We report evidence of a fully established galaxy cluster at z = 2.07 , consisting of a \sim 20 \sigma 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 \gtrsim 1.3 Gyr population observed at z \sim 2.1 .
From an X-ray luminosity of 7.2 \times 10 ^ { 43 } erg s ^ { -1 } and the stellar mass content of the red galaxy population , we estimate a halo mass of 5.3–8 \times 10 ^ { 13 } M _ { \odot } , 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 .