A Survey of Atomic Carbon [C I] in High-redshift Main-Sequence Galaxies

We present the first results of an ALMA survey of the lower fine structure line of atomic carbon [C I]${(}^3{P}_1-{}^3{P}_0)$ in far infrared-selected galaxies on the main sequence at $z\sim 1.2$ in the COSMOS field. We compare our sample with a comprehensive compilation of data available in the literature for local and high-redshift starbursting systems and quasars. We show that the C I luminosity correlates on global scales with the infrared luminosity $L_{\mathrm{I}\mathrm{R}}$ similarly to low-$J$ CO transitions. We report a systematic variation of $L_{[\mathrm{C}\mathrm{I}{]}^3{\mathrm{P}}_1-{}^3{\mathrm{P}}_0}^{\mathrm{\prime }}$/$L_{\mathrm{I}\mathrm{R}}$ as a function of the galaxy type, with the ratio being larger for main-sequence galaxies than for starbursts and sub-millimeter galaxies at fixed $L_{\mathrm{I}\mathrm{R}}$. The $L_{[\mathrm{C}\mathrm{I}{]}^3{\mathrm{P}}_1-{}^3{\mathrm{P}}_0}^{\mathrm{\prime }}$/$L_{\mathrm{C}\mathrm{O}(2-1)}^{\mathrm{\prime }}$ and $M_{\rm{[C I]}}$\mathrm{/}$M_{\rm dust}$ mass ratios are similar for main-sequence galaxies and for local and high-redshift starbursts within a 0.2 dex intrinsic scatter, suggesting that [C I] is a good tracer of molecular gas mass as CO and dust. We derive a fraction of $f_{[\mathrm{C}\mathrm{I}]}=M_{[\mathrm{C}\mathrm{I}]}\mathrm{/}{M}_{\mathrm{C}}\sim 3-13$% of the total carbon mass in the atomic neutral phase. Moreover, we estimate the neutral atomic carbon abundance, the fundamental ingredient to calibrate [C I] as a gas tracer, by comparing $L_{[\mathrm{C}\mathrm{I}{]}^3{\mathrm{P}}_1-{}^3{\mathrm{P}}_0}^{\mathrm{\prime }}$ and available gas masses from CO lines and dust emission. We find lower [C I] abundances in main-sequence galaxies than in starbursting systems and sub-millimeter galaxies, as a consequence of the canonical ${\alpha }_{\mathrm{C}\mathrm{O}}$ and gas-to-dust conversion factors. This argues against the application to different galaxy populations of a universal standard [C I] abundance derived from highly biased samples.