Spectroscopically confirmed large‐scale structures associated to a z = 0.83 cluster

We present a discovery of definitive large‐scale structures around RX J0152.7−1352 at z = 0.83 based on spectroscopic redshifts. In our previous papers, we reported a photometric identification of the large‐scale structures at z ∼ 0.8. A spectroscopic follow‐up observation was carried out on eight selected regions covering the most prominent structures to confirm their association to the main cluster. In six out of the eight fields, a well‐isolated peak is identified in the distribution of spectroscopic redshifts at or near the cluster redshift. This is strong evidence for the presence of large‐scale structures associated to the main cluster at z = 0.83. It seems that there are two large filaments of galaxies at z ∼ 0.837 and ∼0.844 crossing in this field. We then investigate stellar populations of galaxies in the structures. The composite spectra are constructed from a number of red member galaxies on the colour–magnitude sequence. We consider three representative environments – cluster, group and field – to investigate the environmental dependence of their star formation histories. We quantify the strengths of the 4000‐Å break ( D 4000 ) and the Hδ absorption features and compare them with model predictions. The ‘cluster’ red galaxies do not show any sign of on‐going or recent star formation activities and the passive evolution can naturally link them to the present‐day red sequence galaxies in the Sloan Digital Sky Survey . In contrast, the red galaxies in ‘groups’ and in the ‘field’ tend to show signs of remaining and/or recent star formation activities characterized by weak [O  ii ] emissions and/or strong Hδ absorptions. Our current data seem to favour a scenario that star formation is truncated in a short time‐scale (<1 Gyr). This would imply that galaxy–galaxy interactions are responsible for the truncation of star formation.