  We have conducted ultra-deep optical and deep near-infrared observations of a
field around the z=1.226 radio-quiet quasar 104420.8+055739 from the
Clowes-Campusano LQG of 18 quasars at z~1.3 in search of associated galaxy
clustering. Galaxies at these redshifts are distinguished by their
extremely-red colours, with I-K>3.75, and we find a factor ~11 overdensity of
such galaxies in a 2.25'x2.25' field centred on the quasar. In particular, we
find 15-18 galaxies with colours consistent with being a population of
passively-evolving massive ellipticals at the quasar redshift. They form
`fingers' in the V-K/K, I-K/K colour-magnitude plots at V-K~6.9, I-K~4.3
comparable to the red sequences observed in other z~1.2 clusters. We find
suggestive evidence for substructure among the red sequence galaxies in the K
image, in the form of two compact groups, 40 arcsec to the north, and 60 arcsec
to the south-east of the quasar. An examination of the wider optical images
indicates that this substructure is significant, and that the clustering
extends to form a large-scale structure 2-3 Mpc across. We find evidence for a
high (>50%) fraction of blue galaxies in this system, in the form of 15-20
`red-outlier' galaxies with I-K>3.75 and V-I<2.00, which we suggest are dusty,
star-forming galaxies at the quasar redshift. Within 30 arcsec of the quasar we
find a concentration of blue (V-I<1) galaxies in a band that bisects the two
groups of red sequence galaxies. This band of blue galaxies is presumed to
correspond to a region of enhanced star-formation. We explain this distribution
of galaxies as the early-stages of a cluster merger which has triggered both
the star-formation and the quasar.
