Circumnuclear Multi-phase Gas in Circinus Galaxy IV: absorption owing to high-$J$ CO rotational transitions

TL;DR

This study uses 3D non-LTE simulations to analyze high-J CO absorption features in the Circinus galaxy's torus, revealing the structure and conditions of the obscuring material around the AGN.

Contribution

It introduces a detailed radiative transfer model of CO absorption in AGN tori, linking simulations with high-resolution observations to probe the internal structure of the molecular torus.

Findings

Absorption features are prominent along the disk mid-plane at high-J transitions.

Deepest absorption occurs in gas 10-15 pc from the nucleus, in front of hot dust.

Resolution of 0.5-1.0 pc and near edge-on inclination are necessary to detect absorption.

Abstract

We studied the absorption features of CO lines against the continuum originating from the heated dust in the obscuring tori around active galactic nuclei (AGNs). We investigated the formation of absorption lines corresponding to the CO rotational transitions using three-dimensional non-LTE line transfer simulations considering the dust thermal emission. As in Papers I--III of this series, we performed post-processed radiative transfer calculations using the "radiation-driven fountain model" (wada2016}, which yields a geometrically thick obscuring structure around the nucleus. This model is consistent with the spectral energy distribution of the nearest type-2 Seyfert galaxy, the Circinus galaxy. We found that the continuum-subtracted channel maps of $J = 4-3$ and higher transitions show absorption regions along the disk mid-plane for an edge-on viewing angle. The spectra consist of multiple absorption and emission features, reflecting the internal inhomogeneous and turbulent structure of the torus. The deepest absorption feature is caused by the gas on the near-side of the torus between $r =10$ and 15 pc, which is located in front of the AGN-heated dust inside $r \simeq 5$ pc. We also found that a spatial resolution of 0.5--1.0 pc is necessary to resolve the absorption features. Moreover, the inclination angle must be close to the edge-on angle (i.e., $\sim 85^\circ$) to observe the absorption features. The findings of the present study imply that combining our radiation-hydrodynamic model with high-resolution observations of CO (7-6) by ALMA can provide new information about the internal structure of the molecular tori in nearby AGNs.