The Possible Precursors of z = 0 Cluster

We examine the distribution of stellar masses of galaxies in MS 1054.4-0321 and CL 0152.7-1357, two X-ray-selected clusters of galaxies at z = 0.83. Our stellar mass estimates, from spectral energy distribution fitting, reproduce the dynamical masses as measured from velocity dispersions and halflight radii with a scatter of 0.2 dex in the mass for early-type galaxies. When we restrict our sample of members to high stellar masses, those over> 10 M⊙ (M ⋆ in the Schechter mass function for cluster galaxies), we find that the fraction of early-type galaxies is 79% ±6% at z = 0.83 and 87% ±6% at z = 0.023 for the Coma Cluster, consistent with no evolution. Previous work with luminosityselected samples has found that the early-type fraction in rich clusters declines from ≃80% at z = 0 to ≃60% at z = 0.8. The observed evolution in the early-type fraction from luminosity-selected samples must predominantly occur among sub-M⋆ galaxies. As M⋆ for field and group galaxies, especially late-types, is below M⋆ for cluster galaxies, infall could explain most of the recent growth in the early-type fraction. Future surveys could determine the morphological distributions of lower mass systems which would confirm or refute this explanation. Subject headings: galaxies: clusters: general — galaxies: elliptical and lenticular, cD, — galaxies: evolution — galaxies: fundamental parameters — galaxies: photometry — galaxies: clusters: MS 1054.4-0321 — CL 0152.7-1357