Molecular cloud formation as seen in synthetic H I and molecular gas observations
Abstract
We present synthetic H I and CO observations of a numerical simulation of decaying turbulence in the thermally bistable neutral medium. We produce maps of H I, 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 H I, and this in turn being surrounded by warm H I. (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 H I profiles. (iii) We test the suggestion of using the second derivative of the brightness temperature H I profile to trace H I 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 H I 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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- September 2015
- DOI:
- 10.1093/mnras/stv1153
- arXiv:
- arXiv:1403.6417
- Bibcode:
- 2015MNRAS.452.1353H
- Keywords:
-
- turbulence;
- ISM: clouds;
- ISM: evolution;
- ISM: kinematics and dynamics;
- ISM: structure;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Submitted to MNRAS - comments are welcome. 22 pages, 21 figures