Ionised carbon and galaxy activity

We investigate the possibility that the decrease in the relative luminosity of the 158 μm [C ii] line with the far-infrared luminosity in extragalactic sources is due to a larger contribution from the heated dust emission in the more distant sources. Due to the flux limited nature of these surveys, the luminosity of the detected objects increases with distance. However, the fact that the [C ii] luminosity does not climb so steeply as that of the far-infrared, gives the decline in the L[CII]/LFIR ratio with LFIR. Investigating this further, we find that the [C ii] luminosity exhibits similar drops as measured against the carbon monoxide and radio continuum luminosities. The former may indicate that at higher luminosities a larger fraction of the carbon is locked up in the form of molecules and/or that the CO line radiation also contributes to the cooling, due mainly to the [C ii] line at low luminosities. The latter hints at increased activity in these galaxies at larger distances and we suggest that in addition to an underlying heating of the dust by a stellar population, there is also heating of the embedded dusty torus by the ultra-violet emission from the active nucleus, resulting in an excess in the far-infrared emission from the more luminous objects. If thermal equilibrium is to be maintained, the additional heating must be matched by an additional cooling process over and above that supplied by the [C ii] emission line, possibly via [O i], as well as the CO emission.