The CO luminosity and CO-H2 conversion factor of diffuse ISM: does CO emission trace dense molecular gas?
Résumé
Aims: We wish to separate and quantify the CO luminosity and CO-H2 conversion factor applicable to diffuse but partially-molecular ISM when H2 and CO are present but C+ is the dominant form of gas-phase carbon. Methods: We discuss galactic lines of sight observed in Hi, HCO+ and CO where CO emission is present but the intervening clouds are diffuse (locally AV ≲ 1 mag) with relatively small CO column densities NCO ≲ 2 × 1016 cm-2. We separate the atomic and molecular fractions statistically using EB-V as a gauge of the total gas column density and compare N_H_2 to the observed CO brightness. Results: Although there are H2-bearing regions where CO emission is too faint to be detected, the mean ratio of integrated CO brightness to N_H_2 for diffuse ISM does not differ from the usual value of 1K km s-1 of integrated CO brightness per 2 × 1020 H2 cm-2. Moreover, the luminosity of diffuse CO viewed perpendicular to the galactic plane is 2/3 that seen at the Solar galactic radius in surveys of CO emission near the galactic plane. Conclusions: Commonality of the CO-H2 conversion factors in diffuse and dark clouds can be understood from considerations of radiative transfer and CO chemistry. There is unavoidable confusion between CO emission from diffuse and dark gas and misattribution of CO emission from diffuse to dark or giant molecular clouds. The character of the ISM is different from what has been believed if CO and H2 that have been attributed to molecular clouds on the verge of star formation are actually in more tenuous, gravitationally-unbound diffuse gas. Appendix E is only available in electronic form at http://www.aanda.org