Annu. Rev. Astron. Astrophys. 1994. 32: 115-52
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The S0 Problem

For the most part in this review, we have not emphasized the importance of the S0 class. If a spiral loses 90% of its interstellar HI, its potential for future star formation must be greatly diminished and thus its spiral structure should fade (Larson et al. 1980). The resultant object would look like an S0. As pointed out by Giovanelli & Haynes (1985), the S0 fraction at a given density in Dressler's (1980) survey is higher in X-ray luminous clusters, while the corresponding spiral fraction is lower. At the same time though, fundamental differences in B/D and in surface magnitude still distinguish the S0 galaxies from present-day spirals (Dressler 1980). One-fifth of the S0's in the RSA contain significant amounts of HI, while others of similar properties are lacking in HI to much lower limits (Bregman et al. 1992). The HI detection rate is so much higher than that for ellipticals that a capture hypothesis is unlikely. Pogge & Eskridge (1993) find a similar dichotomy in the presence of H II regions in S0 disks, but conclude that the S0's merely represent the early-type end of a continous variation in star formation activity along the spiral sequence. Because of the large dispersion in properties, the S0 class still remains enigmatic.

Classical Galaxies versus Dwarfs

We concur with, among others Sandage et al. (1985) and Binggeli (1993), that there are fundamental differences between bright (``classical'') and faint (``dwarf'') galaxies. The RC3 contains too few dE's for an interpretation of their characteristics in our analysis, but repeatedly in Figures 2 and 3 and Table 1, clear distinctions are seen in the latest types. Sandage (1990) summarizes the results of the Las Campanas surveys of the Virgo and Fornax clusters and selected loose groups conducted by him and his collaborators. In particular, these studies have explored the relative distributions of dE and Im dwarfs. They conclude that:

1. Dwarf galaxies have the same general space distribution as giant galaxies. The dE's are predominantly found in dense groups whereas the Im's are more widely dispersed. The dE's at least seem to form only in the vicinity of more massive galaxies.
2. The faint end slope of the luminosity function for dE's in the field is flatter than that found for similar galaxies in Virgo or Fornax.
3. The dwarf to classical (``giant'') galaxy ratio decreases in low richness regions relative to ratios found in clusters.

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