2004A&A...421..157H

We report the finding of two large-scale structures of galaxies in a 40x35arcmin² field embedded in a 25°² area where two 100h^–1Mpc-scale structures of quasars and MgII absorbers at z≃0.8 and z≃1.2 have been previously detected. Using deep optical (V- and I-band) imaging, we are able to select high-redshift (0.5≲z≲1.3) early-type galaxies as those redder than the cluster red sequence at z=0.5 and having I<23.5. Through comparison with a 35x35arcmin² 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 2.7<V-I<3.1 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 z=0.83±0.08. In follow-up NIR imaging of the four best cluster candidates, we identify three clusters at z=0.8±0.1, each having well defined red sequences of ∼20 galaxies at I-K_s≃3.4, forming a structure 20h^–1Mpc across. The fourth candidate, corresponding to a previously identified cluster at z=1.2±0.1, is confirmed with 28 K_s<20.5 galaxies having VIJK_s colours consistent with passively-evolving galaxies at z≃1.2 located within 1arcmin of the cluster centre. In all four NIR fields we find numerous galaxies with the VIJK_s colours of passively-evolving galaxies both at z≃0.8 and z≃1.2, 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 20-30h^–1Mpc scales explored by the optical data, one ``sheet'' at z≃0.8 with three associated clusters, and a second ``sheet'' at z≃1.2 with one embedded cluster. Our results confirm that, at least inside the probed region, the two known 100h^–1Mpc-scale structures at z∼1 of quasars and MgII absorbers also mark out volumes with an enhanced density of galaxies. The finding of two galaxy structures at z≃1 on scales of 20-30h^–1Mpc within much larger superstructures of quasars and MgII absorbers, firstly establishes the presence of galaxy sheet-like structures at z∼1, and secondly, suggests that they may extend up to 100h^–1Mpc-scales.