title: The obscuration by dust of most of the growth of supermassive black holes.
abstract: Supermassive black holes underwent periods of exponential growth during which we see them as quasars in the distant Universe. The summed emission from these quasars generates the cosmic X-ray background, the spectrum of which has been used to argue that most black-hole growth is obscured. There are clear examples of obscured black-hole growth in the form of 'type-2' quasars, but their numbers are fewer than expected from modelling of the X-ray background. Here we report the direct detection of a population of distant type-2 quasars, which is at least comparable in size to the well-known unobscured type-1 population. We selected objects that have mid-infrared and radio emissions characteristic of quasars, but which are faint at near-infrared and optical wavelengths. We conclude that this population is responsible for most of the black-hole growth in the young Universe and that, throughout cosmic history, black-hole growth occurs in the dusty, gas-rich centres of active galaxies.
authors.affiliation: Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK. a.martinez-sansigre1@physics.oxford.ac.uk
authors.forename: Alejo
authors.initials: A
authors.lastname: Martínez-Sansigre
authors.forename: Steve
authors.initials: S
authors.lastname: Rawlings
authors.forename: Mark
authors.initials: M
authors.lastname: Lacy
authors.forename: Dario
authors.initials: D
authors.lastname: Fadda
authors.forename: Francine R
authors.initials: FR
authors.lastname: Marleau
authors.forename: Chris
authors.initials: C
authors.lastname: Simpson
authors.forename: Chris J
authors.initials: CJ
authors.lastname: Willott
authors.forename: Matt J
authors.initials: MJ
authors.lastname: Jarvis