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

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 (H₂). Here, we report the detection of absorption from H₂ 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 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 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 H₂, a ̃10⁴ 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 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.