Revisiting the "forbidden" region: AGN radiative feedback with radiation trapping

TL;DR

This paper revisits AGN radiative feedback by including radiation trapping effects, revealing an expanded forbidden region in the $N_{ m H} - \lambda$ plane and implications for AGN obscuration and outflows.

Contribution

It introduces the explicit calculation of radiation trapping in AGN feedback models, showing its significant impact on the effective Eddington limit and obscuration properties.

Findings

Radiation trapping enlarges the forbidden region in the $N_{ m H} - \\lambda$ plane.

Even Compton-thick material can be disrupted by sub-Eddington luminosities.

Model results align well with observed AGN properties.

Abstract

Active galactic nucleus (AGN) feedback, driven by radiation pressure on dust, is an important mechanism for efficiently coupling the accreting black hole to the surrounding environment. Recent observations confirm that X-ray selected AGN samples respect the effective Eddington limit for dusty gas in the plane defined by the observed column density versus the Eddington ratio, the so-called $N_{\rm H} - \lambda$ plane. A `forbidden' region occurs in this plane, where obscuring clouds cannot be long-lived, due to the action of radiation pressure on dust. Here we compute the effective Eddington limit by explicitly taking into account the trapping of reprocessed radiation (which has been neglected in previous works), and investigate its impact on the $N_{\rm H} - \lambda$ plane. We show that the inclusion of radiation trapping leads to an enhanced forbidden region, such that even Compton-thick material can potentially be disrupted by sub-Eddington luminosities. We compare our model results to the most complete sample of local AGNs with measured X-ray properties, and find good agreement. Considering the anisotropic emission from the accretion disc, we also expect the development of dusty outflows along the polar axis, which may naturally account for the polar dust emission recently detected in several AGNs from mid-infrared observations. Radiative feedback thus appears to be the key mechanism regulating the obscuration properties of AGNs, and we discuss its physical implications in the context of co-evolution scenarios.