The most extensive morphological classification of BCM's is from the catalogs of Struble and Rood (1984). Within these catalogs BCM's are represented in a variety of forms; gE (giant elliptical), B (binary galaxy); Bp (peculiar binary, usually relating to a common envelope), cD (a gE with evidence of a diffuse and extended halo) and cD, (a "nested" cD of several multiple nuclei embedded in the primary's core). Sketches of several examples are shown in Figure 1, taken from Struble and Rood (1984).
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Figure 1. Examples of BCM's from Struble and Rood (1984), A978 is a flat cluster (F-type) with a D galaxy, A1371 is a Bp cluster and A1927 is an example of a nested cD galaxy. |
Although BCM's come in a variety of flavors, a majority are of the plain cD or the multi-nuclei cD variety where the embedded companions are sufficiently small enough to be ignored for structural analysis. For the rest of this review, I will concentrate on the parent body or primary mass concentration of BCM's, however, one should not forget that a significant fraction of BCM's are of the dumbbell variety (e.g. A400) where the companion is of comparable size and luminosity. An excellent study of the binary BCM's was performed by Valentijn and Casertano (1988).
It is instructive to remember that classification of BCM's is usually performed by visual examination of wide-field photographic plates (e.g glass copies of the Palomar Sky Survey), involving the material from a non-linear detector using a logarithmic device. This places a high weight, in the classification, on the mid-regions of an elliptical's halo. The centers are usually too high in density with little contrast to the eye and the faint, outer envelopes are below eye detection. Thus, it is not too surprising, as detailed in the following section, that a BCM's classification. is strongly influenced by the slope of the radial luminosity distribution at 5 to 10% of sky brightness (Malumuth 1983).