Binary galaxies discussed in chapter 13 are a special case of
multiplicity and clustering among galaxies. Large-scale clustering is
discussed in chapter 15. The present chapter discusses multiple
systems: groups, clusters, and clouds of intermediate size (D
1-3
Mpc) and population (n
10-100). Such systems can be identified and
best studied individually in our immediate neighborhood
(
< 20 Mpc).
The nearer galaxies used in the calibration of the distance scale and
in assessing the detailed properties of galaxies are generally members
of nearby groups.
By ``group'' is meant a number n > 2 of distinct galaxies that are bound gravitationally for periods of the order of 109 years or greater (star cluster analogy) or perhaps are genetically related as products of a common explosive formative event during the past 109 years or so even though they may not be gravitationally bound at present (stellar association analogy).
These concepts, however, are not suitable as operational
definitions. In practice a ``group'' will be a ``small'' number of
galaxies, say n 10,
brighter than M = -16 (perhaps of a specified
class only, either elliptical and lenticular, or spiral and irregular)
occupying a volume of space V in which the average density
n = n /
V, say n 10
per Mpc3, is at least one order of magnitude greater than
the average density
< 1 > in a
surrounding volume of space of diameter
one order of magnitude greater than that of the group. In other words,
the significant factor is the contrast with the surrounding field, not
the absolute densities or numbers. Note that number density is
considered, not mass density (which would exclude the galactic
equivalent of stellar associations).
Space density, however, is not an observable, and the basic observational criterion of clustering is surface density in the apparent distribution. Since dwarf galaxies (dE, dIm) are inconspicuous or invisible in all but the nearest groups, a group is first identified by its brightest members. Further criteria of group membership inc]ude a small velocity range and a general similarity of morphological types, apparent magnitudes, and diameters.