Cold parsec-scale gas in a z_abs ∼ 0.1 sub-damped Lyman α with disparate H₂ and 21-cm absorption

Abstract

We present a detailed analysis of a H₂-bearing metal-rich sub-damped Lyman α system at z_abs = 0.10115 towards the radio-loud quasar J0441-4313, at a projected separation of ∼7.6 kpc from a star-forming galaxy. The H₂, {C I}} and {Na I} absorption are much stronger in the redder of the two components seen in the Hubble Space Telescope/Cosmic Origins Spectrograph spectrum. The best single-component fit to the strong H₂ component gives log N(H₂) = 16.61 ± 0.05. However, possible hidden saturation in the medium-resolution spectrum can allow for log N(H₂) to be as high as 18.9. The rotational excitation temperature of H₂ in this component is 133^{+33}_{-22} K. Photoionization models suggest 30-80 per cent of the total N(H I) is associated with the strong H₂ component that has a density ≤100 cm^-3 and is subject to a radiation field that is ≤0.5 times the Galactic mean field. The Very Long Baseline Array 1.4 GHz continuum image of the radio source contains only 27 per cent of the arcsecond scale emission. Using a previously published spectrum, no 21-cm absorption is found to be associated with the strong H₂ component. This suggests that either the N(H I)) associated with this component is ≤50 per cent of the total N(H I)) or the gas covering factor is ≤0.27. This is consistent with the results of the photoionization model that uses ultraviolet radiation due to stars in the associated galaxy. The 21-cm absorption previously reported from the weaker H₂ component suggests a spin temperature of ≤90 K, at odds with the weakness of H₂, {C I} and {Na I} absorption in this component. From the inferred physical and chemical conditions, we suggest that the gas may be tracing a recent metal-rich outflow from the host galaxy.