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

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 ${\mathrm{H}}_2$-bearing Damped Lyman-$\alpha$ systems. In addition, we measure upper limits on the $\mathrm{H}\mathrm{D}$ column density in twelve recently identified ${\mathrm{H}}_2$-bearing DLAs. We find that the new $\mathrm{H}\mathrm{D}$ detections have similar $N(\mathrm{H}\mathrm{D})\mathrm{/}N({\mathrm{H}}_2)$ 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 $N(\mathrm{H}\mathrm{D})\mathrm{/}N({\mathrm{H}}_2)$ ratios together with priors on the UV flux ($\chi$) and number densities ($n$), obtained from analysis of ${\mathrm{H}}_2$ and associated CI lines, we are able to constrain the cosmic-ray ionization rate (CRIR, $\zeta$) for the new $\mathrm{H}\mathrm{D}$ detections and for eight known HD-bearing systems where priors on $n$ and $\chi$ are available. We find significant dispersion in $\zeta$, from a few $\times 10^{-18}$s$^{-1}$toafew$\times 10^{-15}$s$^{-1}$\mathrm{.}Wealsofindthat$\zeta$stronglycorrelateswith$\chi$ – 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.