Mass Selection and the Evolution of the Morphology‐Density Relation from z = 0.8 to 0

Abstract

We examined the morphology-density relations for galaxy samples selected by luminosity and by mass in each of five massive X-ray clusters from z = 0.023 to 0.83 for 674 spectroscopically confirmed members. Rest-frame optical colors and visual morphologies were obtained primarily from Hubble Space Telescope images. The visual morphologies ensure consistency with the extensive p ublished results on galaxy evolution in dense environments. Morphologydensity relations (MDRs) are derived in each cluster from a complete, luminosity-selected sample of 452 galaxies with a magnitude limit M V < M V * + 1. The change in the early-type fraction with redshift matches previous work for massive clusters of galaxies. We performed a similar analysis, deriving MDRs for complete, mass-selected samples of 441 galaxies with a mass limit of 10 10.6 M ⊙ . Our mass limit includes faint objects, the equivalent of ≃1 mag below L* for the red cluster galaxies, and encompasses ≃70% of the stellar mass in cluster galaxies. The MDRs in the massselected sample at densities of Σ > 50 galaxies Mpc -2 are similar to those in the luminosity-selected sample but show larger early-type fractions, with a weak indication of a shallower slope. However, the trend with redshift in the fraction of elliptical and S0 galaxies with masses > 10 10.6 M ⊙ differs significantly between the mass- and luminosity-selected samples. The clear trend seen in the early-type fraction from z = 0 to ≃0.8 is not found in mass-selected samples. The early-type galaxy fraction changes much less and is consistent with being constant at 92% ±4% at Σ > 500 galaxies Mpc -2 and 83% ± 3% at 50 galaxies Mpc -2 < Σ < 500 galaxies Mpc -2 . Given the mass limit in our sample, this suggests that galaxies of mass lower than > 10 10.6 M ⊙ play a significant role in the evolution of the early-type fraction in luminosity-selected samples; i.e., they are larger contributors to the luminosity-selected samples at higher redshifts than at low redshifts. © 2007. The American Astronomical Society. All rights reserved.