Modern technology has given us high-quality images of more galaxies than ever before. The greater depth of exposures possible, and the all sky coverage due to the PSS, ESO, and SERC surveys, has led to the recognition of new types of galaxies and to more enlightened views of some older classes. In this section I focus on a few of these recent advances.
7.1. Early-type galaxies
There can be little doubt that accurate classification of early-type galaxies requires imaging material of the highest quality. Careful and extensive studies of large numbers of E and S0 galaxies have, over the past 10 years, revealed some of the complexities and inhomogeneities in these two classes of objects.
Several excellent recent reviews of E galaxies already exist (e.g., Nieto, 1988; Kormendy and Djorgovski, 1989, Franx, 1990), as well as a whole I. A. U. Symposium (de Zeeuw, 1987), so I will not list any specific references here. The general consensus of these works, which summarize a great deal of highly focussed research, is that the apparent simplicity of E galaxies is highly deceptive. It is now generally accepted that E's may be characterized more by triaxial intrinsic shapes than oblate shapes, that a significant fraction of E's have dust, that some have formed through merger processes as suggested by boxy isophotes and interleaved shells, that many have weak disks that are hard to detect if face-on, and that many have accreted material since they first formed which manifests itself in unusual HI properties, counter-rotating cores, disks of dust, or polar-type rings. The most interesting aspect of all of this research is just how much can be learned from a concerted and widespread effort on a single type of galaxy.
S0 galaxies have also been studied in great detail, but their relationship to spirals is still controversial. Most interesting has been the detection of neutral gas (e.g., Wardle and Knapp, 1986; van Driel, 1987) and ionized gas (Pogge and Eskridge, 1987) in some S0's. Photometric decompositions (e.g., Simien and de Vaucouleurs, 1986) and bulge studies (Dressier and Sandage, 1983) have favored Hubble's placement of S0's between E's and spirals, rather than in a parallel sequence to spirals, while recent spectroscopic studies (Gregg, 1989) and color analyses (e.g., Bothun and Gregg, 1990) have favored the "burnt-out" spiral theory where S0's are simply spirals that have exhausted their gas supply through astration. However, from a statistical study of S0 luminosities, van den Bergh (1990) has concluded that the S0 class is a "repository of physically quite distinct sorts of objects that exhibit only superficial similarities", indicating that the "various kinds of S0 galaxies might have arrived at their present morphological state along quite different evolutionary tracks." This suggests to me that there is still quite a bit more to be learned about the S0 phenomenon in general.
Finally, the properties of the Morgan D and cD classes, the latter usually found in the centers of rich Abell clusters, have been studied in great detail recently by Schombert (1986, 1987, 1988), who also discusses the classification of these objects in terms of morphology and surface brightness profile properties. The understanding now is that cD galaxies form a unique class of objects that may be related to mergers. "BCM's" are reviewed by Schombert (this conference).