2021MNRAS.505.3810K

We present a systematic study of deuterated molecular hydrogen (HD) at high redshift, detected in absorption in the spectra of quasars. We present four new identifications of HD lines associated with known H_2-bearing Damped Lyman-α systems. In addition, we measure upper limits on the HD column density in 12 recently identified H_2-bearing DLAs. We find that the new HD detections have similar N( HD)/N( H_2) ratios as previously found, further strengthening a marked difference with measurements through the Galaxy. This is likely due to differences in physical conditions and metallicity between the local and the high-redshift interstellar media. Using the measured N(HD)/N(H₂) ratios together with priors on the UV flux (χ) and number densities (n), obtained from analysis of H_2 and associated C I lines, we are able to constrain the cosmic ray ionization rate (CRIR, ζ) for the new HD detections and for eight known HD-bearing systems where priors on n and χ are available. We find significant dispersion in ζ, from a few x 10^–18 s^–1 to a few x 10^–15 s^–1. We also find that ζ strongly correlates with χ - showing almost quadratic dependence, slightly correlates with Z, and does not correlate with n, which probably reflects a physical connection between cosmic rays and star-forming regions.