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

Abstract

We present a statistical study of the [C I] (³P₁ \to ³P₀), [C I] (³P₂ \to 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)}^\prime correlates almost linearly with both {L}_[{{C} {{I}}](1-0)}^\prime and {L}_[{{C} {{I}}](2-1)}^\prime , 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)}^\prime /{L}_CO(1-0)}^\prime , {L}_[{{C} {{I}}](2-1)}^\prime /{L}_CO(1-0)}^\prime , and {L}_[{{C} {{I}}](2-1)}^\prime /{L}_[{{C} {{I}}](1-0)}^\prime 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 {{{H}}}₂ masses using these [C I]-based conversion factors roughly agree with those derived from {L}_CO(1-0)}^\prime and CO-to-H₂ conversion factor.

Based on Herschel observations. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.