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2.8. Galactic Content in Rich Clusters

The fraction of elliptical, S0, and spiral galaxies in rich clusters differs from that in the field, and depends on the classification type, or density, of the cluster (see Section 2.11) (Bahcall 1977; Oemler 1974; Dressler 1980). See Table 4.

Table 4. Typical Galactic Content of Clusters (r ltapprox 1.5h-1 Mpc).

Cluster type E S0 Sp (E+S0)/Sp

Regular clusters (cD) 35% 45% 20% 4.0
Intermediate clusters (spiral-poor) 20% 50% 30% 2.3
Irregular clusters (spiral-rich) 15% 35% 50% 1.0
Field 10% 20% 70% 0.5

The fraction of elliptical (E) and S0 galaxies increases and the fraction of spirals decreases toward the central cores of rich compact clusters. The fraction of spiral galaxies in the dense cores of some rich clusters (e.g., the Coma cluster) may be close to zero.

The galactic content of clusters as represented in Table 4 is part of the general density-morphology relation of galaxies (Dressler 1980; Postman and Geller 1984); as the local density of galaxies increases, the fraction of E and S0 galaxies increases and the fraction of spirals decreases. For local galaxy densities ng ltapprox 5 galaxies Mpc-3, the fractions remain approximately constant at the average "Field" fractions listed above.

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