Neutral Carbon Emission in luminous infrared galaxies The \CI\ Lines as
  Total Molecular Gas Tracers
  We present a statistical study on the [C I]($^{3} \rm P_{1} \rightarrow {\rm
^3 P}_{0}$), [C I] ($^{3} \rm P_{2} \rightarrow {\rm ^3 P}_{1}$) 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 $L'_\mathrm{CO(1-0)}$ correlates almost linearly with both $L'_
\mathrm{[CI](1-0)}$ and $L'_\mathrm{[CI](2-1)}$, suggesting that [C I] lines
can trace total molecular gas mass at least for (U)LIRGs. We also investigate
the dependence of $L'_\mathrm{[CI](1-0)}$/$L'_\mathrm{CO(1-0)}$,
$L'_\mathrm{[CI](2-1)}$/$L'_\mathrm{CO(1-0)}$ and
$L'_\mathrm{[CI](2-1)}$/$L'_\mathrm{[CI](1-0)}$ 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 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.
