ALMA Observations of Multiple-CO and C Lines Toward the Active Galactic Nucleus of NGC 7469: X-Ray-dominated Region Caught in the Act
Abstract
We used the Atacama Large Millimeter/submillimeter Array (ALMA) to map 12CO(J = 1-0), 12CO(J = 2-1), 12CO(J = 3-2), 13CO(J = 2-1), and [CI](3P1-3P0) emission lines around the type 1 active galactic nucleus (AGN) of NGC 7469 (z = 0.0164) at 100 pc resolutions. The CO lines are bright both in the circumnuclear disk (central 300 pc) and the surrounding starburst (SB) ring ( 1 kpc diameter), with two bright peaks on either side of the AGN. By contrast, the [CI](3P1-3P0) line is strongly peaked on the AGN. Consequently, the brightness temperature ratio of [CI](3P1-3P0) to 13CO(2-1) is 20 at the AGN, as compared to 2 in the SB ring. Our local thermodynamic equilibrium (LTE) and non-LTE models indicate that the enhanced line ratios (or CI enhancement) are due to an elevated C0/CO abundance ratio ( 3-10) and temperature ( 100-500 K) around the AGN as compared to the SB ring (abundance ratio 1, temperature 100 K), which accords with the picture of the X-ray-dominated Region (XDR). Based on dynamical modelings, we also provide CO(1-0)-to- and [CI](3P1-3P0)-to-molecular mass conversion factors at the central 100 pc of this AGN as α CO = 4.1 and α CI = 4.4~M (K km s-1 pc2)-1, respectively. Our results suggest that the CI enhancement is potentially a good marker of AGNs that could be used in a new submillimeter diagnostic method toward dusty environments.