Evidence for the heating of atomic interstellar gas by polycyclic aromatichydrocarbons

We report a strong correlation between the [C II] 158 mum cooling line and the mid-infrared flux in the 5-10 mum range in a wide variety of star-formi ng 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 interst ellar grains. The [C II] line is the dominant gas coolant in most regions o f atomic interstellar gas and therefore reflects the heating input to the g as. The ratio of these two quantities, [C II]/AFE, remains nearly constant around 1.5% against variations in the ratio of the IRAS 60 mum band flux to the 100 mum 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 hig her dust temperatures and more intense radiation fields. We interpret the s table [C II]/AFE ratio as evidence that gas heating is dominated by the PAH s or small grains, which are also AFE carriers, over a wide range of condit ions. The trend of decreasing [C II]/FIR and AFE/FIR with increasing radiat ion field suggests a decrease in the importance of PAHs or small grains rel ative to large grains both in gas heating and in dust cooling. We summarize the observed trends and suggest two plausible scenarios.