We report observations with the Atacama Large Millimetre Array (ALMA) of six
submillimetre galaxies (SMGs) within 3 arcmin of the Distant Red Core (DRC) at
z=4.0, a site of intense cluster-scale star formation, first reported by Oteo
et al. (2018). We find new members of DRC in three SMG fields; in two fields,
the SMGs are shown to lie along the line of sight towards DRC; one SMG is
spurious. Although at first sight this rate of association is consistent with
earlier predictions, associations with the bright SMGs are rarer than expected,
which suggests caution when interpreting continuum over-densities. We consider
the implications of all 14 confirmed DRC components passing simultaneously
through an active phase of star formation. In the simplest explanation, we see
only the tip of the iceberg in terms of star formation and gas available for
future star formation, consistent with our remarkable finding that the majority
of newly confirmed DRC galaxies are not the brightest continuum emitters in
their immediate vicinity. Thus while ALMA continuum follow-up of SMGs
identifies the brightest continuum emitters in each field, it does not
necessarily reveal all the gas-rich galaxies. To hunt effectively for
protocluster members requires wide and deep spectral-line imaging to uncover
any relatively continuum-faint galaxies that are rich in atomic or molecular
gas. Searching with short-baseline arrays or single-dish facilities, the true
scale of the underlying gas reservoirs may be revealed.Comment: 8 pages, 2 colour figures; MNRAS, in pres