The galaxy environment of a quasar at z=1.226:: a possible cluster merger

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 similar to 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 similar to 11 overdensity of such galaxies in a 2.25 x 2.25 arcmin(2) 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-K/K, I-K/K colour-magnitude plots at V-K similar or equal to 6.9, I-K similar or equal to 4.3 comparable to the red sequences observed in other z similar or equal to 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 h(-1) Mpc across. We find evidence for a high (greater than or similar to 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 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.