Luminous red mergers in the z=0.83 cluster MS1054-03: Direct evidence for hierarchical formation of massive early-type galaxies

We present results from a morphological study of the distant X-ray cluster MS1054-03 at z=0.83. We have obtained a large, two color HST WFPC2 mosaic of this cluster, and measured redshifts of 186 galaxies in the MS1054-03 field with the 10m Keck telescope. Of 81 spectroscopically confirmed cluster galaxies observerd with HST, 13 are merger systems. Most of these mergers will likely evolve into luminous elliptical galaxies, and some may evolve into S0 galaxies. If the galaxy population in MS1054-03 is typical for its redshift up to 50% of ellipticals may have formed in mergers at z<1. The mergers are generally red and have no detected OII 3727 emission. This result is consistent with the old stellar ages of ellipticals inferred from other studies. The mergers are preferentially found towards the outskirts of the cluster, indicating they probably occur in infalling clumps. A significant overabundance of close pairs of red galaxies detected in the outskirts of MS1054-03 confirms the large number of interacting galaxies in this cluster.