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2.1. Catalogs

The two most extensive and often cited catalogs of rich clusters of galaxies are those of Abell (1958) and Zwicky and his collaborators (Zwicky et al., 1961-1968). As is conventional, in this book Abell clusters will be denoted by giving A and then the number of the cluster in Abell's list. Both of these catalogs were constructed by identifying clusters as enhancements in the surface number density of galaxies on the National Geographic Society-Palomar Observatory Sky Survey (Minkowski and Abell, 1963) and thus are confined to northern areas of sky (declination greater than -20° for Abell and -3° for Zwicky). Abell surveyed only clusters away from the plane of our galaxy.

As clustering exists on a very wide range of angular and intensity scales (Peebles, 1974), it is not possible to give a unique and unambiguous definition of a 'rich cluster'. Thus the membership of a catalog of clusters is determined by the criteria used to define a rich cluster. These criteria must specify the required surface number density enhancement for inclusion and the linear or angular scale of the enhancement. The scale is necessary in order to exclude small groupings of galaxies; for example, a close pair of galaxies can represent a very large enhancement above the background galaxy density on a small angular scale. Alternatively, specifying the surface density and scale is equivalent to specifying the number of galaxies (the 'richness' of the cluster) and the scale size. Because the number of galaxies observed increases as their brightness diminishes (Section 2.4), one must also specify the range of magnitudes of the galaxies included in determining the cluster richness. Finally, because galaxies grow fainter with increasing distance, the catalog can only be statistically complete out to a limiting distance or redshift, and only clusters within this distance range should be included in a statistically complete sample.

Abell's criteria were basically (1) that the cluster contain at least 50 galaxies in the magnitude range m3 to m3 + 2, where m3 is the magnitude of the third brightest galaxy; (2) that these galaxies be contained within a circle of radius RA = 1.7 / z minutes of arc or 3 h50-1 megaparsecs, (2) where z is the estimated redshift of the cluster (Section 2.2); (3) that the estimated cluster redshift be in the range 0.02 leq z leq 0.20. RA is called the Abell radius of the cluster. The Abell catalog contains 2712 clusters, of which 1682 satisfy all these criteria. The other 1030 were discovered during the search and were included to provide a more extensive finding list for clusters. The Abell catalog gives estimates of the cluster center positions (see also Sastry and Rood, 1971), distance, and richness of the clusters, as well as the magnitude of the tenth brightest galaxy m10.

For the Zwicky catalog, the criteria were as follows: (1) the boundary (scale size) of the cluster was determined by the contour (or isopleth) where the galaxy surface density fell to twice the local background density; (2) this isopleth has to contain at least 50 galaxies in the magnitude range m1 to m1 + 3 , where m1 is the magnitude of the first-brightest galaxy. No distance limits were specified, although in practice, very nearby clusters such as Virgo (Figure 1a), were not included because they extended over several Sky Survey plates. Obviously, the Zwicky catalog criteria are much less strict than Abell's, and the Zwicky catalog thus contains many more clusters that are less rich. For each cluster, the Zwicky catalog gives a classification (Section 2.5), and estimates of the coordinates of the center, the diameter, the richness, and the redshift. Finding charts showing the cluster isopleths and positions of brighter galaxies and stars are also presented.

A number of smaller catalogs have been compiled, consisting of clusters in the southern sky or clusters at higher redshifts (z gtapprox 0.2). Early southern cluster catalogs or lists include those of de Vaucouleurs (1956), Klemola (1969), Snow (1970), Sersic (1974), and Rose (1976). Until recently, the search for southern clusters was severely handicapped by the lack of deep survey plates. The first deep optical survey of the south, the European Southern Observatory Quick Blue Survey (ESO-B), was completed in 1978 (West, 1974; Holmberg et al., 1974). A catalog of southern clusters from the first portion of this survey was prepared by Duus and Newell (1977) and a list of southern clusters near X-ray sources was given by Lugger (1978). More recently, portions of the ESO/SRC-J survey of very deep blue plates have been used to compile southern cluster catalogs by Braid and MacGillivray (1978) and White and Quintana (1985). Before his untimely death, Abell was preparing a southern continuation of the Abell catalog in collaboration with Corwin. The red portion of the ESO/SRC survey is currently being done, and these plates are being used to detect high redshift clusters (West and Frandsen, 1981).

The discovery of higher redshift clusters (z gtapprox 0.2) is of great importance to cosmological studies; lists of such clusters include Humason and Sandage (1957), Gunn and Oke (1975), Sandage, Kristian, and Westphal (1976), Spinrad et al. (1985), and Vidal (1980). In addition, the lists of southern clusters from the ESO/SRC surveys discussed in the last paragraph contain many high redshift clusters.

2 For convenience, in this book the Hubble constant Ho will be parameterized as Ho = 50 h50 km/sec/Mpc. Back.

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