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The obscuration by dust of most of the growth of supermassive black holes


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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Figure 1: Diagrammatic representation of the infrared selection criteria.
Figure 2: Probability distributions for the quasar fraction.

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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.

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Correspondence to Alejo Martínez-Sansigre.

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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).

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