The classification of evolving galaxies

Abstract

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 term nation of proto-galactic collapse. An analysis of new homogeneous colour observations of ellipticals by Sandage & Visvanathan suggests that E1 galaxies are slightly redder than are ellipticals of type E5. This situation might have arisen if (1) the mass spectrum of star formation, and hence the rate of heavy element enrichment, depended on turbulence and if (2) turbulence in proto- E1 galaxies was m ore violent than it was in proto- E5 galaxies. Analysis of new data that have recently become available shows that the luminosity distribution of elliptical galaxies contains more luminous objects than does the luminosity distribution of lenticulars. This result suggests that the mass spectrum of lenticular galaxies is deficient in high mass objects when com pared 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 text, 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.