The ESO nearby Abell cluster survey IX. The morphology-radius and morphology-density relations in rich galaxy clusters

We study the morphology-radius and morphology-density relations for a sample of about 850 galaxies (with M-R <= - 19.5) in 23 clusters from the ENACS ( ESO Nearby Abell Cluster Survey). On the basis of their radial distributions we must distinguish: (i) the brightest ellipticals (with M-R < - 22); (ii) the late spirals, and (iii) the ensemble of the less bright ellipticals, the S0 galaxies and the early spirals, which have indistinguishable distributions of projected radial distance R. The brightest ellipticals are most centrally concentrated, while the late spirals are almost absent from the central regions; the radial distribution of the other galaxy classes is intermediate. The previously found radial segregation of the ellipticals thus appears to be due to the brightest ellipticals only, while that of the spirals is due to the late spirals only. The morphology- density (MD-) relation was derived with two measures of projected density: one using the 10 nearest neighbours (Sigma(10)) and another using only the nearest neighbour (Sigma(1)). In the Sigma(10) MD-relation, only the classes of early- and late-type galaxies show a significant difference, but the different galaxy types within those classes are indistinguishable. However, this result is affected by significant cross-talk from the morphology- radius (or MR-) relation, as S10 is strongly correlated with R. Sigma(1) appears much less correlated with R and therefore the crosstalk from the MR- relation is much smaller. As a result, the normal ellipticals (with M-R >= - 22), the S0 galaxies and the early spirals do have different Sigma(1)-distributions. On average, the normal ellipticals populate environments with higher projected density than do the S0 galaxies while the early spirals populate even less dense environments. We conclude that the segregation of the brightest ellipticals and the late spirals is driven primarily by global factors, while the segregation between normal ellipticals, S0 galaxies and early spirals is driven mostly by local factors. We discuss briefly the implications of these results in terms of scenarios for formation and transformation of galaxies in clusters.