From the warm magnetized atomic medium to molecular clouds

TL;DR

This study uses advanced MHD simulations to analyze how large-scale collisions of warm atomic gas streams lead to the formation of inhomogeneous molecular clouds with properties consistent with observations.

Contribution

First self-consistent MHD simulations including cooling and self-gravity of colliding warm atomic flows to study molecular cloud formation.

Findings

Cloud density structures include cold filaments and clumps.

Column density PDF peaks at ~2x10^{21} cm^{-2}.

Magnetic field correlates weakly with density at low levels.

Abstract

{It has recently been proposed that giant molecular complexes form at the sites where streams of diffuse warm atomic gas collide at transonic velocities.} {We study the global statistics of molecular clouds formed by large scale colliding flows of warm neutral atomic interstellar gas under ideal MHD conditions. The flows deliver material as well as kinetic energy and trigger thermal instability leading eventually to gravitational collapse.} {We perform adaptive mesh refinement MHD simulations which, for the first time in this context, treat self-consistently cooling and self-gravity.} {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 $\sim 2 \times 10^{21} \psc$ and decays rapidly at higher values; the magnetic intensity correlates weakly with density from $n \sim 0.1$ to $10^4 \pcc$, and then varies roughly as $n^{1/2}$ for higher densities.} {The global statistical properties of such molecular clouds are reasonably consistent with observational determinations. Our numerical simulations suggest that molecular clouds formed by the moderately supersonic collision of warm atomic gas streams.}