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 ${}^{12}$CO($J$ = 1-0), ${}^{12}$CO($J$ = 2-1), ${}^{12}$CO($J$ = 3-2), ${}^{13}$CO($J$ = 2-1), and [CI](${}^3{P}_1$-${}^3{P}_0$) emission lines around the type 1 active galactic nucleus (AGN) of NGC 7469 ($z=0.0164$) at $\sim 100$ pc resolutions. The CO lines are bright both in the circumnuclear disk (central $\sim 300$ pc) and the surrounding starburst (SB) ring ($\sim 1$ kpc diameter), with two bright peaks on either side of the AGN. By contrast, the [CI](${}^3{P}_1$-${}^3{P}_0$) line is strongly peaked on the AGN. Consequently, the brightness temperature ratio of [CI](${}^3{P}_1$-${}^3{P}_0$) to ${}^{13}$CO(2-1) is $\sim 20$ at the AGN, as compared to $\sim 2$ 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 C${}^0$/CO abundance ratio ($\sim 3-10$) and temperature ($\sim 100-500$ K) around the AGN as compared to the SB ring (abundance ratio $\sim 1$, temperature $\lesssim 100$ 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](${}^3{P}_1$-${}^3{P}_0$)-to-molecular mass conversion factors at the central $\sim 100$ pc of this AGN as $\hat{I}{\pm }_{\mathrm{CO}}=4.1$ and $\hat{I}{\pm }_{\mathrm{CI}}=4.4M_{\odot }$ (K km s${}^{-1}$ pc${}^2$)${}^{-1}$, 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.

25 pages, 12 figures, 8 tables. Accepted for publication in ApJ. Minor updates in a replacement