A multi-wavelength exploration of the [CII]/IR ratio in H-ATLAS/GAMA
  galaxies out to z=0.2
  We explore the behaviour of [CII]-157.74um forbidden fine-structure line
observed in a sample of 28 galaxies selected from ~50deg^2 of the H-ATLAS
survey. The sample is restricted to galaxies with flux densities higher than
S_160um>150mJy and optical spectra from the GAMA survey at 0.02<z<0.2. Far-IR
spectra centred on this redshifted line were taken with the PACS instrument
on-board the Herschel Space Observatory. The galaxies span 10<log(L_IR/Lo)<12
(where L_IR=L_IR[8-1000um]) and 7.3<log(L_[CII]/Lo)<9.3, covering a variety of
optical galaxy morphologies. The sample exhibits the so-called [CII] deficit at
high IR luminosities, i.e. L_[CII]/L_IR (hereafter [CII]/IR) decreases at high
L_IR. We find significant differences between those galaxies presenting
[CII]/IR>2.5x10^-3 with respect to those showing lower ratios. In particular,
those with high ratios tend to have: (1) L_IR<10^11Lo; (2) cold dust
temperatures, T_d<30K; (3) disk-like morphologies in r-band images; (4) a WISE
colour 0.5<S_12um/S_22um<1.0; (5) low surface brightness Sigma_IR~10^8-9 Lo
kpc^-2, (6) and specific star-formation rates of sSFR~0.05-3 Gyr^-1. We suggest
that the strength of the far-UV radiation fields (<G_O>) is main parameter
responsible for controlling the [CII]/IR ratio. It is possible that relatively
high <G_O> creates a positively charged dust grain distribution, impeding an
efficient photo-electric extraction of electrons from these grains to then
collisionally excite carbon atoms. Within the brighter IR population,
11<log(L_IR/Lo)<12, the low [CII]/IR ratio is unlikely to be modified by [CII]
self absorption or controlled by the presence of a moderately luminous AGN
(identified via the BPT diagram).
