A huge filamentary structure at z= 0.55 and star formation histories of galaxies at z < 1

Abstract

Abstract We report a definitive confirmation of a large-scale structure around the super rich cluster CL0016+1609 at z= 0.55. We made spectroscopic follow-up observations with Faint Object Camera and Spectrograph (FOCAS) on Subaru along the large filamentary structure identified in our previous photometric studies, including some subclumps already found by other authors. We have confirmed the physical connection of the huge filament extending over 20h−170 Mpc in the north–south (NS) direction hosting the main cluster and several clumps aligned in a chain-like structure. We have also confirmed a physical association of a new filament extending from the main cluster to the east, which was newly discovered by us. Based on a simple energy argument, we show that it is likely that most of the clumps are bound to the main CL0016 cluster. Given its spatial extent and richness, this structure is surely one of the most prominent confirmed structures ever identified in the distant Universe, which then serves as an ideal laboratory to examine the environmental variation of galaxy properties. We draw star formation histories of galaxies from the composite spectra of red galaxies in field, group and cluster environments. Combining the results from our previous studies, we find that red galaxies in groups at z∼ 0.8 and red field galaxies at z∼ 0.5 show strong Hδ absorption lines for their D4000 indices. These are the environments in which we observed the on-going build-up of the colour–magnitude relation in our previous photometric analyses. The strong Hδ absorption lines observed in their composite spectra imply that their star formation is truncated on a relatively short time-scale in these relatively low density environments. We suggest that galaxy–galaxy interactions are the most likely physical driver of the truncation of star formation and thus responsible for the build-up of the colour–magnitude relation since z∼ 1.