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

TL;DR

This study confirms a galaxy cluster at redshift 2.095 using Keck/MOSFIRE spectroscopy, revealing its properties and potential evolution into a Virgo-like cluster, demonstrating the feasibility of detailed high-redshift cluster studies.

Contribution

First spectroscopic confirmation of a galaxy cluster at z>2 with detailed member mapping and velocity dispersion measurement using MOSFIRE, providing insights into early cluster evolution.

Findings

Cluster at z=2.095 with 57 members confirmed

Velocity dispersion measured at 552 km/s

Likely to evolve into a Virgo-like cluster by z=0

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 $12'\times12'$ 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.