HST/COS Detection of Deuterated Molecular Hydrogen in a DLA at z = 0.18
  We report on the detection of deuterated molecular hydrogen, HD, at $z =
0.18$. HD and H$_{\rm 2}$ are detected in HST/COS data of a low metallicity ($Z
\sim 0.07Z_\odot$) damped Ly$\alpha$ system at $z = 0.18562$ toward QSO
B0120$-$28, with log $N$(H I) = 20.50 $\pm$ 0.10. Four absorption components
are clearly resolved in H$_{\rm 2}$ while two components are resolved in HD;
the bulk of the molecular hydrogen is associated with the components traced by
HD. We find total column densities log $N$(HD) = 14.82 $\pm$ 0.15 and log
$N$(H$_{\rm 2}$) = 20.00 $\pm$ 0.10. This system has a high molecular fraction,
$f$(H$_{\rm 2}$) = 0.39 $\pm$ 0.10 and a low HD to H$_{\rm 2}$ ratio, log
(HD/2H$_{\rm 2}$) $= -5.5 \pm 0.2$ dex. The excitation temperature, $T_{01} =
65 \pm 2$ K, in the component containing the bulk of the molecular gas is lower
than in other DLAs. These properties are unlike those in other higher redshift
DLA systems known to contain HD, but are consistent with what is observed in
dense clouds in the Milky Way.
