Evolution since z = 0.5 of the Morphology-Density Relation for Clusters of Galaxies

Using traditional morphological classifications of galaxies in 10 intermediate-redshift (z ~ 0.5) clusters observed with WFPC2 on the Hubble Space Telescope, we derive relations between morphology and local galaxy density similar to that found by Dressler for low-redshift clusters. Taken collectively, the "morphology-density" relationship, T - Σ, for these more distant, presumably younger clusters is qualitatively similar to that found for the local sample, but a detailed comparison shows two substantial differences: (1) For the clusters in our sample, the T - Σ relation is strong in centrally concentrated "regular" clusters, those with a strong correlation of radius and surface density, but nearly absent for clusters that are less concentrated and irregular, in contrast to the situation for low-redshift clusters, where a strong relation has been found for both. (2) In every cluster the fraction of elliptical galaxies is as large or larger than in low-redshift clusters, but the S0 fraction is 2-3 times smaller, with a proportional increase of the spiral fraction. Straightforward, though probably not unique, interpretations of these observations are (1) morphological segregation proceeds hierarchically, affecting richer, denser groups of galaxies earlier, and (2) the formation of elliptical galaxies predates the formation of rich clusters and occurs instead in the loose-group phase or even earlier, but S0's are generated in large numbers only after cluster virialization.