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

We present syntheticHI and CO observations of a numerical simulation of decaying turbulence in the thermally bistable neutral medium. We produce maps of HI, CO-free molecular gas, and CO, using a simple radiative transfer algorithm, obtaining the following results. (i) The spatial distribution of the gas consists of a layered structure of CO molecular gas being surrounded by CO-free molecular gas, then cold HI, and this in turn being surrounded by warm HI. (ii) The atomic gas is generally flowing towards the molecular gas, and this motion may be reflected in the frequently observed bimodal shape of the HI profiles. (iii) We test the suggestion of using the second derivative of the brightness temperature HI profile to trace HI self-absorption (HISA) and molecular gas, finding significant limitations. On a scale of several parsec, some agreement is obtained between this technique and actual HISA, as well as a correlation between HISA and the molecular gas column density. This correlation, however, quickly deteriorates towards sub-parsec scales. (iv) The column-density probability density functions (PDFs) recovered from the HI line profiles have a cut-off at column densities where the gas becomes optically thick, thus missing the contribution from the HISA-producing gas. (v) The power-law tail typical of gravitational contraction is only observed in the molecular gas and, before the power-law tail develops in the total gas density PDF, no CO is yet present, reinforcing the notion that gravitational contraction is needed to produce this component.