A High Merger Fraction in the Rich Cluster MS 1054–03 at z = 0.83: Direct Evidence for Hierarchical Formation of Massive Galaxies^{* **}

We present a morphological study of the galaxy population of the luminous X-ray cluster MS 1054-03 at z = 0.83. The sample consists of 81 spectroscopically confirmed cluster members in a 3 × 2 h Mpc area imaged in F606W and F814W with WFPC2. We find 13 ongoing mergers in MS 1054-03, comprising 17% of the L ≳ L_* cluster population. Most of these mergers will likely evolve into luminous (~2L_*) elliptical galaxies, and some may evolve into S0 galaxies. Assuming the galaxy population in MS 1054-03 is typical for its redshift, it is estimated that ~50% of present-day cluster elliptical galaxies experienced a major merger at z < 1. The mergers are preferentially found in the outskirts of the cluster and probably occur in small infalling clumps. Morphologies, spectra, and colors of the mergers show that their progenitors were typically E/S0 or early-type spiral galaxies with mean stellar formation redshifts z_* ≳ 1.7. The red colors of the merger remnants are consistent with the low scatter in the color-magnitude relation in rich clusters at lower redshift. The discovery of a high fraction of mergers in this young cluster is direct evidence against formation of elliptical galaxies in a single "monolithic" collapse at high redshift and is in qualitative agreement with predictions of hierarchical models for structure formation.