Copyright © 1982 by Annual Reviews. All rights reserved
|
Annu. Rev. Astron. Astrophys. 1982. 20:
547-85 Copyright © 1982 by Annual Reviews. All rights reserved |
5.2 The Dynamical Stages of Clusters
Table 3 summarizes the characteristics and dynamical indicators for the two cluster families defined by X-ray core radius and dominance of the central galaxy. Details of these observations were reviewed in Sections 3 and 4 (also see Figure 1). The nXD clusters exhibit a range in dynamical indicators between those of Abell 1367 and Coma-like systems. The X-ray double clusters are classed as intermediate based on analogy to the numerical simulations, but they may represent only one of many channels through which relaxing clusters pass. Unevolved XD clusters are typified by Virgo and Abell 262, while the evolved systems include Abell 85, Abell 1795, and Perseus. The intermediate stage in this class is represented by clusters that have cool X-ray gas and intermediate-to-high spiral fractions and galaxy densities. X-ray doubles may also occur in this class although none have been reported. The evolutionary sequence in each family should be viewed as resulting from a smooth continuous process and not as discrete stages.
| Clusters without a central, dominant galaxy | Clusters with a central, dominant galaxy |
| ~ 500 kpc X-ray core radius |
 300 kpc X-ray core radius
|
| Early systems - Abell 1367 | Early systems - Virgo, Abell 262 |
| Low X-ray luminosity (< 1044 erg s-1) | Low X-ray luminosity (< 1044 erg s-1) |
| Cool X-ray gas (1-4 keV) | Cool X-ray gas (1-4 KeV) |
| X-ray emission around galaxies | X-ray emission from halo of single galaxy |
High spiral fraction
( 40%)
| High spiral fraction
( 40%)
|
| Low central galaxy density | Low central galaxy density |
| Irregular X-ray and optical cluster structure | |
| Intermediate systems - X-ray "doubles" | Intermediate systems - Abell 400, Abell 1991, |
| (placed based on X-ray structure) | Abell 2063, Abell 2199 |
| Predominantly cool X-ray gas (1-4 keV) | |
| Multiple nuclei often in dominant galaxy | |
| Intermediate-high spiral fraction (25-60%) | |
| Evolved systems - Coma, Abell 2256, Abell 2255 | Evolved systems - Abell 85, Abell 1795, Perseus |
| High X-ray luminosity (> 1044 erg s-1) | High X-ray luminosity (> 3 x 1044 erg s-1) |
| Hot X-ray gas
( 6 keV)
| Hot X-ray gas
( 6 keV)
|
| No central cooling | Cooling accretion flows onto central dominant galaxy |
| High velocity dispersion | High velocity dispersion |
| Low spiral fraction
( 20%)
| Low spiral fraction
( 20%)
|
| High central galaxy density | High central galaxy density |
| Regular, relaxed, symmetric cluster structure | |
The sequence originates through the dynamical evolution of clusters, which formed from a range of initial conditions. Since the cluster galaxies as well as the cluster are evolving, the conditions in presently unevolved clusters may be very different from the conditions that existed during an earlier epoch in the early dynamical phases of evolved clusters. Therefore while clusters are classified along an evolutionary sequence, a specific cluster may not evolve precisely along that sequence. For example, if some fraction of S0s form initially in high-density regions, then a cluster with a smaller initial density may never have the same population distribution as an initially denser system. Although galaxies may evolve differently from cluster to cluster and the galaxy morphology may depend on initial conditions, the general cluster evolutionary trends should persist for the present epoch.
5.2.1 FRACTIONS OF EVOLVEO AND UNEVOLVED CLUSTERS Based on the dynamical indicators, one can estimate the percentage of young and evolved clusters. Cluster samples drawn from Abell's (1958) catalog of rich clusters have cluster densities about ten times that of the field. This bias toward high-density systems also tends to be a bias toward clusters in relatively advanced evolutionary states. Therefore the following estimated fraction of unevolved clusters is probably a lower limit.
The only evolutionary indicator now available for a considerable
number of clusters is X-ray luminosity. Fortunately, X-ray luminosity
appears to be correlated with other dynamical indicators such as
central galaxy density, cluster velocity dispersion, and X-ray
temperature. A 2-10 keV luminosity threshold dividing cool (2-4 keV)
from hot (> 5 keV) clusters of 3 x 1044 erg s-1 is
obtained from the spectral summary of
Mushotzky & Smith
(1980).
If the correlation of
X-ray temperature and luminosity holds for all clusters, such that
clusters with luminosities above 3 x 1044 erg s-1
are hot and therefore evolved, one finds from the cluster X-ray
luminosity function
(McKee et al. 1980)
that only 9% of Abell clusters exceed this threshold and
thus would be considered evolved systems. Forman & Jones (in
preparation) performed a similar analysis on a sample of nearby
(z 
0.08) Abell clusters observed with Einstein. Again, based on X-ray
temperatures, an X-ray luminosity threshold of 1044 erg
s-1 was chosen
for the 0.5-3.0 keV band. Two thirds of the sample fell below this
threshold and therefore were suggested to be in early evolutionary
stages. For the Einstein survey, this fraction is a lower limit since
several of the clusters with higher luminosities are observed to have
low temperatures. In support of the unevolved nature of most clusters,
Dressler (1980b)
found that the majority of the 55 rich clusters in
his sample had low concentrations of galaxies, and he estimated that
only ~ 20-30% had completed the violent relaxation phase and were
virialized. From these observations we conclude that most rich
clusters are in early stages of their dynamical evolution. The
distinction of rich clusters is important in this conclusion, since
some properties of evolved poor clusters and groups (high spiral
fractions, low velocity dispersion, and low X-ray temperature) would
be interpreted as indicating an unevolved system if these systems are
mistaken for rich clusters.