Neutral Carbon Emission in Luminous Infrared Galaxies: The [CI] Lines as Total Molecular Gas Tracers

We present a statistical study of the [C I] (P-3(1) (R) P-3(0)), [C I] (P-3(2) (R) P-3(1)) 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))/L'(CO (1-0)), L'([C I] (2-1))/L'(CO (1-0)), and L'([C I] (2-1))/L'([C I] (1-0)) on the far-infrared color of 60-to-100 mu m, 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-H-2 mass conversion factors for our sample. The resulting H-2 masses using these [C I]-based conversion factors roughly agree with those derived from L'(CO (1-0)) and CO-to-H-2 conversion factor.