A high molecular fraction in a subdamped absorber at z = 0.56

Neil H.M. Crighton, Jill Bechtold, Robert F. Carswell, Romeel Davé, Craig B. Foltz, Buell T. Jannuzi, Simon L. Morris, John M. O'Meara, J. Xavier Prochaska, Joop Schaye, NICOLÁS ANDRÉS TEJOS SALGADO

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19 Scopus citations


Measuring rest-fram ultraviolet rotational transitions from the Lyman and Werner bands in absorption against a bright background continuum is one of the few ways to directly measure molecular hydrogen (H 2 ). Here, we report the detection of absorption from H 2 at z = 0.56 in a subdamped Lyα system with neutral hydrogen column density N HI = 10 19.5 ±0.2 cm -2 . This is the first H 2 system analysed at a redshift of <1.5 beyond the Milky Way halo. It has a surprisingly high molecular fraction: log 10 fH 2 >-1.93 ± 0.36 based on modelling the line profiles, with a robust model-independent lower limit of fH 2 > 10 -3 . This is higher than fH 2 values seen along sightlines with similar N HI through the Milky Way disc and the Magellanic Clouds. The metallicity of the absorber is 0.19 +0.21 -0.10 solar, with a dust-to-gas ratio of <0.36 of the value in the solar neighbourhood. Absorption from associated low-ionization metal transitions such as O I and Fe II is observed in addition to OVI. Using CLOUDY models, we show that there are three phases present; a ̃100 K phase giving rise to H 2 , a ̃10 4 K phase where most of the low-ionization metal absorption is produced; and a hotter phase associated with OVI. Based on similarities to high-velocity clouds in the MilkyWay halo showing H 2 , and the presence of two nearby galaxy candidates with impact parameters of ̃10 kpc, we suggest that the absorber may be produced by a tidally stripped structure similar to the Magellanic Stream.

Original languageEnglish
Pages (from-to)178-193
Number of pages16
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
StatePublished - 1 Jul 2013


  • Absorption lines
  • Haloes- quasars
  • ISM
  • Molecules- galaxies

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