Butcher & Oemler (1978) discovered that distant rich clusters of galaxies contain more blue spiral galaxies than do similarly rich nearby clusters. This observation provided the first direct evidence for the evolution of galaxy morphology, i.e. that distant galaxies are (from an evolutionary point of view) younger than nearby galaxies. The limited resolution of ground-based telescopes made it difficult to follow this discovery up with more detailed studies of the evolution of galactic structure and morphology with increasing look-back time.

Galaxies viewed at large look-back times

A dramatic improvement of our ability to study distant galaxies is now provided by the HST. Abraham et al. (1996b) find that galaxies in the Medium Deep Survey (Driver, Windhorst & Griffiths 1995), which are typically located at z∼0.5, are basically still quite similar to those in the vicinity of the Milky Way, although the fraction of interacting galaxies (and objects that do not fit naturally within the Hubble classification scheme) is enhanced. Our deepest view into the past is provided by the HST observations in the Hubble Deep Field (Williams et al. 1996). These data are based on observations extending over 150 orbits in four colors of a field in Ursa Major. They provide images of 290 objects with 21<I<25. Abraham et al. (1996a) find that the fraction of asymmetrical and distorted galaxies is larger in the Hubble Deep Field than it is in the Medium Deep Survey.