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 σv1D = 552 ± 52 km s-1. This is the first study of a galaxy cluster in this redshift range (z gsim 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 = 1014.4 ± 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.