Cold parsec-scale gas in a zabs ~ 0.1 sub-damped Lyman α with disparate H2 and 21-cm absorption*

R. Dutta, R. Srianand, S. Muzahid, N. Gupta, E. Momjian, J. Charlton

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We present a detailed analysis of a H2-bearing metal-rich sub-damped Lyman α 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, CI 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, CI 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.

Original languageEnglish (US)
Pages (from-to)3718-3730
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - Jan 1 2015

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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