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 similar to 3600) NIR spectroscopy with MOSFIRE on the Keck 1 telescope and secure 180 redshifts in a 12' x 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 sigma(v1D) = 552 +/- 52 km s(-1). This is the first study of a galaxy cluster in this redshift range (z greater than or similar to 2.0) with the combination of spectral resolution (similar to 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 Lambda CDM cosmological simulation suggests that this cluster will most likely evolve into a Virgo-like cluster with M-vir = 10(14.4 +/- 0.3)M(circle dot) (68% confidence) at z similar to 0. The theoretical probability of finding such a cluster is similar to 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.