Annu. Rev. Astron. Astrophys. 1988. 26: 509-560 Copyright © 1988 by Annual Reviews. All rights reserved

4.3. The Question of Universality

A considerable part of our discussion of galaxian LFs has been focused on the question of whether a universal shape for the LF exists over all magnitudes, independent of Hubble type and environment. Because most cluster studies have not had sufficient type information, the question, until now, has been reduced to comparing LFs over all types in clusters with those in the field. And because the LFs have been mostly modeled by a Schechter function, the problem was further reduced to whether there is a fixed set of parameters and M^* in all environments. Although Oemler (1974), Dressler (1978), and others have shown that there is not a unique cluster LF, the evidence was undervalued because the mean cluster LF seemed so similar to that of the field. From this it was argued that any real differences due to evolution had to be much larger than observed. Until recently the question of universality was therefore emphatically answered in the positive (Felten 1985).

It is now possible to investigate the LF with sufficient morphological type resolution to show that there cannot be a universal LF because every type has a specific LF, varying in form from nearly Gaussian to exponential. Hence, the summed LF must depend on the type mixture and consequently on the environment. But once this Pandora's box is opened, the question then becomes, Is the LF of a given type the same in clusters and in the field? This question is addressed in Section 6.