ALMA Observations of Atomic Carbon [C I] ( ³ P ₁ → ³ P ₀ ) and Low-J CO Lines in the Starburst Galaxy NGC 1808

Abstract

We present [C I] ≤ft({}³{{P}}₁\to {}³{{P}}₀\right), ¹²CO, ¹³CO, and C¹⁸O (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 the Atacama Large Millimeter/submillimeter Array. Radiative transfer analysis of multiline data indicates warm ({T}_{{k}}∼ 40{--}80 K) and dense ({n}_{{{H}}₂}∼ {10}^3{--4} cm^-3) molecular gas with high column density of atomic carbon ({N}_{{C}{{I}}}∼ 3× {10}¹⁸ 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] ≤ft({}³{{P}}₁\to {}³{{P}}₀\right) 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× {10}⁴ {M}_⊙ and ∼ 3× {10}⁵¹ {erg}, respectively.