Supershells as Molecular Cloud Factories: Parsec Resolution Observations
  of HI and 12CO(J=1-0) in GSH 287+04-17 and GSH 277+00+36
  We present parsec-scale resolution observations of the atomic and molecular
ISM in two Galactic supershells, GSH 287+04-17 and GSH 277+00+36. HI synthesis
images from the Australia Telescope Compact Array are combined with 12CO(J=1-0)
data from the NANTEN telescope to reveal substantial quantities of molecular
gas closely associated with both shells. These data allow us to confirm an
enhanced level of molecularization over the volumes of both objects, providing
the first direct observational evidence of increased molecular cloud production
due to the influence of supershells. We find that the atomic shell walls are
dominated by cold gas with estimated temperatures and densities of T ~ 100 K
and n0 ~ 10 cm-3. Locally, the shells show rich substructure in both tracers,
with molecular gas seen elongated along the inner edges of the atomic walls,
embedded within HI filaments and clouds, or taking the form of small CO clouds
at the tips of tapering atomic `fingers'. We discuss these structures in the
context of different formation scenarios, suggesting that molecular gas
embedded within shell walls is well explained by in-situ formation from the
swept up medium, whereas CO seen at the ends of fingers of HI may trace
remnants of molecular clouds that pre-date the shells. A preliminary assessment
of star formation activity within the shells confirms ongoing star formation in
the molecular gas of both GSH 287+04-17 and GSH 277+00+36.
