Luminosity effects on morphology

From a study of galaxies in the Virgo cluster Holmberg (1958, p. 69) noted that giant galaxies have a higher surface brightness than dwarfs. When the prints of the Palomar Sky Survey first became available in the late 1950s it was immediately obvious that this large, and very uniform, database of galaxy images enabled one to segregate lowluminosity dwarfs from the much more numerous galaxies of average luminosity. Based on inspection of the prints of the Palomar Sky Survey, van den Bergh (1959, 1966) was able to compile catalogs of 243 DDO dwarf galaxies north of δ= −27°. The entries in these catalogs showed that the distribution of such dwarfs on the sky is broadly similar to that of nearby giant galaxies. This conclusion was confirmed for Virgo dwarfs by Reaves (1956, 1967). Furthermore, observations in the Local Group clearly show that dwarfs cluster around giants. Van den Bergh's data also showed that (1) the fraction of all galaxies classified as irregular increases dramatically with decreasing luminosity, and (2) the fraction of all spirals that are barred is much lower among giants than it is among dwarfs. In a subsequent study van den Bergh (1960a,b,c) was able to show that both the surface brightnesses of spiral galaxies and their morphologies are functions of luminosity. Supergiant spirals were found to have long and well-developed ‘grand design’ spiral arms, whereas low-luminosity spirals tend to have poorly developed ‘scraggily’ spiral arms.