  Today, almost every galaxy spheroid contains a massive black hole: a remnant
of, and testament to, a period in its evolution when it contained an active
galactic nucleus (AGN). However, the sequence and timescales of the formation
of the black hole and surrounding spheroid of stars are completely unknown,
leaving a large gap in our knowledge of how the Universe attained its present
appearance. Here we present submillimeter observations of matched samples of
X-ray absorbed and unabsorbed AGN which have luminosities and redshifts
characteristic of the sources responsible for most of the mass in present day
black holes. Strong submillimeter emission, an isotropic signature of copious
star formation, is found only in the X-ray absorbed sample, ruling out
orientation effects as the cause of the absorption. The space density and
luminosity range of the X-ray absorbed AGN indicate that they are undergoing
the transition between a hidden growth phase and an unabsorbed AGN phase, and
implies that the X-ray absorbed period in the AGN's evolution coincides with
the formation of the galaxy spheroid.
