Circumnuclear Multi-phase Gas in Circinus Galaxy III: Structure of the Nuclear Ionized Gas

TL;DR

This study models the ionized gas structure around the AGN in the Circinus galaxy, revealing conical morphologies and properties similar to observed narrow line regions without assuming an optically thick torus.

Contribution

It introduces a radiation-driven fountain model to explain the ionized gas morphology and spectral properties in the Circinus galaxy's nucleus, challenging traditional torus-based models.

Findings

Ionized gas forms conical structures around the rotation axis.

Most regions exhibit properties similar to narrow line regions in AGNs.

Central 10-pc regions show LINER-like spectral characteristics.

Abstract

We investigate the properties of the ionized gas irradiated by an active galactic nucleus (AGN) based on our "radiation-driven fountain" model for the nearest type-2 Seyfert galaxy, the Circinus galaxy (Wada et al. 2016). We conducted "quasi-three dimensional" spectral analysis using the spectral synthesis code CLOUDY and obtained the surface brightness distributions of lines, such as H$\alpha$, H$\beta$, [OIII], [NII], and [SII] for the central 16-parsec region. The ionized regions observed based on these lines show a conical morphology around the rotation axis, even if we do not phenomenologically postulate the presence of an optically thick "torus". This region also shows non-uniform internal structures, reflecting the inhomogeneous structure of fountain flows. Using ionization diagnostic diagrams, we investigated the spectral properties of the ionized gas. The diagrams based on the line ratios of [NII]/H$\alpha$ and [SII]/H$\alpha$ show that most regions of the cone have the same properties as those in the narrow line regions (NLRs) in AGNs, whereas using [OI]/H$\alpha$, the central 10-pc regions are rather LINER-like. The gas density, temperature, and ionizing parameter in regions identified as "NLR" are typically $n \sim 300-1500$ cm$^{-3}$, $T \sim 1-3 \times 10^4 $ K, and $ U \sim 0.01$, respectively. The morphology and [OIII] intensity are similar to the base of the observed [OIII] cone in the Circinus galaxy, implying some physical connections with the origin of the $\sim100$ parsec scale NLR.