Keck/MOSFIRE Spectroscopic Confirmation of a Virgo-like Cluster Ancestor at z = 2.095

We present spectroscopic confirmation of a galaxy cluster at z = 2.095 in the COSMOS field. This galaxy cluster was first reported in the ZFOURGE survey as harboring evolved massive galaxies using photometric redshifts derived with deep near-infrared (NIR) medium-band filters. We obtain medium-resolution (R ~ 3600) NIR spectroscopy with MOSFIRE on the Keck 1 telescope and secure 180 redshifts in a 12' × 12' region. We find a prominent spike of 57 galaxies at z = 2.095 corresponding to the galaxy cluster. The cluster velocity dispersion is measured to be σ_v1D = 552 ± 52 km s^-1. This is the first study of a galaxy cluster in this redshift range (z >~ 2.0) with the combination of spectral resolution (~26 km s^-1) and the number of confirmed members (>50) needed to impose a meaningful constraint on the cluster velocity dispersion and map its members over a large field of view. Our ΛCDM cosmological simulation suggests that this cluster will most likely evolve into a Virgo-like cluster with M _vir = 10^{14.4 ± 0.3} M _⊙ (68% confidence) at z ~ 0. The theoretical probability of finding such a cluster is ~4%. Our results demonstrate the feasibility of studying galaxy clusters at z > 2 in the same detailed manner using multi-object NIR spectrographs as has been done in the optical in lower-redshift clusters.

Based on data obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA, and was made possible by the generous financial support of the W. M. Keck Foundation.