What Determines the Morphological Fractions in Clusters of Galaxies?

A reexamination of Dressler's sample of nearly 6000 galaxies in 55 clusters shows that the morphology-clustercentric radius relation is more fundamental than the morphology-local density relation. This conclusion is supported by improved correlations when the clustercentric radius is used as the independent parameter, and by a comparison of galaxies with the same normalized clustercentric radii but different values of the local density. The morphology-radius relation, when normalized by a characteristic cluster radius, R_c_^opt^, does not vary as a function of the number density within 0.5 Mpc, N_0.5_, the X-ray luminosity, L_x_, or the velocity dispersion of the cluster, V_disp_. This surprising result means that only one parameter is needed to determine the morphological fractions in clusters, namely R/R_c_^opt^. The elliptical fraction in the outer regions of clusters is relatively constant for all types of clusters, with a slight rise from about 10% in the outermost region to about 16% at R/R_c_^opt^ = 1. For radii smaller than R/R_c_^opt^= 1 (generally about 0.5 Mpc for most clusters) the elliptical fraction rises rapidly, reaching values of 60%-70% near the center of the cluster. The S0 fraction rises moderately as the center is approached, and then falls sharply within about 0.2 Mpc of the center. The spiral fraction falls moderately as the clustercentric radius decreases and then falls rapidly near the center. The spiral fraction is essentially zero at the cluster center, even though spirals dominate everywhere else in the universe. These results indicate that some property of the cluster center plays the key role in determining the morphological fractions in clusters, and suggests the possibility that a destructive mechanism is controlling the morphological fractions rather than a formation mechanism. Using this basic idea as a starting point we developed the following simple model. The three basic assumptions are (1) the intrinsic morphological mix of galaxies is E/(S0 + S + I) = 10%/90%, (2) elliptical galaxies form first, followed by the cluster collapse, S0 galaxies, and finally spiral and irregular galaxies, and (3) during the cluster collapse, galaxies which are still protogalactic clouds of gas are destroyed, and the gas from these failed galaxies is added to the intracluster medium. The destruction of the late-forming spiral and S0 protogalaxies near the cluster centers results in the increase in the elliptical fraction. This simple model can explain a wide range of observations. Besides explaining the morphological fractions, it also suggests that roughly half of the intracluster gas resulted from "failed" galaxies. The model is also consistent with the tidal radii that would be imposed by the mean tidal field of the clusters.