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

We present the first results of an Atacama Large Millimeter Array 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 ∼ 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] (³ P ₁ → ³ P ₀) luminosity correlates on global scales with the infrared luminosity {L}_IR}, similar to low-J CO transitions. We report a systematic variation of {L}_{[{{C}{{I}}]}³{P}₁ {--}{}³{P}₀}^{\prime }/{L}_IR} as a function of the galaxy type, with the ratio being larger for main-sequence galaxies than for starbursts and submillimeter galaxies at fixed {L}_IR}. The {L}_{[{{C}{{I}}]}³{P}₁ {--}{}³{P}₀}^{\prime }/{L}_CO(2-1)}^{\prime } and {M}_[{{C}{{I}}]}/{M}_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}_[{{C}{{I}}]}={M}_[{{C}{{I}}]}/{M}_{{C}}∼ 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}_{[{{C}{{I}}]}³{P}₁ {--}{}³{P}₀}^{\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 submillimeter galaxies as a consequence of the canonical α _CO 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.