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