The close environments of accreting massive black holes are shaped by
  radiative feedback
  The large majority of the accreting supermassive black holes in the Universe
are obscured by large columns of gas and dust. The location and evolution of
this obscuring material have been the subject of intense research in the past
decades, and are still highly debated. A decrease in the covering factor of the
circumnuclear material with increasing accretion rates has been found by
studies carried out across the electromagnetic spectrum. The origin of this
trend has been suggested to be driven either by the increase in the inner
radius of the obscuring material with incident luminosity due to the
sublimation of dust; by the gravitational potential of the black hole; by
radiative feedback; or by the interplay between outflows and inflows. However,
the lack of a large, unbiased and complete sample of accreting black holes,
with reliable information on gas column density, luminosity and mass, has left
the main physical mechanism regulating obscuration unclear. Using a systematic
multi-wavelength survey of hard X-ray-selected black holes, here we show that
radiation pressure on dusty gas is indeed the main physical mechanism
regulating the distribution of the circumnuclear material. Our results imply
that the bulk of the obscuring dust and gas in these objects is located within
the sphere of influence of the black hole (i.e., a few to tens of parsecs), and
that it can be swept away even at low radiative output rates. The main physical
driver of the differences between obscured and unobscured accreting black holes
is therefore their mass-normalized accretion rate.
