ALMA Observations of Atomic Carbon [C I] (${^3\mathrm{P}}_1\rightarrow{^3\mathrm{P}}_0$) and Low-$J$ CO Lines in the Starburst Galaxy NGC 1808

Abstract: We present C I, $^{12}$CO, $^{13}$CO, and C$^{18}$O ($J=2\rightarrow1$) observations of the central region (radius 1 kpc) of the starburst galaxy NGC 1808 at 30-50 pc resolution conducted with Atacama Large Millimeter/submillimeter Array. Radiative transfer analysis of multiline data indicates warm ($T_\mathrm{k}\sim40\mathrm{-}80$ K) and dense ($n_\mathrm{H_2}\sim10^{3\mathrm{-}4}$ cm$^{-3}$) molecular gas with high column density of atomic carbon ($N_\mathrm{CI}\sim3\times10^{18}$ cm$^{-2}$) in the circumnuclear disk (central 100 pc). The C I/H$2$ abundance in the central 1 kpc is $\sim3-7\times10^{-5}$, consistent with the values in luminous infrared galaxies. The intensity ratios of [C I]/CO(1-0) and [C I]/CO(3-2), respectively, decrease and increase with radius in the central 1 kpc, whereas [C I]/CO(2-1) is uniform within statistical errors. The result can be explained by excitation and optical depth effects, since the effective critical density of CO (2-1) is comparable to that of [C I]. The distribution of [C I] is similar to that of $^{13}$CO (2-1), and the ratios of [C I] to $^{13}$CO (2-1) and C$^{18}$O (2-1) are uniform within $\sim30\%$ in the central $<400$ pc starburst disk. The results suggest that C I luminosity can be used as a CO-equivalent tracer of molecular gas mass, although caution is needed when applied in resolved starburst nuclei (e.g., circumnuclear disk), where the [C I]/CO(1-0) luminosity ratio is enhanced due to high excitation and atomic carbon abundance. The [C I]/CO(1-0) intensity ratio toward the base of the starburst-driven outflow is $\lesssim0.15$, and the upper limits of the mass and kinetic energy of the atomic carbon outflow are $\sim1\times10^4~M\odot$ and $\sim3\times10^{51}$ erg, respectively.