From the warm magnetized atomic medium to molecular clouds

Context. It has been proposed that giant molecular complexes form at the sites of streams of diffuse warm atomic gas that collide at transonic velocities. Aims. We study the global statistics of molecular clouds formed by large scale colliding flows of warm neutral atomic interstellar gas under pure hydrodynamic and ideal MHD conditions. The flows deliver material as well as kinetic energy and trigger thermal instability leading eventually to gravitational collapse. Methods. We perform adaptive mesh refinement MHD simulations that, for the first time in this context, treat cooling and self-gravity self-consistently. Results. The clouds formed in the simulations develop a highly inhomogeneous density and temperature structure, with cold dense filaments and clumps condensing from converging flows of warm atomic gas. In the clouds, the column density probability density distribution (PDF) peaks at similar to 2 x 10(21) cm(-2) and decays rapidly at higher values; the magnetic intensity correlates weakly with density between n similar to 0.1 and 10(4) cm(-3), and then varies roughly as n(1/2) for higher densities. Conclusions. The global statistical properties of such molecular clouds are reasonably consistent with observational measurements. Our numerical simulations suggest that molecular clouds form by the moderately supersonic collision of warm atomic gas streams.