We present the 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 \sim 3600 ) NIR spectroscopy with MOSFIRE on the Keck 1 telescope and secure 180 redshifts in a 12 ^ { \prime } \times 12 ^ { \prime } 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 _ { v 1 D } = 552 \pm 52 km/s .
This is the first study of a galaxy cluster in this redshift range ( z \gtrsim 2.0 ) with the combination of spectral resolution ( \sim 26 km/s ) 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 \pm 0.3 } { M _ { \odot } } ( 68 \% confidence ) at z \sim 0 .
The theoretical expectation of finding such a cluster is \sim 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 .