The classification of evolving galaxies

A review of available evidence favours the view that most lenticular galaxies are former spirals that have been stripped of gas rather than galaxies in which all gas was used up at the termination of proto-galactic collapse. An analysis of new homogeneous colour observations of ellipticals suggests that E1 galaxies are slightly redder than are ellipticals of type E5. Analysis of new data shows that the luminosity distribution of elliptical galaxies contains more luminous objects than does that of lenticulars which suggests that the mass spectrum of lenticular galaxies is deficient in high mass objects when compared with that of ellipticals. The frequency with which barred spirals occur is found to depend on both Hubble type and on galaxy luminosity but not on environmental factors such as membership in clusters or binary systems. It is shown that statistically selected samples of 'binary' galaxies are heavily contaminated by cluster members. This result, and a selection effect described in the test, lower the credibility of galaxy mass: light ratios determined from studies of binary galaxies. Finally it is noted that the brightest galaxies in clusters of BM type I are much more likely to be binaries than is the case for first-ranked galaxies in clusters of BM type III. (author)