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 (H-2). Here, we report the detection of absorption from H-2 at z = 0.56 in a subdamped Ly alpha 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) f(H2) > -1.93 +/- 0.36 based on modelling the line profiles, with a robust model-independent lower limit of f(H2) > 10(-3). This is higher than f(H2) 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(-0.10)(+0.21) 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 similar to 100 K phase giving rise to H-2, a similar to 10(4) 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 H-2, and the presence of two nearby galaxy candidates with impact parameters of similar to 10 kpc, we suggest that the absorber may be produced by a tidally stripped structure similar to the Magellanic Stream.