Detection of 20–30 h⁻¹ Mpc-scale galaxy structures embedded in 100 h⁻¹ Mpc-scale structures of quasars and MgII absorbers at z0.8 and z 1.2

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We report the finding of two large-scale structures of galaxies in a field embedded in a area where two 100-scale structures of quasars and absorbers at and have been previously detected. Using deep optical (V- and I-band) imaging, we are able to select high-redshift () early-type galaxies as those redder than the cluster red sequence at and having . Through comparison with a control field, we find a 30% excess of these red galaxies, corresponding to 563 galaxies across the field. The colour distribution of the excess galaxies shows a coherent peak at and the magnitude distribution is well described by a Schechter function, showing unambiguously that the galaxy excess is primarily due to a population of early-type galaxies at . In follow-up NIR imaging of the four best cluster candidates, we identify three clusters at , each having well defined red sequences of ~20 galaxies at , forming a structure across. The fourth candidate, corresponding to a previously identified cluster at , is confirmed with 28 galaxies having colours consistent with passively-evolving galaxies at located within 1 arcmin of the cluster centre. In all four NIR fields we find numerous galaxies with the colours of passively-evolving galaxies both at and , including regions well away from the clusters. The results of this study support the presence of two sheet-like large-scale galaxy structures extending across the 20–3 scales explored by the optical data, one “sheet” at with three associated clusters, and a second “sheet” at with one embedded cluster. Our results confirm that, at least inside the probed region, the two known 100-scale structures at of quasars and absorbers also mark out volumes with an enhanced density of galaxies. The finding of two galaxy structures at on scales of 20–30 within much larger superstructures of quasars and absorbers, firstly establishes the presence of galaxy sheet-like structures at , and secondly, suggests that they may extend up to 100-scales.