An Excess of Submm Sources near 4c 41.17: a Candidate Proto-cluster at Z=3.8?

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 submillimeter (submm) wavelengths we can therefore test both galaxyand structure-formation models. We have undertaken such a search in the field of a z = 3.8 radio galaxy, 4C 41.17, with the SCUBA submm camera. Our extremely deep 450and 850-μm 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 < 2× 10−3). The SCUBA galaxies have bolometric luminosities, > 1013 L⊙, which imply star-formation rates (SFRs) consistent with those required to form a massive galaxy in only a few 108 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. Subject headings: cosmology: observations — galaxies: evolution — galaxies: formation — infrared: galaxies — radio: galaxies