Cold parsec-scale gas in a zabs~0.1 sub-DLA with disparate H2 and 21-cm absorption

Abstract

We present a detailed analysis of a H2-bearing metal-rich sub-damped Lyman-alpha system at zabs = 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% 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 Large Baseline Array 1.4 GHz continuum image of the radio source contains only 27% 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% 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 UV 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.

arXiv comment13 pages, 5 figures, 4 tables, Accepted for publication in the MNRAS

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