Ionized Carbon in Galaxies: The [C ii] 158 μm Line as a Total Molecular Gas Mass Tracer Revisited

In this paper, we present a statistical study of the [C ii ] 158 μ m 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 G 0 / 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 ≳ 10 11 L ⊙ 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.