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 interstellar medium in two Galactic supershells, GSH 287+04-17 and GSH 277+00+36. H I 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 ∼ 10cm ^-3, respectively. Locally, the shells show rich substructure in both tracers, with molecular gas seen elongated along the inner edges of the atomic walls, embedded within H I 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 H I 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.