A multiwavelength exploration of the [C II]/IR ratio in H-ATLAS/GAMA galaxies out to z=0.2

We explore the behaviour of [C II] lambda 157.74 mu m forbidden fine-structure line observed in a sample of 28 galaxies selected from similar to 50 deg(2) of the Herschel-Astrophysical Terahertz Large Area Survey survey. The sample is restricted to galaxies with flux densities higher than S-160 mu m > 150 mJy and optical spectra from the Galaxy and Mass Assembly survey at 0.02 <z <0.2. Far-IR spectra centred on this redshifted line were taken with the Photodetector Array Camera and Spectrometer instrument on-board the Herschel Space Observatory. The galaxies span 10 <log(L-IR/L-circle dot) <12 (where L-IR = L-IR[8-1000 mu m]) and 7.3 <log(L-[C II]/L-circle dot) <9.3, covering a variety of optical galaxy morphologies. The sample exhibits the so-called [CII] deficit at high-IR luminosities, i. e. L-[C II]/L-IR (hereafter [CII]/IR) decreases at high L-IR. We find significant differences between those galaxies presenting [C II]/IR > 2.5x10(-3) with respect to those showing lower ratios. In particular, those with high ratios tend to have: (1) L-IR <10(11) L-circle dot ; (2) cold dust temperatures, T-d <30 K; (3) disc-like morphologies in r-band images; (4) a Wide-field Infrared Survey Explorer colour 0.5 less than or similar to S-12 mu m/S-22 mu m less than or similar to 1.0; (5) low surface brightness Sigma(IR) approximate to 10(8-9) L-circle dot kpc(-2), (6) and specific star formation rates of sSFR approximate to 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 [C II]/IR ratio. It is possible that relatively high <G(O)> creates a positively charged dust grain distribution, impeding an efficient photoelectric extraction of electrons from these grains to then collisionally excite carbon atoms. Within the brighter IR population, 11 <log(L-IR/L-circle dot) <12, the low [C II]/IR ratio is unlikely to be modified by [C II] self-absorption or controlled by the presence of a moderately luminous AGN (identified via the BPT diagram).