The close environments of accreting massive black holes are shaped by radiative feedback

TL;DR

This study demonstrates that radiation pressure on dusty gas primarily governs the distribution of obscuring material around accreting supermassive black holes, influencing their observed obscuration properties.

Contribution

It provides the first large-scale, unbiased evidence that radiation pressure is the key mechanism shaping the circumnuclear environment of black holes.

Findings

Obscuring material is located within a few to tens of parsecs of the black hole.

Radiation pressure can clear gas and dust even at low accretion rates.

Differences in obscuration are mainly driven by mass-normalized accretion rates.

Abstract

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.