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

Measuring rest-frame 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 (H2). Here, we report the detection of absorption from H2 at z = 0.56 in a subdamped Lyα system with neutral hydrogen column density NHI = 1019.5 ± 0.2 cm−2. This is the first H2 system analysed at a redshift of <1.5 beyond the Milky Way halo. It has a surprisingly high molecular fraction: log10 fH2 > −1.93 ± 0.36 based on modelling the line profiles, with a robust model-independent lower limit of fH2 > 10−3. This is higher than fH2 values seen along sightlines with similar NHI 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 O vi. Using cloudy models, we show that there are three phases present; a ∼100 K phase giving rise to H2, a ∼104 K phase where most of the low-ionization metal absorption is produced; and a hotter phase associated with O vi. Based on similarities to high-velocity clouds in the Milky Way halo showing H2, 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.