Detection of 20–30 h$\mathsf{^{-1}}$ Mpc-scale galaxy structures embedded in 100 h$\mathsf{^{-1}}$ Mpc-scale structures of quasars and MgII absorbers at $\textit{z}\simeq\mathsf{0.8}$ and $\textit{z} \simeq\mathsf{1.2}$

We report the finding of two large-scale structures of galaxies in a $40\times35~{\rm arcmin}^{2}$ field embedded in a $25\deg^2$ area where two 100$~h^{-1}\,{\rm Mpc}$-scale structures of quasars and $\ion{Mg}{ii}$ absorbers at $z\simeq0.8$ and $z\simeq1.2$ have been previously detected. Using deep optical ( V - and I -band) imaging, we are able to select high-redshift ($0.5\la z\la 1.3$) early-type galaxies as those redder than the cluster red sequence at $z=0.5$ and having $I 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 $I-K_{\rm s}\simeq3.4$, forming a structure $20~h^{-1}\,{\rm Mpc}$ across. The fourth candidate, corresponding to a previously identified cluster at $z=1.2\pm0.1$, is confirmed with 28 $K_{\rm s}<20.5$ galaxies having $\textit{VIJK}_{\rm s}$ colours consistent with passively-evolving galaxies at $z\simeq1.2$ located within 1 arcmin of the cluster centre. In all four NIR fields we find numerous galaxies with the $\textit{VIJK}_{\rm s}$ colours of passively-evolving galaxies both at $z\simeq0.8$ and $z\simeq1.2$, 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 20–3$0~h^{-1}\,{\rm Mpc}$ scales explored by the optical data, one “sheet” at $z\simeq0.8$ with three associated clusters, and a second “sheet” at $z\simeq1.2$ with one embedded cluster. Our results confirm that, at least inside the probed region, the two known 100$~h^{-1}\,{\rm Mpc}$-scale structures at $z\sim1$ of quasars and $\ion{Mg}{ii}$ absorbers also mark out volumes with an enhanced density of galaxies. The finding of two galaxy structures at $z\simeq 1$ on scales of 20–30$~h^{-1}\,{\rm Mpc}$ within much larger superstructures of quasars and $\ion{Mg}{ii}$ absorbers, firstly establishes the presence of galaxy sheet-like structures at $z\sim1$, and secondly, suggests that they may extend up to 100$~h^{-1}\,{\rm Mpc}$-scales.