KECK/MOSFIRE spectroscopic confirmation of a Virgo-like cluster ancestor at z=2.095

Abstract

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 ${\sigma }_{v1D}$ = 552 $\pm$ 52 km/s. This is the first study of a galaxy cluster in this redshift range ($z>2.0$) 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 ${\mathrm{M}}_{\mathrm{vir}}=10^{14.4\pm 0.3}{\mathrm{M}}_{\odot }$ ($68\%$ 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.

6 pages, 5 figures; accepted for publication in ApJ Letters

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