A Compact Cluster of Massive Red Galaxies at a Redshift of 1.5

We describe a compact cluster of massive red galaxies at z=1.51 discovered inone of the Gemini Deep Deep Survey (GDDS) fields. Deep imaging with the NearInfrared Camera and Multi Object Spectrometer (NICMOS) on the Hubble SpaceTelescope reveals a high density of galaxies with red optical to near-IR colorssurrounding a galaxy with a spectroscopic redshift of 1.51. Mid-IR imaging withInfrared Array Camera (IRAC) on the Spitzer Space telescope shows that thesegalaxies have spectral energy distributions that peak between 3.6 and 4.5microns. Fits to 12-band photometry reveal 12 or more galaxies with spectralshapes consistent with z = 1.51. Most are within ~170 co-moving kpc of the GDDSgalaxy. Deep F814W images with the Advanced Camera for Surveys (ACS) on HSTreveal that these galaxies are a mix of early-type galaxies, disk galaxies andclose pairs. The total stellar mass enclosed within a sphere of 170 kpc inradius is > 8E+11 solar masses. The colors of the most massive galaxies areclose to those expected from passive evolution of simple stellar populations(SSP) formed at much higher redshifts. We suggest that several of thesegalaxies 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 clustercore typical of the progenitors of massive clusters in the present day andsuggests the red sequence was in place in over-dense regions at early times.Comment: 14 pages, 6 figures, submitted to ApJ Letter