2021MNRAS.501.2573D

We present Atacama Large Millimetre Array observations of a small but statistically complete sample of 12 250-µm-selected galaxies at z = 0.35 designed to measure their dust submillimeter continuum emission as well as their ¹²CO(1-0) and atomic carbon [C I](³P₁-³P₀) spectral lines. This is the first sample of galaxies with global measures of all three H₂-mass tracers and that shows star formation rates (4-26 M_☉ yr^–1) and infrared luminosities (1-6×10¹¹ L_☉) typical of star-forming galaxies in their era. We find a surprising diversity of morphology and kinematic structure; one third of the sample have evidence for interaction with nearby smaller galaxies, several sources have disjoint dust and gas morphology. Moreover, two galaxies have very high L^′_CI/ L^′^_CO ratios for their global molecular gas reservoirs; if confirmed, such extreme intensity ratios in a sample of dust-selected, massive star-forming galaxies present a challenge to our understanding of interstellar medium. Finally, we use the emission of the three molecular gas tracers, to determine the carbon abundance, X_CI, and CO- H₂ conversion α_CO in our sample, using a weak prior that the gas-to-dust ratio is similar to that of the Milky Way for these massive and metal-rich galaxies. Using a likelihood method that simultaneously uses all three gas tracer measurements, we find mean values and errors on the mean of <α_CO>= 3.0±0.5 M_☉ (K kms^–1 pc²)^–1 and <X_CI >=1.6±0.1×10^–5 (or α_CI = 18.8 M_☉ (K kms^–1 pc²)^–1) and δ_GDR = 128±16 (or α₈₅₀ = 5.9×10¹² W Hz^–1 M_☉^–1), where our starting assumption is that these metal-rich galaxies have an average gas-to-dust ratio similar to that of the Milky Way centred on δ_ GDR_ =135.