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 obscured1,2. There are clear examples of obscured black-hole growth in the form of ‘type-2’ quasars3,4,5, 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.
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We thank C. Wolf, L. Clewley, H.-R. Klöckner and G. Cotter for discussions. A.M.-S. is grateful to the Council of the European Union for financial support. S.R. and C.S. are grateful to the UK PPARC for a Senior Research Fellowship and an Advanced Fellowship respectively.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
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Martínez-Sansigre, A., Rawlings, S., Lacy, M. et al. The obscuration by dust of most of the growth of supermassive black holes. Nature 436, 666–669 (2005). https://doi.org/10.1038/nature03829
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