CO-dark gas and molecular filaments in Milky Way-type galaxies - II. The temperature distribution of the gas

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.