2017ApJ...840L..18J

We present a statistical study of the [C i] (³P₁ →³P₀), [C i] (³P₂ →³P₁) lines (hereafter [C i] (1-0) and [C i] (2-1), respectively) and the CO(1-0) line for a sample of (ultra-)luminous infrared galaxies ((U)LIRGs). We explore the correlations between the luminosities of CO(1-0) and [C i] lines, and find that L_CO(1–0)^′^ correlates almost linearly with both L_{[C I](1–0)}^′^ and L_{[C I](2–1)}^′^, suggesting that [C i] lines can trace total molecular gas mass, at least for (U)LIRGs. We also investigate the dependence of L_{[C I](1–0)}^′^/LCO(1-0)_^′^, L_{[C I](2–1)}^′^/LCO(1-0)_^′^, and L_{[C I](2–1)}^′^/L[C I](1-0)_^′^ on the far-infrared color of 60-to-100 µm, and find non-correlation, a weak correlation, and a modest correlation, respectively. Under the assumption that these two carbon transitions are optically thin, we further calculate the [C i] line excitation temperatures, atomic carbon masses, and mean [C i] line flux-to-H₂ mass conversion factors for our sample. The resulting H₂ masses using these [C i]-based conversion factors roughly agree with those derived from L_CO(1–0)^′^ and CO-to-H₂ conversion factor.