Dust Continuum, Co, And [C-I] 1-0 Lines: Self-Consistent H-2 Mass Estimates And The Possibility Of Globally Co-'Dark' Galaxies At Z=0.35

We present Atacama Large Millimetre Array observations of a small but statistically complete sample of 12 250-mu m-selected galaxies at z = 0.35 designed to measure their dust submillimeter continuum emission as well as their (CO)-C-12(1 - 0) and atomic carbon [CI](P-3(1) -P-3(0)) spectral lines. This is the first sample of galaxies with global measures of all three H-2-mass tracers and that shows star formation rates (4-26M(circle dot) yr(-1)) and infrared luminosities (1-6 x 10(11) L-circle dot) 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-H-2 conversion alpha(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 = 3.0 +/- 0.5 M-circle dot (K kms(-1) pc(2))(-1) and < X-CI > = 1.6 +/- 0.1 x 10(-5) (or alpha(CI) = 18.8 M-circle dot (K kms(-1) pc(2))(-1)) and delta(GDR) = 128 +/- 16 (or alpha(850) = 5.9 x 10(12) WHz(-1)M(circle dot)(-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 delta(GDR) = 135.