Subaru Weak-Lensing Survey II: Multi-Object Spectroscopy and Cluster Masses

Abstract

Abstract We present the first results of a multi-object spectroscopic campaign to follow up cluster candidates located via weak lensing. Our main goals are to search for spatial concentrations of galaxies that are plausible optical counterparts of the weak-lensing signals, and to determine the cluster redshifts from those of member galaxies. Around each of 36 targeted cluster candidates, we obtained 15–32 galaxy redshifts. For 28 of these targets, we confirmed a secure cluster identification, with more than five spectroscopic galaxies within a velocity of $\pm$3000 kms$^{-1}$. This includes three cases where two clusters at different redshifts are projected along the same line-of-sight. In 6 of the 8 unconfirmed targets, we found multiple small galaxy concentrations at different redshifts, each containing at least three spectroscopic galaxies. The weak-lensing signal around those systems was thus probably created by the projection of groups or small clusters along the same line-of-sight. In both of the remaining two targets, a single small galaxy concentration was found. In some candidate super-cluster systems, we found additional evidence of filaments connecting the main density peak to an additional nearby structure. For a subsample of our most cleanly measured clusters, we investigated the statistical relation between their weak-lensing mass ($M_{\rm NFW}$, $\sigma_{\rm SIS}$) and the velocity dispersion of their member galaxies ($\sigma_{v}$), comparing our sample with optically and X-ray selected samples from the literature. Our lensing-selected clusters are consistent with $\sigma_v$$=$$\sigma_{\rm SIS}$, with a similar scatter to that of optically and X-ray selected clusters. We also derived an empirical relation between the cluster mass and the galaxy velocity dispersion, $M_{200}E$($z$) $=$ 11.0 $\times$ 10$^{14}$$\times$ ($\sigma_v/$1000 kms$^{-1}$)$^{3.0}h^{-1}M_\odot$, which is in reasonable agreement with predictions of $N$-body simulations in the $\Lambda$CDM cosmology.