Publication Date:
2016-08-26
Description:
We investigate the temperature distribution of CO-dark molecular hydrogen (H 2 ) in a series of disc galaxies simulated using the arepo moving-mesh code. In conditions similar to those in the Milky Way, we find that H 2 has a flat temperature distribution ranging from 10 to 100 K. At T 〈 30 K, the gas is almost fully molecular and has a high CO content, whereas at T 〉 30 K, the H 2 fraction spans a broader range and the CO content is small, allowing us to classify gas in these two regimes as CO-bright and CO-dark, respectively. The mean sound speed in the CO-dark H 2 is c s, dark = 0.64 km s –1 , significantly lower than the value in the cold atomic gas ( c s, CNM = 1.15 km s –1 ), implying that the CO-dark molecular phase is more susceptible to turbulent compression and gravitational collapse than its atomic counterpart. We further show that the temperature of the CO-dark H 2 is highly sensitive to the strength of the interstellar radiation field, but that conditions in the CO-bright H 2 remain largely unchanged. Finally, we examine the usefulness of the [C ii ] and [O i ] fine-structure lines as tracers of the CO-dark gas. We show that in Milky Way-like conditions, diffuse [C ii ] emission from this gas should be detectable. However, it is a problematic tracer of this gas, as there is only a weak correlation between the brightness of the emission and the H 2 surface density. The situation is even worse for the [O i ] line, which shows no correlation with the H 2 surface density.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
