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 ClowesCampusano 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 2.25×2.25arcmin 2 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 h 1 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.