ALMA Observations of Multiple CO and C Lines toward the Active Galactic Nucleus of NGC 7469: An X-Ray-dominated Region Caught in the Act
Baba, Shunsuke; Meier, David S.; Turner, Jean L.; Imanishi, Masatoshi; Kohno, Kotaro; Kawamuro, Taiki; Nakai, Naomasa; Martín, Sergio; Matsushita, Satoki; Nguyen, Dieu D.; Izumi, Takuma; Wiklind, Tommy; Nakanishi, Kouichiro; Michiyama, Tomonari; Harada, Nanase; Takano, Shuro; Hsieh, Pei-Ying; Nakano, Suzuka
United States, Japan, Taiwan, China, Chile
Abstract
We used the Atacama Large Millimeter/submillimeter Array to map 12CO(J = 1-0), 12CO(J = 2-1), 12CO(J = 3-2), 13CO(J = 2-1), and [C I](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 in both 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 [C I](3P1-3P0) line is strongly peaked on the AGN. Consequently, the brightness temperature ratio of [C I](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 C I 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. Based on dynamical modelings, we also provide CO(1-0)-to- and [C I](3P1-3P0)-to-molecular mass conversion factors at the central ∼100 pc of this AGN as αCO = 4.1 and αC I = 4.4 M⊙ (K km s-1 pc2)-1, respectively. Our results suggest that the C I enhancement is potentially a good marker of AGNs that could be used in a new submillimeter diagnostic method toward dusty environments.