Evolution of the Early-Type Galaxy Fraction in Clusters since z = 0.8
  We study the morphological content of a large sample of high-redshift
clusters to determine its dependence on cluster mass and redshift. Quantitative
morphologies are based on bulge+disk decompositions of cluster and field
galaxies on deep VLT/FORS2 images of 18 optically-selected clusters at 0.45 < z
< 0.80 from the ESO Distant Cluster Survey (EDisCS). Morphological content is
given by the early-type galaxy fraction f_et, and early-type galaxies are
selected based on their bulge fraction and image smoothness. A set of 158 SDSS
clusters is analyzed exactly as the EDisCS sample to provide a robust local
comparison. Our main results are: (1) f_et values for the SDSS and EDisCS
clusters exhibit no clear trend as a function of sigma. (2) Mid-z EDisCS
clusters around sigma = 500 km/s have f_et ~= 0.5 whereas high-z EDisCS
clusters have f_et ~= 0.4 (~25% increase over 2 Gyrs). (3) There is a marked
difference in the morphological content of EDisCS and SDSS clusters. None of
the EDisCS clusters have f_et greater than 0.6 whereas half of the SDSS
clusters lie above this value. This difference is seen in clusters of all
velocity dispersions. (4) There is a strong correlation between morphology and
star formation in SDSS and EDisCS clusters. This correlation holds independent
of sigma and z even though the fraction of [OII] emitters decreases from z~0.8
to z~0.06 in all environments. Our results pose an interesting challenge to
structural transformation and star formation quenching processes that strongly
depend on the global cluster environment and suggest that cluster membership
may be of lesser importance than other variables in determining galaxy
properties. (ABRIDGED)
