HD molecules at high redshift: cosmic-ray ionization rate in the diffuse interstellar medium

Abstract

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 twelve 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 (H 2 ) 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 ×10 −18 s −1 to a few ×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.