A high molecular fraction in a sub-damped 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 Lyman-Werner absorption from H2 at z=0.56 in a sub-damped Ly-alpha
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 < 1.5 beyond the Milky Way
halo. It has a surprisingly high molecular fraction: log 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 disk and the Magellanic clouds. The
metallicity of the absorber is 0.19 +0.21 -0.10 solar, with a dust-to-gas ratio
< 0.36 times the value in the solar neighbourhood. Absorption from associated
low-ionisation metal transitions such as OI and FeII 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 ~10^4 K phase where most of the low-ionisation metal
absorption is produced; and a hotter phase associated with OVI. 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.
