Introduction

Galaxies are complex, many-body systems. Typically, a galaxy can consist of hundreds of millions or billions of stars, it can contain considerable quantities of interstellar gas and dust and can be subject to environmental influences through interactions with other galaxies and with the intergalactic gas. Star formation takes place in dense regions of the interstellar gas. To complicate matters further, it is certain that dark matter is present in galaxies and in clusters of galaxies and that its mass is considerably greater than the mass in baryonic matter. Consequently, the dynamics of galaxies are dominated by this invisible dark component, the nature of which is unknown.

Traditionally, galaxies have been classified by meticulous morphological studies of samples of bright galaxies. These morphological classification schemes had to encompass a vast amount of detail and this was reflected in Hubble's pioneering studies, as elaborated by de Vaucouleurs, Kormendy, Sandage, van den Bergh and others. The Hubble sequence of galaxies has real astrophysical significance because a number of physical properties are correlated with Hubble type. While the detailed study of individual galaxies was feasible for reasonably large samples, a different approach had to be adopted for massive surveys of galaxies such as the Anglo-Australian 2dF survey (AAT 2dF) and the Sloan Digital Sky Survey (SDSS) which have provided enormous quantitative databases for the studies of galaxies.