The ESO Nearby Abell Cluster Survey IX. The morphology-radius and morphology-density relations in rich galaxy clusters

Abstract

We study the morphology-radius (MR-) and morphology-density (MD-) relations for a sample of about 850 galaxies (with M <= -19.5) in 23 clusters from the ENACS (ESO Nearby Abell Cluster Survey). On the basis of their radial distributions we must distinguish: 1. brightest ellipticals (with M < -22), 2. late spirals, and 3. the ensemble of less bright ellipticals, S0 galaxies and early spirals. The latter have indistinguishable distributions of projected radial distance R. The brightest ellipticals are most centrally concentrated, the late spirals are almost absent from the central regions and the other classes are intermediate. Radial segregation of the ellipticals is due to the brightest ellipticals only, that of the spirals to the late spirals only. We derive the MD-relation with two measures of projected density: one using the 10 nearest neighbours (Sigma10) and another using only the nearest neighbour (Sigma1). The Sigma10 MD-relation only shows a significant difference between early- and late-type galaxies, but the different galaxy types within those classes are indistinguishable. However, The Sigma1 MD-relation shows that the normal 'ellipticals' (with M >= -22), the S0 galaxies and the early spirals have different Sigma1-distributions. The reason for this is that Sigma1 is much less correlated with R than is Sigma10, and thus has much less cross-talk from the (MR-) relation. 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. The segregation of the brightest ellipticals and the late spirals is driven mostly by global factors, while the segregation between 'normal' ellipticals, S0 galaxies and early spirals is driven primarily by local factors.

8 pages, 6 figures; accepted for publication in Astronomy&Astrophysics

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• A JavaScript Passive Evolution Calculator
• The ESO Nearby Abell Cluster Survey. XIII. The orbits of the different types of galaxies in rich clusters
• The CFHT Legacy Survey: The Morphology-Density Relation of Galaxies out to Z~1
• The ESO Nearby Abell Cluster Survey VIII. Morphological and spectral classification of galaxies

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