Preprint typeset using LATEX style emulateapj v. 5/2/11 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 evi-dence 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 ∼ 1.7×105 M), and is likely pressure confined by the colliding flows, implying that self-gravity was not a necessary ingredient for its for-mation. 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