Next Contents Previous

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)).

Table 5. Cluster Classification and Related Characteristics.

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

Next Contents Previous