High-Resolution [CI] (3 P 2 to 3 P 1) and CO (J=7 to6) Observations of Circumnuclear Disks in Nearby Seyfert Galaxies

We present [CI] (3 P 2- 3 P 1), CO (J=7-6), and 800-GHz continuum observations of the circumnuclear disks (CNDs) in the Seyfert galaxies NGC 613 and NGC 1808 with the Atacama Large Millimeter/submillimeter Array. The images reveal the distributions of neutral gas and dust in nearby galaxy nuclei at a resolution of 0.2'' (10 pc) which is unprecedented at these frequencies. Both lines and continuum exhibit peaks at the position of molecular tori. The maximum [CI] (3 P 2- 3 P 1)/(3 P 1- 3 P 0) integrated-intensity ratio is about1.1 (K km s-1 scale) in the region of the torus, yielding an excitation temperature of T ex about60 K under the approximation of local thermodynamic equilibrium (LTE) and optically thin emission. A non-LTE radiative transfer analysis of the [CI] line ratio in the CND constrains the density and kinetic temperature of molecular gas to n H 2>10 3 cm-3 and T k>50 K, respectively. To estimate the C/CO abundance ratio and physical conditions, we simultaneously modeled the [CI] and multiple CO lines, and obtained solutions with densities and temperatures of n H 2 about10 3-4 cm-3 and T k about80-250 K in the central 63 pc. The C/CO ratios are about2 in NGC 613 and about0.6 in NGC 1808. C/CO tends to increase with T k and decrease with n H 2 in the CND, which is consistent with models of photodissociation regions. Based on the kinematics of gas traced by [CI] (3 P 2- 3 P 1) and CO (J=7-6), the mass of the supermassive black hole at the center of NGC 613 is estimated to be about2 times10 7 M.