Luminosity Functions of the Galaxy Cluster MS 1054-0321 at z=0.83 based on ACS Photometry

We present new measurements of the galaxy luminosity function (LF) and its dependence on local galaxy density, color, morphology, and clustocentric radius for the massive z=0.83 cluster MS 1054-0321. Our analyses are based on imaging performed with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) in the F606W, F775W, and F850LP passbands and extensive spectroscopic data obtained with the Keck Low-Resolution Imaging Spectrograph. Our main results are based on a spectroscopically selected sample of 143 cluster members with morphological classifications derived from the ACS observations. Our three primary findings are (1) the faint-end slope of the LF is steepest in the bluest filter, (2) the LF in the inner part of the cluster (or highest density regions) has a flatter faint-end slope, and (3) the fraction of early-type galaxies is higher at the bright end of the LF, and gradually decreases toward fainter magnitudes. These characteristics are consistent with those in local galaxy clusters, indicating that, at least in massive clusters, the common characteristics of cluster LFs are established at z=0.83. These results provide additional support for the hypothesis that the formation of galaxies in MS 1054-0321 began at redshifts considerably greater than unity. We also find a 2 σ deficit of intrinsically faint, red galaxies (i₇₇₅-z₈₅₀>=0.5, M_i>-19) in this cluster. Although the significance is marginal, this trend may suggest that faint, red galaxies (which are common in z<0.1 rich clusters) have not yet been created in this cluster at z=0.83. The giant-to-dwarf ratio in MS 1054-0321 starts to increase inwards of the virial radius or when Σ>30 Mpc^-2, coinciding with the environment where the galaxy star formation rate and the morphology-density relation start to appear. A physical process that begins to become effective at around the virial radius or Σ~30 Mpc^-2 may thus be responsible for the evolution of color and luminosity of cluster galaxies.