Detection of 20-30 Mpc-scale Galaxy Structures Embedded in 100 Mpc-scale Structures of Quasars and MgII Absorbers at z~0.8 and z~1.2

We report the finding of two large-scale structures of galaxies in a 40'x35' field embedded in a 25deg^2 area where two 100 Mpc-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 35'x35' 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\pm0.08. In follow-up NIR imaging of the four best cluster candidates, we identify three clusters at z=0.8\pm0.1, each having well defined red sequences of ~20 galaxies at I-K~3.4, forming a structure 20 Mpc across. The fourth candidate, corresponding to a previously identified cluster at z=1.2\pm0.1 is confirmed with 28 K<20.5 galaxies having VIJK colours consistent with passively-evolving galaxies at z~1.2 located within 1' of the cluster centre. The results of this study support the presence of two sheet-like large-scale galaxy structures extending across the 20-30 Mpc 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 100 Mpc-scale structures at z~1 of quasars and MgII absorbers also mark out volumes with an enhanced density of galaxies.