Neutral Carbon Emission in luminous infrared galaxies The \CI\ Lines as Total Molecular Gas Tracers

Abstract: We present a statistical study on the C I, C I lines (hereafter C I and C I, 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 $L'\mathrm{CO(1-0)}$ correlates almost linearly with both $L' \mathrm{CI}$ and $L'\mathrm{CI}$, suggesting that [C I] lines can trace total molecular gas mass at least for (U)LIRGs. We also investigate the dependence of $L'\mathrm{CI}$/$L'\mathrm{CO(1-0)}$, $L'\mathrm{CI}$/$L'\mathrm{CO(1-0)}$ and $L'\mathrm{CI}$/$L'\mathrm{CI}$ 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 the mean [C I] line flux-to-H$_2$ mass conversion factors for our sample. The resulting $\mathrm{H_2}$ masses using these [C I]-based conversion factors roughly agree with those derived from $L'\mathrm{CO(1-0)}$ and CO-to-H$_2$ conversion factor.