2024ApJ...977...46Z

In this paper, we present a statistical study of the [C II] 158 μm line and the CO(1-0) emission for a sample of ∼200 local and high-z (32 sources with z > 1) galaxies with very different physical conditions. We explore the correlation between the luminosities of [C II] and CO(1-0) lines and obtain a strong linear relationship, confirming that [C II] is able to trace total molecular gas mass, with a small difference between (U)LIRGs and less-luminous galaxies. The tight and linear relation between [C II] and CO(1-0) is likely determined by the average value of the observed visual extinction A_V and the range of G₀/n in galaxies. Further investigations into the dependence of L_[C ii]_/L_CO(1–0) on different physical properties show that L_[C ii]_/L_CO(1–0) (1) anticorrelates with Σ_IR, and the correlation becomes steeper when Σ_IR ≳ 10¹¹L_⊙ kpc^–2; (2) correlates positively with the distance from the main sequence Δ(MS) when Δ(MS) ≲ 0; and (3) tends to show a systematically smaller value in systems where the [C II] emission is dominated by ionized gas. Our results imply that caution needs to be taken when applying a constant [C II]-to-M_mol conversion factor to estimate the molecular gas content in extreme cases, such as galaxies having low-level star formation activity or high star formation rate surface density.