1. Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
2. Institute for Computational Cosmology, University of Durham, South Road, Durham DH1 3LE, UK
3. California Institute of Technology, Pasadena, California 91125, USA
4. Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Laboratory, University Park, Pennsylvania 16802, USA
5. Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
6. Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK

Correspondence to: D. M. Alexander¹ Correspondence and requests for materials should be addressed to D.M.A. (Email: dma@ast.cam.ac.uk).

Abstract

The tight relationship between the masses of black holes and galaxy spheroids in nearby galaxies¹ implies a causal connection between the growth of these two components. Optically luminous quasars host the most prodigious accreting black holes in the Universe, and can account for 30 per cent of the total cosmological black-hole growth^{2, 3}. As typical quasars are not, however, undergoing intense star formation and already host massive black holes (> 10⁸ M , where M is the solar mass)^{4, 5}, there must have been an earlier pre-quasar phase when these black holes grew (mass range (10⁶–10⁸)M ). The likely signature of this earlier stage is simultaneous black-hole growth and star formation in distant (redshift z > 1; >8 billion light years away) luminous galaxies. Here we report ultra-deep X-ray observations of distant star-forming galaxies that are bright at submillimetre wavelengths. We find that the black holes in these galaxies are growing almost continuously throughout periods of intense star formation. This activity appears to be more tightly associated with these galaxies than any other coeval galaxy populations. We show that the black-hole growth from these galaxies is consistent with that expected for the pre-quasar phase.

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