Giant Metrewave Radio Telescope detection of two new HI 21cm absorbers
  at $z \approx 2$
  I report the detection of HI 21cm absorption in two high column density
damped Lyman-$\alpha$ absorbers (DLAs) at $z \approx 2$ using new wide-band
$250-500$ MHz receivers onboard the Giant Metrewave Radio Telescope. The
integrated HI 21cm optical depths are $0.85 \pm 0.16$ km/s (TXS1755+578) and
$2.95 \pm 0.15$ km/s (TXS1850+402). For the $z=1.9698$ DLA towards TXS1755+578,
the difference in HI 21cm and CI profiles and the weakness of the radio core
suggest that the HI 21cm absorption arises towards radio components in the jet,
and that the optical and radio sightlines are not the same. This precludes an
estimate of the DLA spin temperature. For the $z = 1.9888$ DLA towards
TXS1850+402, the absorber covering factor is likely to be close to unity, as
the background source is extremely compact, with all the 5 GHz emission arising
from a region of size $\leq 1.4$ mas. This yields a DLA spin temperature of
${\rm T_s} = (372 \pm 18) \times (f/1.0)$ K, lower than typical ${\rm T_s}$
values in high-$z$ DLAs. This low spin temperature and the relatively high
metallicity of the $z = 1.9888$ DLA ([Zn/H] $= (-0.68 \pm 0.04)$) are
consistent with the anti-correlation between metallicity and spin temperature
that has been earlier found in damped Lyman-$\alpha$ systems.
