Galaxy gas as obscurer: II. Separating the galaxy-scale and nuclear obscurers of Active Galactic Nuclei

TL;DR

This paper distinguishes galaxy-scale and nuclear obscurers in AGN, showing galaxy gas causes some obscuration but not Compton-thick levels, and proposes a radiation-lifted torus model consistent with various observations.

Contribution

It introduces a model separating galaxy-scale and nuclear obscurers, quantifies their roles, and provides a sub-grid recipe for simulations, advancing understanding of AGN obscuration mechanisms.

Findings

Galaxy gas causes luminosity-independent Compton-thin obscuration.

Galaxy gas does not produce Compton-thick columns.

The proposed torus model includes both Compton-thick and thin components, dependent on black hole mass and luminosity.

Abstract

The "torus" obscurer of Active Galactic Nuclei (AGN) is poorly understood in terms of its density, substructure and physical mechanisms. Large X-ray surveys provide model boundary constraints, for both Compton-thin and Compton-thick levels of obscuration, as obscured fractions are mean covering factors $f_{\text{cov}}$. However, a major remaining uncertainty is host galaxy obscuration. In Paper I we discovered a relation of $N_H \propto M_{\star}^{1/3}$ for the obscuration of galaxy-scale gas. Here we apply this observational relation to the AGN population, and find that galaxy-scale gas is responsible for a luminosity-independent fraction of Compton-thin AGN, but does not produce Compton-thick columns. With the host galaxy obscuration understood, we present a model of the remaining, nuclear obscurer which is consistent with a range of observations. Our radiation-lifted torus model consists of a Compton-thick component ($f_{\text{cov}}\sim35\%$) and a Compton-thin component ($f_{\text{cov}}\sim40\%$), which depends on both black hole mass and luminosity. This provides a useful summary of observational constraints for torus modellers who attempt to reproduce this behaviour. It can also be employed as a sub-grid recipe in cosmological simulations which do not resolve the torus. We also investigate host-galaxy X-ray obscuration inside cosmological, hydro-dynamic simulations (EAGLE, Illustris). The obscuration from ray-traced galaxy gas can agree with observations, but is highly sensitive to the chosen feedback assumptions.