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
Takuma Izumi, Dieu D. Nguyen, Masatoshi Imanishi, Taiki Kawamuro, Shunsuke Baba, Suzuka Nakano, Kotaro Kohno, Satoki Matsushita, David S. Meier, Jean L. Turner, Tomonari Michiyama, Nanase Harada, Sergio Martín, Kouichiro Nakanishi, Shuro Takano, Tommy Wiklind, Naomasa Nakai, Pei-Ying Hsieh
Abstract
Abstract We used the Atacama Large Millimeter/submillimeter Array to map 12 CO( J = 1–0), 12 CO( J = 2–1), 12 CO( J = 3–2), 13 CO( J = 2–1), and [C i ]( 3 P 1 – 3 P 0 ) 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 ]( 3 P 1 – 3 P 0 ) line is strongly peaked on the AGN. Consequently, the brightness temperature ratio of [C i ]( 3 P 1 – 3 P 0 ) to 13 CO(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 C 0 /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 ]( 3 P 1 – 3 P 0 )-to-molecular mass conversion factors at the central ∼100 pc of this AGN as α CO = 4.1 and α C i = 4.4 M ⊙ (K km s −1 pc 2 ) −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.