2001MNRAS.323..688H

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 2.25x2.25arcmin² field centred on the quasar. In particular, we find 15-18 galaxies that have colours consistent with being a population of passively evolving massive ellipticals at the quasar redshift. They form `fingers' in the V-KK, I-KK 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, 40arcsec to the north, and 60arcsec 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 2-3h<sup>–1</sup>Mpc across. We find evidence for a high (≳50 per cent) 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 30arcsec 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.