Detection of Large Scale Structures at z ≃ 0 . 8 and z ≃ 1 . 2 ⋆

We report the finding of two large-scale structures in a wide-field (40 × 35 arcmin) optical (V and I-band) imaging survey of a field towards two known concentrations of quasars and Mg ii absorbers at z ≃ 0.8 and z ≃ 1.2. We are able to select high-redshift (0.6∼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 × 35 arcmin control field, we find a 30% excess of these red galaxies corresponding to 563 galaxies across the field. The colour distribution of the excess shows a coherent peak at 2.7 < V − I < 3.1, and the magnitude distribution is well described by a Schechter function. Together they show unambiguously that the galaxy excess is primarily due to a population of passively-evolving early-type galaxies at z = 0.83 ± 0.08. In follow-up NIR imaging of the four best cluster candidates in the field we identify three clusters at z = 0.8± 0.1 each having clear red sequences of ∼ 20 galaxies at I−Ks ≃ 3.4, forming a structure 20h −1 Mpc across. A previously identified cluster at z = 1.2±0.1 is confirmed with 28 galaxies having V IJKs colours consistent with passively-evolving galaxies at z ≃ 1.2 within 1 arcmin of the cluster centre. In all four fields red sequences are apparent at both I −Ks ≃ 3.4 and I −Ks ≃ 4.1 indicating large-scale structures at z ≃ 0.8 and z ≃ 1.2 extending across the whole optical field (20–30 h Mpc). The full extents of the two large-scale structures are believed to be much greater, both being traced by superstructures of quasars and Mg ii absorbers, including the Clowes-Campusano Large Quasar Group (LQG) of 23 quasars at z ≃ 1.2, the largest known structure in the early Universe having a maximal extent of ∼ 200 h Mpc. In both cases, there is evidence directly connecting the large-scale structures of galaxies found here and the quasar / Mg ii superstructures that trace them, with a cluster and group of galaxies associated with two z ≃ 1.2 quasars from the Clowes-Campusano LQG, and one of the z ≃ 0.8 clusters found near a z = 0.8016 Mg ii absorber.