Nature 549, 488–491 (2017)

Shy monsters lurk at the centres of galaxies. Such accreting supermassive black holes, or active galactic nuclei, lie concealed behind thick curtains of gas and dust. But the origin and exact location of these obscuring shrouds has long been debated, and different physical mechanisms leading to their formation have been proposed. Using a multi-wavelength survey of over eight hundred accreting supermassive black holes, Claudio Ricci and colleagues uncovered the role of radiation pressure as the main driving force responsible for shaping the density and distribution of dust and gas haze.

From the observational data, Ricci et al. could estimate the gas and dust densities and the black hole X-ray luminosities. These suggest a relation between the degree of obscuration and the black hole accretion rates. Ricci and co-workers also found that the bulk of gas and dust material lies a few to tens of parsecs from the black hole, where its gravitational potential is stronger than the galactic one. This explains how sometimes black holes can shed their shroud to peek out, unobscured.