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∼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 2.25×2.25 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−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, 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 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.