Multi-phase Nature of a Radiation-Driven Fountain with Nuclear Starburst in a Low-mass Active Galactic Nucleus

TL;DR

This study uses advanced 3D radiation hydrodynamic simulations to reveal the complex, multi-phase structure of gas around a low-mass AGN, challenging the traditional torus model and aligning with recent observations.

Contribution

First to incorporate non-equilibrium chemistry in a radiation-driven fountain model with supernova feedback for low-luminosity AGNs.

Findings

Formation of a double hollow cone structure without a thick torus

Presence of hot and cold molecular hydrogen phases with significant scale height

Polar mid-infrared emission consistent with bipolar outflows and observed galaxy SEDs

Abstract

The structures and dynamics of molecular, atomic, and ionized gases are studied around a low-luminosity active galactic nucleus (AGN) with a small ($2\times 10^6 M_\odot$) black hole using 3D radiation hydrodynamic simulations. We studied, for the first time, the non-equilibrium chemistry for the X-ray dominated region in the "radiation-driven fountain" (Wada 2012) with supernova feedback. A double hollow cone structure is naturally formed without postulating a thick "torus" around a central source. The cone is occupied with an inhomogeneous, diffuse ionized gas and surrounded by a geometrically thick ($h/r \gtrsim 1$) atomic gas. Dense molecular gases are distributed near the equatorial plane, and energy feedback from supernovae enhances their scale height. Molecular hydrogen exists in a hot phase ( > 1000 K) as well as in a cold ( < 100 K), dense ( >$10^3$ cm$^{-3}$) phase. The velocity dispersion of H$_2$ in the vertical direction is comparable to the rotational velocity, which is consistent with near infrared observations of nearby Seyfert galaxies. Using 3D radiation transfer calculations for the dust emission, we find polar emission in the mid-infrared band (12$\mu m$), which is associated with bipolar outflows, as suggested in recent interferometric observations of nearby AGNs. If the viewing angle for the nucleus is larger than 75 deg, the spectral energy distribution (~ 2 -- 60 $\mu m$) of this model is consistent with that of the Circinus galaxy. The multi-phase interstellar medium observed in optical/infrared and X-ray observations is also discussed.