A search for HI 21cm absorption towards a radio-selected quasar sample
  II: a new low spin temperature DLA at high redshift
  We report results from a deep search for redshifted HI 21cm absorption from
eight damped Lyman-$\alpha$ absorbers (DLAs) detected in our earlier optical
survey for DLAs towards radio-loud quasars. HI 21cm absorption was detected
from the $z = 2.192$ DLA towards TXS2039+187, only the sixth case of such a
detection at $z > 2$, while upper limits on the HI 21cm optical depth were
obtained in six other DLAs at $z > 2$. Our detection of HI 21cm absorption in
the eighth system, at $z = 2.289$ towards TXS0311+430, has been reported
earlier. We also present high spatial resolution images of the background
quasars at frequencies close to the redshifted HI 21cm line frequency, allowing
us to estimate the covering factor of each DLA, and then determine its spin
temperature $T_s$. For three non-detections of HI 21cm absorption, we obtain
strong lower limits on the spin temperature, $T_s \gtrsim 790$ K, similar to
the bulk of the high-$z$ DLA population; three other DLAs yield weak lower
limits, $T_s > 140-400$ K. However, for the two DLAs with detections of HI 21cm
absorption, the derived spin temperatures are both low $T_s = (160 \pm 35)
\times (f/0.35)$ K for the $z = 2.192$ DLA towards TXS2039+187 and $T_s = (72
\pm 18) \times (f/0.52)$ K for the $z = 2.289$ DLA towards TXS0311+430. These
are the first two DLAs at $z > 1$ with $T_s$ values comparable to those
obtained in local spiral galaxies. Based on the observed correlations between
spin temperature and metallicity and velocity spread and metallicity in DLAs,
we suggest that the hosts of the two absorbers are likely to be massive,
high-metallicity galaxies.
