2015MNRAS.449.2498I

We explore the behaviour of [Cii] λ157.74 µm forbidden fine-structure line observed in a sample of 28 galaxies selected from ∼ 50 deg2 of the Herschel-Astrophysical Terahertz Large Area Survey survey. The sample is restricted to galaxies with flux densities higher than S160µm > 150mJy 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(LIR/L_☉) < 12 (where LIR ≡ LIR[8-1000µm]) and 7.3<log (L_[C II]/L_☉) <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]/LIR (hereafter [Cii]/IR) decreases at high LIR. 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) LIR <1011 L_☉; (2) cold dust temperatures, Td < 30 K; (3) disc-like morphologies in r-band images; (4) a Wide-field Infrared Survey Explorer colour 0.5 ≲ S12µm/S22µm ≲ 1.0; (5) low surface brightness ΣIR ~ 10^8–9L_☉/kpc2, (6) and specific star formation rates of sSFR ~0.05-3 G/yr. We suggest that the strength of the far-UV radiation fields (<GO>) is main parameter responsible for controlling the [Cii]/IR ratio. It is possible that relatively high <GO> 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(LIR/L_☉) < 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).