HST/COS detection of deuterated molecular hydrogen in a damped Lyα system at z = 0.18

We report on the detection of deuterated molecular hydrogen, HD, at z = 0.18. HD and H₂ are detected in HST/COS data of a low-metallicity (Z ∼ 0.07 Z _☉) damped Lyα (DLA) system at z = 0.18562 toward QSO B0120–28, with log N(H I) = 20.50 ± 0.10. Four absorption components are clearly resolved in H₂, 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 ± 0.15 and log N(H₂) = 20.00 ± 0.10. This system has a high molecular fraction, f(H₂) = 0.39 ± 0.10, and a low HD-to-H₂ ratio, log (HD/2H₂) = –5.5 ± 0.2 dex. The excitation temperature, T ₀₁ = 65 ± 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.