2007ApJ...664L..17M

We describe a compact cluster or group of massive red galaxies at z=1.5 discovered in one of the Gemini Deep Deep Survey (GDDS) fields. Deep H-band imaging from the Hubble Space Telescope (HST) reveals a high density of red galaxies associated with a galaxy with a spectroscopic redshift of 1.51. These galaxies have spectral energy distributions (SEDs) that peak between 3.6 and 4.5 µm, and fits to 12-band photometry reveal 12 or more galaxies with spectral shapes consistent with z=1.5. Most are within ∼170 comoving kpc of the GDDS galaxy, and the enclosed stellar mass is >6x10¹¹ M_☉. The colors of the most massive galaxies are close to those expected from passive evolution of simple stellar populations (SSPs) formed at much higher redshifts. We suggest that several of these galaxies will merge to form a single, very massive galaxy by the present day. This system may represent an example of a short-lived dense group or cluster core typical of the progenitors of massive clusters in the present day and suggests that the red sequence was in place in overdense regions at early times.