|| © CAMBRIDGE UNIVERSITY PRESS 1999
2.1. Typical Properties of Clusters and Groups
Clusters of galaxies are bound, virialized, high overdensity systems of galaxies, held together by the clusters self gravity. Rich clusters contain, by traditional definition (Abell 1958), at least 30 galaxies brighter than m3 + 2m (where m3 is the magnitude of the third brightest cluster member) within a radius of R 1.5h-1 Mpc of the cluster center. This galaxy count is generally defined as the richness of the cluster. The galaxies in rich clusters move with random peculiar velocities of typically ~ 750 km s-1 (median line-of-sight velocity dispersion). This motion corresponds to a typical rich cluster mass (within 1.5h-1 Mpc) of ~ 5 × 1014h-1 M. In addition to galaxies, all rich clusters contain an intracluster medium of hot plasma, extending as far as the main galaxy concentration (R ~ 1.5h-1 Mpc). The typical temperature of the hot intracluster gas is ~ 5 kev, with a range from ~ 2 to 14 kev; the central gas density is ~ 10-3 electrons cm-3. The hot plasma is detected through the luminous X-ray emission it produces by thermal bremsstrahlung radiation, with Lx ~ 1044 erg s-1.
Like mountain peaks on earth, the high density rich clusters are relatively "rare" objects; they exhibit a spatial number density of ~ 10-5 clusters Mpc-3, as compared with ~ 10-2 galaxies Mpc-3 for the density of bright galaxies.
The main properties of clusters and groups of galaxies are summarized in Table 1 (Bahcall 1996). The table lists the typical range and/or median value of each observed property. Groups and poor clusters, whose properties are also listed, provide a natural and continuous extension to lower richness, mass, size, and luminosity from the rich and rare clusters.
|Property a||Rich clusters||Groups and poor clusters|
|Richness b||30-300 galaxies||3-30 galaxies|
|Radius c||(1-2) h-1 Mpc||(0.1-1) h-1 Mpc|
|Radial velocity dispersion d||400-1400 km s-1||100-500 km s-1|
|Radial velocity dispersiond (median)||~ 750 km s-1||~ 250 km s-1|
|Mass (r 1.5h-1 Mpc) e||(1014-2 × 1015)h-1||(1012.5-1014)h-1|
|Luminosity (B) f||(6 ×
1011-6 × 1012)h-2
h-1 Mpc) ||< /
LB> g ||~ 300h
||X-ray temperature h ||2-14 keV
|| 2 keV
||X-ray luminosity h
1043h-2 erg s-1
||Cluster number density i
||Cluster correlation scale j
||(22 ± 4)h-1 Mpc
||(13 ± 2)h-1 Mpc
||Fraction of galaxies in clusters or groups k
||~ 5% ||~ 55%
||a In most entries, the typical range in the
listed property or the median value is given. Groups and poor clusters
are a natural and continuous extension to lower richness, mass, size,
and luminosity from the rich and rare clusters.
||b Cluster richness: the number of cluster galaxies
brighter than m3 + 2m (where
m3 is the magnitude of the third
brightest cluster galaxy), and located within a
1.5h-1 Mpc radius of the cluster center
||c The radius of the main concentration of
typically, the galaxy surface density drops to ~ 1% of the central
core density). Many clusters and groups are embedded in larger
scale structures (to tens of Mpc).
||d Typical observed range and median value for
the radial (line-of-sight) velocity dispersion in groups and clusters
||e Dynamical mass range of clusters within
1.5h-1 Mpc radius
||f Luminosity range (blue) of clusters within
1.5h-1 Mpc radius
||g Typical mass-to-light ratio of clusters and
groups (median value) (Section 2.10).
||h Typical observed ranges of the X-ray
temperature and 2-10-keV X-ray luminosity of the hot intracluster gas
||i The number density of clusters decreases
sharply with cluster richness (Section 7).
||j The cluster correlation scale for rich
50, nc = 0.6 × 10-5
h3 Mpc-3) and poor (NR
nc = 2.4 × 10-5
h3 Mpc-3) clusters
||k The fraction of bright galaxies
L*) in clusters and groups
within 1.5 h-1 Mpc.
In the following subsections I discuss in more detail some of these intrinsic cluster properties.