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

Monthly Notices of the Royal Astronomical Society. 2021. Vol. 505. No. 3. P. 3810–3822.

Kosenko D. N., Balashev S., Noterdaeme P., Krogager J. -., Srianand R., Ledoux C.

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 H2H2-bearing Damped Lyman-α systems. In addition, we measure upper limits on the HDHD column density in 12 recently identified H2H2-bearing DLAs. We find that the new HDHD detections have similar N(HD)/N(H2)N(HD)/N(H2) 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 H2H2 and associated C I lines, we are able to constrain the cosmic ray ionization rate (CRIR, ζ) for the new HDHD 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.

Keywords: cosmic rays, космические лучи, absorption spectroscopy, абсорбционная спектроскопия, галактики, galaxies, quasars, квазары