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

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](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 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](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 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.