On the evolution of gas clouds exposed to AGN radiation. I. Three-dimensional radiation hydrodynamic simulations

TL;DR

This study uses 3D radiation hydrodynamic simulations to explore how dusty gas clouds near AGN evolve under different ionization conditions, revealing distinct photo-evaporation and radiation pressure-driven behaviors.

Contribution

It provides the first detailed 3D simulation-based analysis of gas cloud evolution under AGN radiation, highlighting the dependence on ionization parameters and implications for clumpy torus stability.

Findings

Low ionization parameter leads to photo-evaporation and filament formation.

High ionization parameter results in radiation pressure-driven gas disks.

Clumps are destroyed faster than their orbital periods unless radiation is weakened.

Abstract

We perform three-dimensional radiation hydrodynamic simulations of uniform dusty gas clouds irradiated by an active galactic nucleus (AGN) to investigate the dependence of evolution of clouds on the ionization parameter $\mathcal{U}$ and the Str{\"o}mgren number $\mathcal{N}_{S}$. We find that the evolution can be classified into two cases depending on $\mathcal{U}$. In low $\mathcal{U}$ cases ($\mathcal{U}\approx 10^{-2}$), the evolution is mainly driven by photo-evaporation. A approximately spherically-symmetric evaporation flow with velocity of $100\operatorname{-}150\;\mathrm{km\;s^{-1}}$ is launched from the irradiated face. The cloud is compressed by a D-type shock with losing its mass due to photo-evaporation and is finally turned into a dense filament by $t\lesssim 1.5t_{\mathrm{sc}}$. In high $\mathcal{U}$ cases ($\mathcal{U}\approx 5\times 10^{-2}$), radiation pressure suppresses photo-evaporation from the central part of the irradiated face, reducing photo-evaporation rate. A evaporation flow from the outskirts of the irradiated face is turned into a high velocity ($\lesssim 500\;\mathrm{km\;s^{-1}}$) gas wind because of radiation pressure on dust. The cloud is swept by a radiation pressure-driven shock and becomes a dense gas disk by $t\approx t_{\mathrm{sweep}}$. Star formation is expected in these dense regions for both cases of $\mathcal{U}$. We discuss the influences of the AGN radiation on the clumpy torus. A simple estimate suggests that the clumps are destroyed in timescales shorter than their orbital periods. For the clumpy structure to be maintained over long period, the incident radiation field needs to be sufficiently weaken for most of the clumps, or, some mechanism that creates the clumps continuously is needed.