| Abstract
| - We report evidence of a fully established galaxy cluster at z = 2.07, consisting of a ~20 σ overdensity of red, compact spheroidal galaxies spatially coinciding with extended X-ray emission detected with XMM-Newton. We use VLT VIMOS and FORS2 spectra and deep Subaru, VLT and Spitzer imaging to estimate the redshift of the structure from a prominent z = 2.07 spectroscopic redshift spike of emission-line galaxies, concordant with the accurate 12-band photometric redshifts of the red galaxies. Using NICMOS and Keck AO observations, we find that the red galaxies have elliptical morphologies and compact cores. While they do not form a tight red sequence, their colours are consistent with that of a ≳1.3 Gyr population observed at z ~ 2.1. From an X-ray luminosity of 7.2 × 10 43 erg s -1 and the stellar mass content of the red galaxy population, we estimate a halo mass of 5.3-8 × 10 13M⊙, comparable to the nearby Virgo cluster. These properties imply that this structure could be the most distant, mature cluster known to date and that X-ray luminous, elliptical-dominated clusters are already forming at substantially earlier epochs than previously known.
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