Mass Selection and the Evolution of the Morphology-Density Relation from z = 0.8 to 0^{* **}

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 published results on galaxy evolution in dense environments. Morphology-density relations (MDRs) are derived in each cluster from a complete, luminosity-selected sample of 452 galaxies with a magnitude limit M_V < M + 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 mass-selected 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.