  We report a definitive confirmation of a large-scale structure around the
super rich cluster CL0016+1609 at z=0.55. We made spectroscopic follow-up
observations with FOCAS on Subaru along the large filamentary structure
identified in our previous photometric studies. We have confirmed the physical
connection of the huge filament extending over 20 Mpc in the N-S direction, and
another filament extending from the main cluster to the East. Based on a simple
energy argument, we show that it is likely that most of the clumps are bound to
the main CL0016 cluster. This structure is surely one of the most prominent
confirmed structures ever identified in the distant Universe, which then serves
as an ideal laboratory to examine the environmental variation of galaxy
properties. We draw star formation histories of galaxies from the composite
spectra of red galaxies in field, group, and cluster environments. Combining
the results from our previous studies, we find that red galaxies in groups at
z~0.8 and red field galaxies at z~0.5 show strong Hd absorption lines for their
D4000 indices. These are the environments in which we observed the on-going
build-up of the colour-magnitude relation in our previous photometric analyses.
The strong Hd absorption lines imply that their star formation is truncated on
a relatively short time scale. We suggest that a galaxy-galaxy interaction is
the most likely physical driver of the truncation of star formation and thus
responsible for the build-up of the colour-magnitude relation since z~1.
(Abridged)
