ALMA Observations of Atomic Carbon [C i] (3P1 → 3P0) and Low-J CO Lines in the Starburst Galaxy NGC 1808

Abstract

Abstract We present [C i] , 12CO, 13CO, and C18O ( ) 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 ( K) and dense ( cm−3) molecular gas with high column density of atomic carbon ( cm−2) in the circumnuclear disk (central 100 pc). The C i/H2 abundance in the central 1 kpc is , 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 in the central 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 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 , and the upper limits of the mass and kinetic energy of the atomic carbon outflow are and , respectively.