2019ApJ...887..143S

We present [C I] (³P₁→³P₀), ¹²CO, ¹³CO, and C¹⁸O (J=2→1) observations of the central region (radius 1 kpc) of the starburst galaxy NGC 1808 at 30-50 pc resolution conducted with the Atacama Large Millimeter/submillimeter Array. Radiative transfer analysis of multiline data indicates warm (T_k∼40–80 K) and dense (n_H2_∼10^3–4 cm^–3) molecular gas with high column density of atomic carbon (N_CI∼3×10¹⁸ cm^–2) in the circumnuclear disk (central 100 pc). The C I/H₂ abundance in the central 1 kpc is ∼3–7×10^–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 ¹³CO (2-1), and the ratios of [C I] to ¹³CO (2-1) and C¹⁸O (2-1) are uniform within ∼30 % in the central < 400 pc starburst disk. The results suggest that [C I] (³P₁→³P₀) 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 owing to high excitation and atomic carbon abundance. The [C I]/CO (1-0) intensity ratio toward the base of the starburst-driven outflow is ≲0.15, and the upper limits of the mass and kinetic energy of the atomic carbon outflow are ∼1×10⁴M_☉ and ∼3×10⁵¹erg, respectively.