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| © CAMBRIDGE UNIVERSITY PRESS 1999 |
2.11. Cluster Classification
Rich clusters are classified in a sequence ranging from early- to late-type clusters, or equivalently, from regular to irregular clusters. Many cluster properties (shape, concentration, dominance of brightest galaxy, galactic content, density profile, and radio and X-ray emission) are correlated with position in this sequence. A summary of the sequence and its related properties is given in Table 5. Some specific classification systems include the Bautz-Morgan (BM) System (Bautz and Morgan 1970), which classifies clusters based on the relative contrast (dominance in extent and brightness) of the brightest galaxy to the other galaxies in the cluster, ranging from type I to III in decreasing order of dominance; and the Rood-Sastry (RS) system (Rood and Sastry 1971) which classifies clusters based on the distribution of the ten brightest members (from cD, to binary (B), core (C), line(L), flat (F), and irregular (I)).
| Regular (Early) | Intermediate | Irregular (late) | |
| Property | type clusters | clusters | type clusters |
| Zwicky type | Compact | Medium-compact | Open |
| BM type | I, I-II, II | (II), II-III | (II-III), III |
| RS type | cD,B,(L,C) | (L),(F),(C) | (F),I |
| Shape symmetry | Symmetrical | Intermediate | Irregular shape |
| Central concentration | High | Moderate | Low |
| Galactic content | Elliptical-rich | Spiral-poor | Spiral-rich |
| E fraction | 35% | 20% | 15% |
| S0 fraction | 45% | 50% | 35% |
| Sp fraction | 20% | 30% | 50% |
| E:S0:Sp | 3:4:2 | 2:5:3 | 1:2:3 |
| Radio emission | ~ 50% detection | ~ 50% detection | ~ 25% detection |
| X-ray luminosity | High | Intermediate | Low |
| Fraction of clusters | ~ 1/3 | ~ 1/3 | ~ 1/3 |
| Examples | A401, Coma | A194 | Virgo, A1228 |