Probing the physical environment of the most high-redshift H2-DLAs through numerical models

Abstract Damped Lyman-α absorbers (DLAs) with molecular hydrogen have been probed in detail through both spectroscopic observations and numerical modelling. However, such H2 absorbers are quite sparse at very high redshifts. We identify six of the most distant known H2-DLAs (redshift between 3 and 4.5), with medium/high-resolution spectroscopic observations reported in the literature, and perform detailed numerical modelling followed by Bayesian analysis to constrain their physical properties mainly using the H2 rotational level population and C i fine structure levels. Our modelling approach involves setting up a constant-pressure multiphase cloud irradiated from both sides, in comparison to most models which employ constant density. This enables us to use all observed atomic and molecular species as constraints to build a more realistic model of the DLA. Our results indicate high interstellar radiation field strength ∼ 102 to 103 G0 for some sightlines, which is suggestive of in situ star formation. The cosmic ray ionization rate for all DLAs is constrained between 10−17 and 10−14 s−1, consistent with recent estimates for high-redshift sightlines. Total hydrogen density and temperature lie in the ranges 50 to 4 × 104 cm−3 and 35–200 K in the innermost part of the absorbers. The corresponding gas pressure in our DLA models lies between 103.5 and 106.4 cm−3 K, with three sightlines having a higher pressure than the range typical of high-redshift H2-DLAs.