Evidence for the heating of atomic interstellar gas by polycyclic aromatic hydrocarbons

We report a strong correlation between the [C II] 158 μm cooling line and the mid-infrared flux in the 5-10 μm range in a wide variety of star-forming galaxies. The mid-infrared flux is dominated by the aromatic features in emission (AFEs), which are thought to arise from large polycyclic aromatic hydrocarbons (PAHs) and are generally associated with the smallest interstellar grains. The [C II] line is the dominant gas coolant in most regions of atomic interstellar gas and therefore reflects the heating input to the gas. The ratio of these two quantities, [C II]/AFE, remains nearly constant around 1.5% against variations in the ratio of the IRAS 60 μm band flux to the 100 μm band flux, R(60/100). This is in contrast to the drop in the [C II]/far-infrared (FIR) ratio with increasing R(60/100), which signals higher dust temperatures and more intense radiation fields. We interpret the stable [C II]/AFE ratio as evidence that gas heating is dominated by the PAHs or small grains, which are also AFE carriers, over a wide range of conditions. The trend of decreasing [C II]/FIR and AFE/FIR with increasing radiation field suggests a decrease in the importance of PAHs or small grains relative to large grains both in gas heating and in dust cooling. We summarize the observed trends and suggest two plausible scenarios.