Brightness distributions are a useful measure of the mass distribution in the visible components of galaxies. Together with kinematic measurements they provide input data for dynamical studies of galaxy structure and evolution. However, it is important to recognize their limitations. Color maps and spectrophotometry show that stellar populations and hence total mass-to-light ratios vary from place to place in a galaxy. As a result, the interpretation of surface brightnesses as mass densities becomes progressively more complicated toward later Hubble types. Furthermore, the visible galaxy is only a small fraction of the total mass: dark halos contribute significantly even over the visible part of the structure, and account for most of the mass at large radii. Only near the centers of ellipticals and bulges are we reasonably certain that the halo is gravitationally negligible; even in these objects the radius at which it becomes significant is probably small compared to a Holmberg (1958) radius. At late Hubble types the total mass of gas is also important. Thus the part of a galaxy which we study with surface photometry is only the "tip of an iceberg" of total mass. How fundamental this tip is depends on the unknown composition of the halo. e.g., if the halo consists of low-mass stars, then the fraction of material that we can study depends on relatively unfundamental limitations in our observing technique. If the halo consists of neutrinos, then the light distribution (together with the gas density) measures a more fundamental quantity, namely the total baryon density.
This section brings up to date a recent review (Kormendy 1980) of surface photometry studies of ordinary galaxies. It will complement this review as much as possible. There will be overlap on basic topics, but I will put more emphasis here on galaxy interactions and mergers, and on the cores of elliptical galaxies. I will not discuss modeling (see Binney 1982a), spiral structure, star formation or active galaxies.