  The tight relationship between the masses of black holes and galaxy spheroids
in nearby galaxies 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% of the total cosmological
black-hole growth. As typical quasars are not, however, undergoing intense star
formation and already host massive black holes [>10^(8) M(Sun)], there must
have been an earlier pre-quasar phase when these black holes grew [mass range
  ~10^(6)-10^(8) M(Sun)]. The likely signature of this earlier stage is
simultaneous black-hole growth and star formation in distant (i.e., 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.
