7.3. Amorphous galaxies
This class was introduced by Sandage and Brucato (1979) to encompass galaxies "which are not E, S0, or any type of spiral but which have an amorphous appearance to the unresolved light, sometimes with imbedded resolved stars... All members of the class have well-developed early-type absorption spectra spread throughout the disk." These galaxies are related to Holmberg's Irr II class and de Vaucouleurs I0 class. A detailed study of a prototype amorphous galaxy, NGC 1800, was made by Gallagher, Hunter, and Knapp (1981). They suggested that the properties of this object favor a system that for some reason has a flatter IMF than normal and has been extremely efficient in forming stars. The color properties and star formation histories of a larger sample of amorphous galaxies are discussed by Gallagher and Hunter (1987).
7.4. Dwarf and low surface brightness galaxies
A detailed photographic study of low-luminosity members of the Virgo Cluster, which contains galaxies of every known morphological type, has been presented by Sandage and Binggeli (1984). With this large body of morphological data they propose a refined classification of dwarf galaxies that covers late and early-type galaxies. Particularly interesting is their recognition of a dwarf S0 class (dS0), that is, a class of low surface brightness dwarfs which morphologically are distinguishable from dwarf E's in showing direct evidence for a disk or an inflection in the brightness distribution. The recognition of dwarf S0's, however, does not imply the existence of dwarf spirals of comparable luminosity to the dS0's. Sandage and Binggeli comment that "there are ... no convincing candidates in the Virgo Cluster for dSa", and that "there is no equivalent in the spirals of the Hubble sequence at faint absolute magnitudes."
Other interesting findings from this paper are a new class of galaxies which are dwarfs in luminosity but not in size ("huge" Im and dE types), and of variations in the properties of the dwarf ellipticals (e.g., with or without nuclei, see also Kunth et al., 1988). Sandage and Binggeli find that 80% of the Virgo Cluster galaxies fainter than 14th magnitude are dE's while 20% are Sm or Im types. A more detailed study of a subset of dE's in Virgo is given by Impey, Bothun, and Malin (1988).
Perhaps one of the most interesting realizations the past few years is that there exist disk galaxies which are low in surface brightness but which are nevertheless neither dwarfs in luminosity nor in size. Bothun et al. (1987, 1990) discuss two examples discovered so far, Malin 1 and F 568-6, and the implications of their properties with regard to the evolutionary time-scales of disks. These galaxies are examples of massive, low surface brightness disk galaxies.