A multi - wavelength exploration of the [ C ii ] / IR ratio in H - ATLAS / GAMA galaxies out to z = 0 . 2

We explore the behaviour of [C ii]λ157.74μm forbidden fine-structure line observed in a sample of 28 galaxies selected from ∼ 50 deg of the H-ATLAS survey. The sample is restricted to galaxies with flux densities higher than S160μm > 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(LIR/L⊙) < 12 (where LIR ≡ LIR[8− 1000μm]) and 7.3 < log(L[CII]/L⊙) < 9.3, covering a variety of optical galaxy morphologies. The sample exhibits the so-called [C ii] deficit at high IR luminosities, i.e. L[C II]/LIR (hereafter [C ii]/IR) decreases at high LIR. We find significant differences between those galaxies presenting [C ii]/IR> 2.5 × 10 with respect to those showing lower ratios. In particular, those with high ratios tend to have: (1) LIR < 10 11 L⊙; (2) cold dust temperatures, Td < 30K; (3) disk-like morphologies in r-band images; (4) aWISE colour 0.5 ∼ S12μm/S22μm ∼ 1.0; (5) low surface brightness ΣIR ≈ 10 8−9 L⊙ kpc , (6) and specific star-formation rates of sSFR≈ 0.05 − 3Gyr. We suggest that the strength of the far-UV radiation fields () is main parameter responsible for controlling the [C ii]/IR ratio. It is possible that relatively high 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(LIR/L⊙) < 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).