2020ApJ...890...24V

We report Atacama Large Millimeter Array observations of the neutral atomic carbon transitions [C I] and multiple CO lines in a sample of ∼30 main-sequence galaxies at z∼1, including novel information on [C I](³P₂–³P₁) and CO (7–6) for 7 of such normal objects. We complement our observations with a collection of >200 galaxies with coverage of similar transitions, spanning the z = 0-4 redshift interval and a variety of ambient conditions from local to high-redshift starbursts. We find systematic variations in the [C I]/IR and [C I]/high-J_upper (J_upper = 7) CO luminosity ratios among the various samples. We interpret these differences as increased dense molecular gas fractions and star formation efficiencies in the strongest high-redshift starbursts with respect to normal main-sequence galaxies. We further report constant L_[CI]3P_2–3P₁^′^/L[CI]^3P1-^3P0_^′^ ratios across the galaxy populations and redshifts, suggesting that gas temperatures T_exc traced by [C I] do not strongly vary. We find only a mild correlation with T_dust and that, generally, T_exc <= T_dust. We fit the line ratios with classical photodissociation region models, retrieving consistently larger densities and intensities of the UV radiation fields in submillimeter galaxies than in main-sequence and local objects. However, these simple models fall short in representing the complexity of a multiphase interstellar medium and should be treated with caution. Finally, we compare our observations with the Santa Cruz semi-analytical model of galaxy evolution, recently extended to simulate submillimeter emission. While we confirm the success in reproducing the CO lines, we find systematically larger [C I] luminosities at fixed IR luminosity than predicted theoretically. This highlights the necessity of improving our understanding of the mechanisms regulating the [C I] emission on galactic scales. We release our data compilation to the community.