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

TL;DR

This study tests a photoionization-based unification of active galactic nuclei by integrating two-dimensional radiative hydrodynamic simulations with one-dimensional photoionization equilibrium calculations to derive neutral hydrogen covering factors. By computing $C_{22}$ and $C_{24}$ for H I (and H I/H II mixtures) across four Eddington ratios, the work finds a robust Compton-thick factor around $\sim$30% and a Compton-thin factor near $\sim$30%–70% that depends on $R_{\rm Edd}$, with inner gas ($\lesssim 0.1$ pc) being photoionized and largely independent of $R_{\rm Edd}$. The results agree with X-ray observations for $C_{24}$, but suggest that to reproduce observed $C_{22}$ at low $R_{\rm Edd}$, Compton-thin gas at larger radii (e.g., $\sim 10$–$30$ pc) must exist; this motivates an extended, photoionization-dominated AGN unified model where ionization state, not just geometry, governs obscuration. The work highlights the role of photoionization in setting inner obscuration and connects the radiation-driven fountain scenario with observed AGN diversity, offering a framework for interpreting X-ray obscuration in terms of radial and angular gas structure.

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.

Figures (8)

• Figure 1: The spectral energy distribution (SED). The red, green, light blue, and purple lines correspond to the SED of the logarithmic Eddington ratio $\log R_{\mathrm{Edd}} = 0,-1,-2,\ \mathrm{and}\ -3$, respectively.
• Figure 2: (a) The distribution of the hydrogen number density $\log n_{\mathrm{H}}/\mathrm{cm}^{-3}$ obtained from the radiative hydrodynamic simulation with logarithmic Eddington ratio $\log R_{\mathrm{Edd}}$ of $-3$. (b) The $\log n_{\mathrm{H}}/\mathrm{cm}^{-3}$ distribution with $\log R_{\mathrm{Edd}} = -2$. (c) The $\log n_{\mathrm{H}}/\mathrm{cm}^{-3}$ distribution with $\log R_{\mathrm{Edd}} = -1$. (d) The $\log n_{\mathrm{H}}/\mathrm{cm}^{-3}$ distribution with $\log R_{\mathrm{Edd}} = 0$.
• Figure 3: The averaged hydrogen column density as a function of the polar angle. The red, green, light blue, and purple lines correspond to the radiative hydrodynamic simulations of $\log R_{\mathrm{Edd}} = -3$, $-2$, $-1$, and $0$, respectively. The solid, and dashed lines represent the hydrogen column density of $10^{22} \ \mathrm{cm}^{-2}$ and $10^{24} \ \mathrm{cm}^{-2}$, respectively.
• Figure 4: (a) The distribution of the ionization parameter $\log \xi/\mathrm{erg} \ \mathrm{cm} \ \mathrm{s}^{-1}$ obtained from the radiative hydrodynamic simulation with logarithmic Eddington ratio $\log R_{\mathrm{Edd}}$ of $-3$. (b) The $\log \xi/\mathrm{erg} \ \mathrm{cm} \ \mathrm{s}^{-1}$ distribution with $\log R_{\mathrm{Edd}} = -2$. (c) The $\log \xi/\mathrm{erg} \ \mathrm{cm} \ \mathrm{s}^{-1}$ distribution with $\log R_{\mathrm{Edd}} = -1$. (d) The $\log \xi/\mathrm{erg} \ \mathrm{cm} \ \mathrm{s}^{-1}$ distribution with $\log R_{\mathrm{Edd}} = 0$.
• Figure 5: (a) The distribution of H1 number density $\log n_{\mathrm{HI}}/\mathrm{cm}^{-3}$ obtained from the radiative hydrodynamic simulation with logarithmic Eddington ratio $\log R_{\mathrm{Edd}}$ of $-3$. (b) The $\log n_{\mathrm{HI}}/\mathrm{cm}^{-3}$ distribution with $\log R_{\mathrm{Edd}} = -2$. (c) The $\log n_{\mathrm{HI}}/\mathrm{cm}^{-3}$ distribution with $\log R_{\mathrm{Edd}} = -1$. (d) The $\log n_{\mathrm{HI}}/\mathrm{cm}^{-3}$ distribution with $\log R_{\mathrm{Edd}} = 0$.