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

TL;DR

This study investigates deuterated molecular hydrogen (HD) in high-redshift quasars, measuring cosmic-ray ionization rates and their relation to physical conditions, revealing significant variability and correlations with UV flux and metallicity.

Contribution

First systematic measurement of HD in high-redshift DLAs, constraining cosmic-ray ionization rates and their dependence on environmental factors.

Findings

HD/ H2 ratios differ from local measurements, indicating different physical conditions.

Cosmic-ray ionization rate varies widely, from 10^{-18} to 10^{-15} s^{-1}.

CRIR correlates strongly with UV flux, suggesting a link to star-forming activity.

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-$\alpha$ systems. In addition, we measure upper limits on the $\rm HD$ column density in twelve 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 $N({\rm HD})/N({\rm H_2})$ ratios together with priors on the UV flux ($\chi$) and number densities ($n$), obtained from analysis of $\rm H_2$ and associated CI lines, we are able to constrain the cosmic-ray ionization rate (CRIR, $\zeta$) for the new $\rm HD$ 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$^{-1}$ to a few $\times 10^{-15}$ s$^{-1}$. We also find that $\zeta$ strongly correlates with $\chi$ -- 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.