Two spectroscopically confirmed galaxy structures at z = 0.61 and 0.74 in the CFHTLS Deep 3 field

Context. Galaxy evolution is known to depend on environment since it differs in clusters and in the field, but studies are sometimes limited to the relatively nearby Universe (z < 0.5). It is still necessary to increase our knowledge of cluster galaxy properties above z = 0.5.
Aims: In a previous paper we have detected several cluster candidates at z> 0.5 as part of a systematic search for clusters in the Canada France Hawaii Telescope Legacy Survey by applying the Adami & MAzure Cluster FInder (AMACFI), based on photometric redshifts. We focus here on two of them, located in the Deep 3 (hereafter D3) field: D3-6 and D3-43.
Methods: We have obtained spectroscopy with Gemini/GMOS instrument and measured redshifts for 23 and 14 galaxies in the two structures. These redshifts were combined with those available in the literature. A dynamical and a weak lensing analysis were also performed, together with the study of X-ray Chandra archive data.
Results: Cluster D3-6 is found to be a single structure of eight spectroscopically confirmed members at an average redshift z = 0.607, with a velocity dispersion of 423 km s^-1. It appears to be a relatively low-mass cluster. D3-43-S3 has 46 spectroscopically confirmed members at an average redshift z = 0.739. The cluster can be decomposed into two main substructures, having a velocity dispersion of about 600 and 350 km s^-1. An explanation of the fact that D3-43-S3 is detected through weak lensing (only marginally, at the ~3σ level) but not in X-rays could be that the two substructures are just beginning to merge more or less along the line of sight. We also show that D3-6 and D3-43-S3 have similar global galaxy luminosity functions, stellar mass functions, and star formation rate (SFR) distributions. The only differences are that D3-6 exhibits a lack of faint early-type galaxies, a deficit of extremely high stellar mass galaxies compared to D3-43-S3, and an excess of very high SFR galaxies.
Conclusions: This study shows the power of techniques based on photometric redshifts to detect low to moderately massive structures, even at z ~ 0.75. Combined-approach cluster surveys such as EUCLID are crucial to find and study these clusters at these relatively high redshifts. Finally, we show that photometric redshift techniques are also well suited to study the galaxy content and properties of the clusters (galaxy types, SFRs, etc.).

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