  The most massive galaxies in the present-day Universe are found to lie in the
centres of rich clusters. They have old, coeval stellar populations suggesting
that the bulk of their stars must have formed at early epochs in spectacular
starbursts - luminous phenomena at submillimetre wavelengths. The most popular
model of galaxy formation predicts that these galaxies form in proto-clusters
at high-density peaks in the early Universe. Such peaks are signposted by
massive high-redshift radio galaxies. Here we report deep submillimetre mapping
of seven high-redshift radio galaxies and their environments. These data
confirm not only the presence of spatially extended massive star-formation
activity in the radio galaxies themselves, but also in companion objects
previously undetected at any wavelength. The prevalence, orientation, and
inferred masses of these submillimetre companion galaxies suggest that we are
witnessing the synchronous formation of the most luminous elliptical galaxies
found today at the centres of rich galaxy clusters.
