7.6 Population Types
There is a remarkable segregation of galaxy types among nearby groups and clouds; this phenomenon, which was first noted in the Grus cloud (de Vaucouleurs 1956a), the UMa I cloud (Morgan 1958), and the ``M94 group'' (van den Bergh 1960d), is probably related to differences in age and/or physical conditions (gas density, temperature, composition) at the time of formation of the groups.
A quantitative index of population type can be obtained by means of a numerical scale attached to the classification stage as follows:
In principle accurate color indices could be used, but in addition to the fact that color indices are not yet available for all galaxies in nearby groups, the color scale is too compressed in the early types (t < 0) and it is sensitive to absorption and emission. Table 7 gives the mean type index t, range and average deviation St (a ``purity'' index) of the four to six brightest galaxies in nearby groups.
Nearly all the nearer groups are ``S'' type, i.e., have a large majority of spirals and Magellanic irregulars with an occasional lenticular or I0 system and practically no giant ellipticals. Among all the nearby groups only the ``E'' core of the Virgo cluster and the Fornax I cluster have a dominant population of ellipticals.