line of six
0.6<z<1.7 HI 21cm absorbers. The dataset is complemented by both VLBA
observations (to derive the absorber covering factor, f), and optical echelle
spectra from which metal abundances are determined. Our dataset therefore not
only offers the largest statistical study of HI 21cm absorbers to date, and
bridges the redshift desert, but is also the first to use a fully f-corrected
dataset to look for metallicity-based trends. In agreement with trends found in
Galactic sightlines, we find that the lowest N(HI) absorbers tend to be
dominated by warm gas. In the DLA regime, spin temperatures show a wider range
of values than Galactic data, as may be expected in a heterogenous galactic
population. However, we find that low metallicity DLAs are dominated by small
cold gas fractions and only absorbers with relatively high metallicities
exhibit significant fractions of cold gas. Using a compilation of HI 21cm
absorbers which are selected to have f-corrected spin temperatures, we confirm
an anti-correlation between metallicity and Ts at 3.4 sigma significance.
Finally, one of the DLAs in our sample is a newly-discovered HI 21cm absorber
(at z=0.602 towards J1431+3952), which we find to have the lowest f-corrected
spin temperature yet reported in the literature: Ts=90+-23 K. The observed
distribution of Ts and metallicities in DLAs and the implications for
understanding the characteristics of the interstellar medium in high redshift
galaxies are discussed.
