  We investigate the effects of dense environments on galaxy evolution by
examining how the properties of galaxies in the z = 1.6 protocluster Cl
0218.3-0510 depend on their location. We determine galaxy properties using
spectral energy distribution fitting to 14-band photometry, including data at
three wavelengths that tightly bracket the Balmer and 4000A breaks of the
protocluster galaxies. We find that two-thirds of the protocluster galaxies,
which lie between several compact groups, are indistinguishable from field
galaxies. The other third, which reside within the groups, differ significantly
from the intergroup galaxies in both colour and specific star formation rate.
We find that the fraction of red galaxies within the massive protocluster
groups is twice that of the intergroup region. These excess red galaxies are
due to enhanced fractions of both passive galaxies (1.7 times that of the
intergroup region) and dusty star-forming galaxies (3 times that of the
intergroup region). We infer that some protocluster galaxies are processed in
the groups before the cluster collapses. These processes act to suppress star
formation and change the mode of star formation from unobscured to obscured.
