Ionised carbon and galaxy activity

TL;DR

This paper explores how increased galaxy activity and dust heating contribute to the observed decline in the [CII] line's relative luminosity with far-infrared luminosity, suggesting active galactic nuclei influence in distant sources.

Contribution

It proposes that galaxy activity, including nuclear UV heating, explains the [CII] luminosity decline with increasing far-infrared luminosity in extragalactic sources.

Findings

[CII] luminosity declines with increasing L_FIR.

Correlation between [CII], CO, and radio luminosities suggests molecular gas involvement.

Enhanced galaxy activity and nuclear heating contribute to far-infrared excess.

Abstract

We investigate the possibility that the decrease in the relative luminosity of the 158 micron [CII] line with the far-infrared luminosity in extragalactic sources stems from a stronger 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 [CII] luminosity does not climb as steeply as that of the far-infrared, giving the decline in the L_[CII]/L_FIR ratio with L_FIR. Investigating this further, we find that the [CII] 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, done mainly by the [CII] line at low luminosities. The latter hints at increased activity in these galaxies at greater distances, so 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.