A Young GMC Formed at the Interface of Two Colliding Supershells:
  Observations Meet Simulations
  Dense, star-forming gas is believed to form at the stagnation points of
large-scale ISM flows, but observational examples of this process in action are
rare. We here present a giant molecular cloud (GMC) sandwiched between two
colliding Milky Way supershells, which we argue shows strong evidence of having
formed from material accumulated at the collision zone. Combining 12CO, 13CO
and C18O(J=1-0) data with new high-resolution, 3D hydrodynamical simulations of
colliding supershells, we discuss the origin and nature of the GMC
(G288.5+1.5), favoring a scenario in which the cloud was partially seeded by
pre-existing denser material, but assembled into its current form by the action
of the shells. This assembly includes the production of some new molecular gas.
The GMC is well interpreted as non-self-gravitating, despite its high mass (MH2
~ 1.7 x 10^5 Msol), and is likely pressure confined by the colliding flows,
implying that self-gravity was not a necessary ingredient for its formation.
Much of the molecular gas is relatively diffuse, and the cloud as a whole shows
little evidence of star formation activity, supporting a scenario in which it
is young and recently formed. Drip-like formations along its lower edge may be
explained by fluid dynamical instabilities in the cooled gas.
