Molecular cloud formation as seen in synthetic Hi and molecular gas observations

We present synthetic Hi and CO observations of a numerical simulation of decaying turbulence in the thermally bistable neutral medium. We first present the simulation, which produces a clumpy medium, with clouds initially consisting of clustered clumps. Self-gravity causes these clump clusters to merge and form more homogeneous dense clouds. We apply a simple radiative transfer algorithm, throwing rays in many direc-tions from each cell, and defining every cell with 〈Av 〉> 1 as molecular. We then produce maps of Hi, CO-free molecular gas, and CO, and investigate the following aspects: i) The spatial distribution of the warm, cold, and molecular gas, finding the well-known layered structure, with molecular gas being surrounded by cold Hi and this in turn being surrounded by warm Hi. ii) The velocity of the various components, find-ing that the atomic gas is generally flowing towards the molecular gas, and that this motion is reflected in the frequently observed bimodal shape of the Hi profiles. This conclusion is, however, tentative, because we do not include feedback that may pro-duce Hi gas receding from molecular regions. iii) The production of Hi self-absorptio