Zusammenfassung
We report the detection of Lyman-Werner absorption from molecular hydrogen
(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, the Magellanic clouds, or towards most higher redshift QSOs.
The metallicity of the absorber is 0.14 +0.10 -0.06 solar, with a dust-to-gas
ratio 0.04 +0.06 -0.03 of 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.
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