On the interpretation of the morphology-density relation for galaxies in clusters

A reexamination of Dressler's sample of nearly 6000 galaxies in 55 clusters shows that galactic morphology correlates as well with the projected distance from the center of the cluster, R(clus), as it does with the projected local density of the 10 nearest galaxies. Most of the variation occurs within 0.5 Mpc of the cluster center. The effect is strongest in the 34 clusters with D galaxies, with the percentage of ellipticals rising from 18 + or - 3 percent at R(clus) = 0.6 Mpc to 65 + or - 7 percent at R(clus) = 0.1 Mpc. Various correlations suggest that the physical mechanisms which result in the population gradients for the spiral and S0 galaxies are different than the mechanism responsible for the preponderance of ellipticals near the cluster centers. Tentative evidence is also found that the fraction of S0 galaxies declines sharply within 0.2 Mpc of the cluster center, especially around central D galaxies. 19 refs.