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2.4. Fraction of Galaxies in Clusters
The fraction of galaxies in rich R 0 clusters is ~ 5% (within the Abell radius RA = 1.5h-1 Mpc). The fraction of all galaxies that belong in clusters increases with increasing radius RA and with decreasing cluster richness threshold.
The average number of galaxies per cluster for R 0 clusters within 1.5h-1 Mpc radius and m m3 + 2m is <NR>median 50, or <NR>mean 56. For R 1 clusters the average is <NR>median 60, or <NR>mean 75. The number of galaxies increases to fainter luminosities following the Schechter (1976) luminosity function.
2.5. Galaxy Overdensity in Rich Clusters
The average number density of bright ( L*) galaxies in R 0 clusters (within RA = 1.5h-1 Mpc) is
The average overall (field) number density of bright ( L*) galaxies is
The average galaxy overdensity in rich (R 0) clusters (within 1.5h-1 Mpc radius) is thus
The typical threshold galaxy overdensity in clusters (within 1.5h-1 Mpc radius) is
The galaxy overdensity increases at smaller radii from the cluster center. The galaxy overdensity in the cores of typical compact rich clusters is approximately