ABSTRACT
A graph of surface brightness versus position along a specified line drawn on the image of a galaxy is called a luminosity profile, or simply a profile. The prime interest in the analysis of the luminosity profile of nearly face-on galaxies lies in studying the radial luminosity distribution, that is, how the luminous matter is distributed with distance from the rotation axis of a galaxy. The abscissa is the radial distance from the galactic center for the mean and the averaged profiles, and the equivalent radius for the equivalent profile. Elliptical galaxies consist only of an ellipsoidal component without any flat disk component, while in late-type spiral galaxies the disk component is dominant and the central bulge component of ellipsoidal shape, if present, is very weak. The intrinsic sizes and surface brightnesses of galaxies cover an extremely wide range. Furthermore, the apparent size of a galaxy depends upon the distance, which is a quantity very difficult to determine.
