The Birth of Molecular Clouds: Formation of Atomic Precursors in Colliding Flows

Molecular Cloud Complexes (MCCs) are highly structured and ``turbulent''.Observational evidence suggests that MCCs are dynamically dominated systems,rather than quasi-equilibrium entities. The observed structure is more likely aconsequence of the formation process rather than something that is imprintedafter the formation of the MCC. Converging flows provide a natural mechanism togenerate MCC structure. We present a detailed numerical analysis of thisscenario. Our study addresses the evolution of a MCC from its birth incolliding atomic hydrogen flows up until the point when H$_2$ may begin toform. A combination of dynamical and thermal instabilities breaks up coherentflows efficiently, seeding the small-scale non-linear density perturbationsnecessary for local gravitational collapse and thus allowing (close to)instantaneous star formation. Many observed properties of MCCs come as anatural consequence of this formation scenario. Since converging flows areomnipresent in the ISM, we discuss the general applicability of this mechanism,from local star formation regions to galaxy mergers.Comment: 15 pages, 20 figures, accepted by Ap