  We describe a compact cluster of massive red galaxies at z=1.51 discovered in
one of the Gemini Deep Deep Survey (GDDS) fields. Deep imaging with the Near
Infrared Camera and Multi Object Spectrometer (NICMOS) on the Hubble Space
Telescope reveals a high density of galaxies with red optical to near-IR colors
surrounding a galaxy with a spectroscopic redshift of 1.51. Mid-IR imaging with
Infrared Array Camera (IRAC) on the Spitzer Space telescope shows that these
galaxies have spectral energy distributions that peak between 3.6 and 4.5
microns. Fits to 12-band photometry reveal 12 or more galaxies with spectral
shapes consistent with z = 1.51. Most are within ~170 co-moving kpc of the GDDS
galaxy. Deep F814W images with the Advanced Camera for Surveys (ACS) on HST
reveal that these galaxies are a mix of early-type galaxies, disk galaxies and
close pairs. The total stellar mass enclosed within a sphere of 170 kpc in
radius is > 8E+11 solar masses. The colors of the most massive galaxies are
close to those expected from passive evolution of simple stellar populations
(SSP) 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 the red sequence was in place in over-dense regions at early times.
