Abstract

We report the finding of two large-scale structures of galaxies in a 40 x 35 arcmin(2) field embedded in a 25 deg(2) area where two 100 h(-1) Mpc-scale structures of quasars and Mg II absorbers at z similar or equal to 0.8 and z similar or equal to 1.2 have been previously detected. Using deep optical (V- and I-band) imaging, we are able to select high-redshift (0.5 less than or similar to z less than or similar to 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 35 x 35 arcmin(2) 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 similar to 20 galaxies at I - K-s similar or equal to 3.4, forming a structure 20 h(-1) Mpc 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 similar or equal to 1.2 located within 1 arcmin of the cluster centre. In all four NIR fields we find numerous galaxies with the VIM, colours of passively-evolving galaxies both at z similar or equal to 0.8 and z similar or equal to 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 20-30 h(-1) Mpc scales explored by the optical data, one "sheet" at z similar or equal to 0.8 with three associated clusters, and a second "sheet" at z similar or equal to 1.2 with one embedded cluster. Our results confirm that, at least inside the probed region, the two known 100 h(-1) Mpc-scale structures at z similar to 1 of quasars and Mg II absorbers also mark out volumes with an enhanced density of galaxies. The finding of two galaxy structures at z similar or equal to 1 on scales of 20-30 h(-1) Mpc within much larger superstructures of quasars and Mg it absorbers, firstly establishes the presence of galaxy sheet-like structures at z similar to 1, and secondly, suggests that they may extend up to 100 h(-1) Mpc-scales.