2026/07/23 by Dragan Salak, Suphakorn Suphapolthaworn, Yusuke Miyamoto +3
Physics and Astronomy · #astro-ph.GA
We present [CI] (3P2-3P1), 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](3P2-3P1)/(3P1-3P0) integrated-intensity ratio is ≈1.1 (K~km~s-1 scale) in the region of the torus, yielding an excitation temperature of Tex≈60~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 nH2>103~cm-3 and Tk>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 nH2∼103-4~cm-3 and Tk∼80-250~K in the central 63 pc. The C/CO ratios are ≈2 in NGC 613 and ≈0.6 in NGC 1808. C/CO tends to increase with Tk and decrease with nH2 in the CND, which is consistent with models of photodissociation regions. Based on the kinematics of gas traced by [CI] (3P2-3P1) and CO (J=7-6), the mass of the supermassive black hole at the center of NGC 613 is estimated to be ∼2×107~M_\odot.