Dust continuum, CO and [Ci] 1-0 lines: Self-consistent H2 mass estimates and the possibility of globally CO-‘dark’ galaxies at z=0.35
Abstract We present ALMA observations of a small but statistically complete sample of twelve 250μm selected galaxies at z = 0.35 designed to measure their dust submillimeter continuum emission as well as their $\rm {^{12}CO(1-0)}$ and atomic carbon [Ci](3P1-3P0) spectral lines. This is the first sample of galaxies with global measures of all three H2-mass tracers and which show star formation rates (4–26 $\rm M_{\odot }$ yr−1) and infra-red luminosities ($1-6\times 10^{11}\, {\,\rm L_{\odot }\,}$) 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 $L^{\prime }_{\rm CI}$ / $L^{\prime }_{\rm {CO}}$ ratios for their global molecular gas reservoirs; if confirmed, such extreme intensity ratios in a sample of dust selected, massive star forming galaxies presents a challenge to our understanding of ISM. Finally, we use the emission of the three molecular gas tracers, to determine the carbon abundance, $\rm {X_{CI}}$ , and CO–$\rm {H_2}$ conversion αCO 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 which simultaneously uses all three gas tracer measurements, we find mean values and errors on the mean of $\langle {\,\alpha _{\rm {CO}}\,}\rangle =3.0\pm 0.5\, \rm {{\,\rm M_{\odot }\,}\, (K\, kms^{-1}\, pc^2)^{-1}}$ and $\langle {\,\rm {X_{CI}}\,}\rangle =1.6\pm 0.1\times 10^{-5}$ (or ${\,\alpha _{\rm {CI}}\,}=18.8\, {\,\rm {M_{\odot }\, (K\, kms^{-1}\, pc^2)^{-1}}\,}$) and ${\,\delta _{\rm {GDR}}\,}=128\pm 16$ (or ${\,\alpha _{\rm {850}}\,}=5.9\times 10^{12}\, \rm {W\, Hz^{-1}\, {\,\rm M_{\odot }\,}^{-1}}$), where our starting assumption is that these metal rich galaxies have an average gas-to-dust ratio similar to that of the Milky Way centered on ${\,\delta _{\rm {GDR}}\,}=135$.