What Is the Primary Source of the Morphological Segregation?

In 1880, Wolf first noted the existence of a segregation of nebulae of different types toward the Virgo cluster. In 1926, Hubble & Humason (1926) recognized the general tendency for Es to reside in the core of clusters. Dressler (1980) and Dressler et al. (1997) claim that the morphological types are primarily segregated in galaxy local density, whereas Whitmore, Gilmore & Jones (1993) claim that they are segregated with respect the clustercentric distance. However, as stressed by Sanromá & Salvador-Solè (1990) and by Dressler et al. (1997), galaxy density and clustercentric distance are almost degenerate in galaxy clusters, except perhaps in irregular clusters, so any correlation with one of these quantities is likely to be also found with the other one. The existence of a observational relation between galaxy properties and environment imposes the existence of a physical mechanism relating galaxies and environment. The exact nature of the morphological type segregation, in density, in clustercentric radius or in whatever parameter, gives us informations on the nature of the physical mechanism producing the observed change in the morphological composition of the clusters. A clustercentric segregation points out a global mechanism, whereas a segregation with respect to the local density a local one. Fig. 1 shows the morphology–density (left panels) and the morphology– clustercentric distance (right panels) relations. Points mark our data for a complete sample of ∼ 200 galaxies in Coma (Andreon et al. 1996, 1997), lines in the left panels present the morphology–density relation as determined in 10 nearby centrally concentrated clusters (Dressler et al. 1997), including Coma. Lines in the right panels show the morphology–clustercentric distance relation as determined by Whitmore, Gilmore & Jones (1993) on 55 nearby clusters (again including Coma). In the plot, Dressler density units are transformed