THE STRUCTURE AND DYNAMICS OF AN ACTIVE GALACTIC NUCLEUS TORUS: CO LINE PREDICTIONS FOR ALMA FROM THREE-DIMENSIONAL HYDRODYNAMICAL SIMULATIONS WITH X-RAY-DRIVEN CHEMISTRY

摘要

Several attempts have been made to model the mass distribution and dynamical evolution of the circumnuclear gas in active galactic nuclei (AGNs). However, chemical evolution is not included in detail in three-dimensional (3D) hydrodynamic simulations. The X-ray radiation from the AGN can drive the gas chemistry and affect the thermodynamics, as well as the excitation of the interstellar medium. Therefore, we estimate the effects (on chemical abundances and excitation) of X-ray irradiation by the AGN for atomic and molecular gas in a 3D hydrodynamic model of an AGN torus. We obtain the abundances of various species from an X-ray chemical model. A 3D radiative transfer code estimates the level populations, which result in line intensity maps. Predictions for the CO J = 1 → 0 to J = 9 → 8 lines indicate that mid-J CO lines are excellent probes of density and dynamics in the central (60 pc) region of the AGN in contrast to the low-J CO lines. Analysis of the X CO/α conversion factors shows that only the higher J CO lines can be used for gas mass determination in AGN tori. The [C II] 158 μm emission traces mostly the hot (Tk 1000 K) central region of the AGN torus. The [C II] 158 μm line will be useful for ALMA observations of high-redshift (z 1) AGNs. The spatial scales (≥0.25 pc) probed with our simulations match the size of the structures that ALMA will resolve in nearby (≤45 Mpc at 001) galaxies.