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

Abstract

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 $\rm H_2$-bearing Damped Lyman-α systems. In addition, we measure upper limits on the $\rm HD$ column density in 12 recently identified $\rm H_2$-bearing DLAs. We find that the new $\rm HD$ detections have similar $N({\rm HD})/N(\rm H_2)$ 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 $\rm H_2$ and associated C i lines, we are able to constrain the cosmic ray ionization rate (CRIR, ζ) for the new $\rm HD$ 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.