  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 $12'\times12'$ 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 $\sigma_{\rm v1D}$ = 552 $\pm$ 52 km/s. This is
the first study of a galaxy cluster in this redshift range ($z \gt 2.0$) 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 ${\rm M_{vir}}{=}10^{14.4\pm0.3}
{\rm M_\odot}$ ($68\%$ 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.

Astrophysics of Galaxies;Cosmology and Nongalactic Astrophysics