Abstract
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).
Highlights
Understanding the chemical and physical evolution of a galaxy is far from trivial
We have used recent Photodetector Array Camera and Spectrometer (PACS) spectroscopic [C ii] observations to describe its relation to the IR luminosity in a sample of 28 galaxies selected from the H-ATLAS survey
We find that galaxies with high L[CII]/LIR > 2.5 × 10−3 luminosity ratios tend to: have LIR < 1011 L⊙, dust temperatures lower than 30 K, high WISE colours in the range 0.5 < S12μm/S22μm < 1.0, present disk-like morphologies, have low surface brightness ΣIR ≈ 108−9 L⊙/kpc2, and got a range of starformation rate efficiencies
Summary
Understanding the chemical and physical evolution of a galaxy is far from trivial. Newly born stars consume and process the available gas, whilst heating the interstellar medium (ISM), and supernovae enrich the environment with heavy elements, contributing to potentially complex feedback processes. A good description of ISM physics under the influence of stellar radiation fields was achieved using photo-dissociation region (PDR) modelling (e.g. Tielens & Hollenbach 1985). These models can explain the origin of most of the dense ISM emission from starforming galaxies, including the major cooling fine-structure lines of carbon/nitrogen/oxygen (C/N/O) and the underlying Infrared (IR) continuum emission produced by interstellar dust. As a fine-structure line, [C ii] is an excellent tracer of all the different stages of evolution of the ISM: it can be excited by collisions with electrons in the warm ionised medium; H i in the warm or cold diffuse media; and H2 in the warm and dense molecular gas. Its intensity is sensitive to the column density, the volume density and the kinetic temperature of the ISM (Pineda et al 2013)
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