Resolved Neutral Carbon Emission in Nearby Galaxies: [C I] Lines as Total Molecular Gas Tracers

Abstract

We present maps of atomic carbon [C I]({}³{{{P}}}₁\to {{}³{{P}}}₀) and [C I]({}³{{{P}}}₂\to {{}³{{P}}}₁) emission (hereafter [C I] (1-0) and [C I] (2-1), respectively) at a linear resolution ∼1 kpc scale for a sample of one H II, six LINER, three Seyfert, and five starburst galaxies observed with the Herschel Space Observatory. We compare spatial distributions of two [C I] lines with that of CO J=1\to 0 (hereafter CO (1-0)) emission, and find that both [C I] lines distribute similarly to CO (1-0) emission in most galaxies. We present luminosity ratio maps of {L}_[{{C} {{I}}](1-0)}^{\prime }/{L}_CO(1-0)}^{\prime }, {L}_[{{C} {{I}}](2-1)}^{\prime }/{L}_CO(1-0)}^{\prime }, {L}_[{{C} {{I}}](2-1)}^{\prime }/{L}_[{{C} {{I}}](1-0)}^{\prime } (hereafter {R}_[{{C}{{I}}]}) and 70-to-160 μm far-infrared color of f ₇₀/f ₁₆₀. {L}_[{{C} {{I}}](2-1)}^{\prime }/{L}_CO(1-0)}^{\prime }, {R}_[{{C}{{I}}]} and {f}₇₀/{f}₁₆₀ are centrally peaked in starbursts; whereas they remain relatively constant in LINERs, indicating that star-forming activity can enhance carbon emission, especially for [C I] (2-1). We explore the correlations between the luminosities of CO (1-0) and [C I] lines, and find that {L}_CO(1-0)}^{\prime } correlates tightly and almost linearly with both {L}_[{{C} {{I}}](1-0)}^{\prime } and {L}_[{{C} {{I}}](2-1)}^{\prime }, suggesting that [C I] lines, similar to CO (1-0), can trace total molecular gas in H II, LINER, Seyfert, and starburst galaxies on kpc scales. We 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 [C I] excitation temperature, T _ex, on dust temperature, T _dust, and find noncorrelation and a weak and modest correlation, respectively. The ratio of {L}_[{{C} {{I}}](1-0)}^{\prime }/{L}_CO(1-0)}^{\prime } stays a smooth distribution in most galaxies, indicating that the conversion factor of [C I] (1-0) luminosity to H₂ mass ({X}_[{CI](1-0)}) changes with CO (1-0) conversion factor ({α }_CO}) proportionally. Under optically thin and local thermodynamical equilibrium assumptions, we derive a galaxy-wide average carbon excitation temperature of {T}_ex}∼ 19.7+/- 0.5 {{K}}, and an average neutral carbon abundance of X[{CI}]/X[{{{H}}}₂]∼ 2.5+/- 1.0× {10}^-5 in our resolved sample, which is comparable to the usually adopted value of 3 × 10^-5, but ∼3 times lower than the carbon abundance in local (ultra)luminous infrared galaxies. We conclude that the carbon abundance varies in different galaxy types.