The parameters in most cluster classification systems can be related to dynamical indicators of cluster evolution. This provides a unification of a variety of properties in a simple framework. In such a scheme, long dynamical times are associated with clusters having low mass densities, low velocity dispersions, a cool (few times 107 K) intracluster medium, irregular galaxy and gas distributions, and usually large populations of spiral galaxies. More dynamically evolved systems (those with shorter dynamical timescales) have higher mass densities, higher velocity dispersions, hot intracluster gas temperatures, regular galaxy and gas distributions, and small populations of spiral galaxies. Some of the classification parameters are tied closely to dynamical indicators. For example, increases in central galaxy density and X-ray luminosity are related to the deepening of the gravitational potential during cluster collapse and relaxation. Other properties such as spiral fraction are related less directly through correlations such as those of Dressler (1980) who showed that galactic populations are related to the local density which in turn is related to dynamical evolution. Classification systems based on cluster morphology (Zwicky et al. 1961-1968; Rood and Sastry 1971), the dominance of the central galaxy (Bautz and Morgan 1970 and Hausman and Ostriker 1978), the galaxy population (Oemler 1974), and others (see Bahcall 1977a for an excellent summary) can all be interpreted in terms of a dynamical indicator.
The X-ray observations of rich clusters have demonstrated the need for additional complexity in this otherwise simple, linear classification scheme. The added complication arises from the presence of massive dominant galaxies in clusters whose other properties - low X-ray luminosities, cool gas temperatures, high spiral fractions, low velocity dispersion, irregular galaxy distributions - are indicative of dynamical youth. Examples are Virgo, Centaurus, A262, and A1060 whose dominant galaxies are M87, N4696, N708, and N3311. These clusters are often of late Bautz-Morgan type with two or more bright galaxies of comparable optical luminosity (e.g., M87 and N4472 in Virgo), but X-ray observations suggest one of the galaxies is a unique object and is centrally located in the cluster potential. The best studied examples of massive galaxies in otherwise dynamically young clusters are M87 and N4696 which were studied in detail by Fabricant and Gorenstein (1983) and Matilsky, Jones, and Forman (1985). Table 1 lists the properties of the galaxies and their respective clusters. Very large masses are found within even a few hundred kpc of these galaxies.
|M87 / Virgo||N4696 / Centaurus|
|X-ray Luminosity a ( ergs sec-1)||3 x 1043||7 x 1043|
|Gas Temperature b (keV)||2.4||2.1|
|Mass Deposition Rate c (M yr-1)||1-10||~ 50|
|Gas Massd (M) within 200 kpc||1 x 1012||2 x 1012|
|Spiral Fraction e||55%||45%|
|Velocity Dispersion f(l-o-s; km sec-1)||673||507|
|Total Mass d (M) within 200 kpc||3 x 1013||2 x 1013|
|a Jones and Forman 1978 scaled to 2-6 keV; Lea et al. 1982.|
|b Matilsky Matilsky, Jones, and Forman 1985 and Fabricant and Gorenstein 1983.|
|c Matilsky Matilsky, Jones, and Forman 1985 and Stewart et al. 1984.|
|d Matilsky, Jones, and Forman 1985 and Fabricant and Gorenstein 1983.|
|e Bahcall 1977b.|
|f Lucey, Dickens, and Dawe, 1980 and Danese, DeZotti, and di Tullio 1980.|
The presence of massive central galaxies in clusters whose other properties are those of dynamically young systems suggests the addition of a second dimension to the cluster classification system, related to the dominance of a central galaxy (Jones and Forman 1984). Figure 4 and Table 2 illustrate the two-dimensional classification system.
Figure 4. X-ray contours of four clusters (A1367, A262, A2256, and A85) are shown superposed on optical photographs. These illustrate the cluster classification described in Table 2. The top and bottom pairs of clusters are at comparable distances. The pair of clusters on the right have bright central galaxy (XD). The X-ray emission is centrally concentrated with a small core radius for the clusters on the right compared to the nXD systems on the left.
|No X-ray |
|Low X-Ray Luminosity (< 1044 ergs sec-1)|
|High Spiral Fraction (> 40%)|
|A1367||Low Central Density||A262|
|Cool Gas (~ few keV)|
|Irregular Gas and Galaxy||Distributions|
|High X-ray Luminosity (> 1044 ergs sec-1)|
|Low Spiral Fraction (< 40%)|
|A2256||High Central Density||A85|
|Hot Gas (>6 keV)|
|Regular Gas and Galaxy Distributions|
|Large core radii||Small core radii|