Circumnuclear Multi-phase Gas in the Circinus Galaxy II: The Molecular and Atomic Obscuring Structures Revealed with ALMA

TL;DR

This study uses ALMA to map molecular and atomic gas in the Circinus galaxy's nucleus, revealing the structure, composition, and dynamics of the obscuring torus around its active galactic nucleus, supporting the radiation-driven fountain model.

Contribution

First detailed ALMA observations of the multi-phase gas in the Circinus galaxy's nucleus, confirming the radiation-driven fountain as the origin of the AGN torus's structure.

Findings

The circumnuclear disk has a molecular mass of ~3 million solar masses.

The atomic gas shows a more extended vertical distribution than molecular gas.

Atomic outflows contribute to the torus' geometrical thickness.

Abstract

We used the Atacama Large Millimeter/Submillimeter Array (ALMA) to map the CO(3-2) and [CI](1-0) lines, as well as their underlying continuum emission, from the central $\sim 200$ pc region of the Circinus galaxy that hosts the nearest type 2 Seyfert-class active galactic nucleus (AGN), with a spatial resolution of $\sim 6-15$ pc. The lines and continuum-emitting regions consist of a circumnuclear disk (CND; 74 pc $\times$ 34 pc) and spiral arms. The distribution of the continuum emission revealed a temperature-dependent dust geometry and possibly polar dust elongation in the torus region. The molecular mass of the CND is $M_{\rm H2} \sim 3 \times 10^6~M_\odot$ with a beam-averaged H$_2$ column density of $\sim 5 \times 10^{23}$ cm$^{-2}$ toward the AGN position, which contributes significantly to the nuclear obscuration. The [CI](1-0)/CO(3-2) ratio at the AGN position is unusually high, suggesting an X-ray dominated region-type chemistry. We decomposed the observed velocity fields into rotational and dispersion components, and revealed multi-phase dynamic nature in the $r \lesssim 10$ pc torus region, i.e., the diffuse atomic gas is more spatially extended along the vertical direction of the disk than the dense molecular gas. Through comparisons with our model predictions based on the radiation-driven fountain scheme, we indicate that atomic outflows are the driver of the geometrical thickness of the atomic disk. This supports the validity of the radiation-driven fountain scheme in the vicinity of this AGN, which would explain the long-lasting mystery, the physical origin of the AGN torus.