2006MNRAS.365.1392T

We present a discovery of definitive large-scale structures around RXJ0152.7-1352 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-Åbreak (D₄₀₀₀) 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 [Oii] emissions and/or strong Hδ absorptions. Our current data seem to favour a scenario that star formation is truncated in a short time-scale (<1Gyr). This would imply that galaxy-galaxy interactions are responsible for the truncation of star formation.