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

Abstract

We present [C I] (${{}^3\mathrm{P}}_1\to {{}^3\mathrm{P}}_0$), ${}^{12}$CO, ${}^{13}$CO, and C${}^{18}$O ($J=2\to 1$) 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 ($T_{\mathrm{k}}\sim 40-80$ K) and dense ($n_{{\mathrm{H}}_2}\sim 10^{3-4}$ cm${}^{-3}$) molecular gas with high column density of atomic carbon ($N_{\mathrm{CI}}\sim 3\times 10^{18}$ cm${}^{-2}$) in the circumnuclear disk (central 100 pc). The C I/H${}_2$ abundance in the central 1 kpc is $\sim 3-7\times 10^{-5}$, 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 $\sim 30\%$ in the central $<400$ pc starburst disk. The results suggest that [C I] (${{}^3\mathrm{P}}_1\to {{}^3\mathrm{P}}_0$) 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 $\lesssim 0.15$, and the upper limits of the mass and kinetic energy of the atomic carbon outflow are $\sim 1\times 10^4{M}_{\odot }$ and $\sim 3\times 10^{51}$ erg, respectively.

Accepted for publication in ApJ (30 pages, 15 figures)