  Biased galaxy-formation theories predict that massive galaxies at high
redshifts should act as signposts to high-density environments in the early
Universe, which subsequently evolve into the cores of the richest clusters seen
at the present day. These regions are characterised by over-densities of young
galaxies, perhaps including a population of dusty, interaction-driven
starbursts - the progenitors of massive cluster ellipticals. By searching for
this population at submm wavelengths we can therefore test both galaxy- and
structure-formation models. We have undertaken such a search in the field of a
z=3.8 radio galaxy, 4C41.17, with the SCUBA submm camera. Our extremely deep
450- and 850-micron maps reveal an order-of-magnitude over-density of luminous
submm galaxies compared to typical fields (the likelihood of finding such an
over-density in a random field is <2E-3). The SCUBA galaxies have bolometric
luminosities, >10^13 L(solar), which imply star-formation rates consistent with
those required to form a massive galaxy in only a few 10^8 years. We also note
that this field exhibits an over-density of extremely red objects (EROs), some
of which may be associated with the submm sources, and Lyman-break galaxies. We
propose that the over-densities of both submm and ERO sources in this field
represent young dusty, starburst galaxies forming within a proto-cluster
centered on the radio galaxy at z=3.8, which is also traced by a less-obscured
population of Lyman-break galaxies.
