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