Unification Model of Active Galactic Nuclei by Photoionization Equilibrium Calculation Based on Radiative Hydrodynamic Simulations

TL;DR

This study uses radiative hydrodynamic simulations and photoionization calculations to analyze the covering factors of HI in active galactic nuclei, revealing their dependence on Eddington ratio and consistency with X-ray observations.

Contribution

It introduces a combined simulation and photoionization approach to quantify AGN covering factors, highlighting their independence from Eddington ratio and the need for extended gas regions.

Findings

C_{22} of HI and HII is about 70%, independent of Eddington ratio.

C_{24} of HI matches C_{22} and aligns with X-ray data.

Discrepancies at low Eddington ratios suggest extended gas regions are needed.

Abstract

To investigate the origin of the dependence of the covering factor on the Eddington ratio suggested by X-ray observations, we examined the angular distribution of HI and HII based on two-dimensional radiative hydrodynamic simulations. To calculate the Compton-thin covering factor $C_{22}$ and Compton-thick covering factor $C_{24}$ of HI alone, we performed one-dimensional photoionization equilibrium calculations with the XSTAR code based on radiative hydrodynamic simulations. The results obtained are as follows. (1) The Compton-thin covering factor $C_{22}$ of HI and HII is independent of the Eddington ratio and is approximately $70\%$, while $C_{22}$ of HI alone is also independent of the Eddington ratio and is approximately $30\%$. (2) The Compton-thick covering factor $C_{24}$ of HI has the same value as $C_{22}$ of HI. (3) Our $C_{24}$ is consistent with that obtained from X-ray observations. (4) Our $C_{22}$ agrees with that obtained from X-ray observations in a high Eddington ratio, while our $C_{22}$ is smaller than that from X-ray observations in a low Eddington ratio. (5) To explain the difference between $C_{22}$ obtained from theoretical calculations and that inferred from X-ray observations, a Compton-thin gas is required in regions extending at least $10~\mathrm{pc}$ beyond the current computational regions.