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 x 2 h(50)(-1) Mpc area imaged in F606W and F814W with WFPC2. We find 13 ongoing mergers in MS 1054-03, comprising 17% of the L greater than or similar to L-* cluster population. Most of these mergers will likely evolve into luminous (similar to 2L(*)) elliptical galaxies, and some may evolve into SO galaxies. Assuming the galaxy population in MS 1054-03 is typical for its redshift, it is estimated that similar to 50% of present-day cluster elliptical galaxies experienced a major merger at