Conventionally, two major components of spiral galaxies are distinguished: (1) a “spheroidal component”, which includes the central bulge and the surrounding halo, and (2) a “disc component”, which is basically flat and circular but is often warped at the edges. Other structures, such as bars or rings, may also be present, but they are probably of less fundamental significance for the processes of galaxy formation, and therefore they will not be considered here. The distinction between bulge and disc components corresponds approximately to the classical distinction between Population II, as exemplified by the globular clusters which have a nearly spherical distribution, and Population I, which is confined to the galactic plane and is relatively young and metal-rich. However, unlike the “classical” Population II objects which are old and metal-poor, the inner part of the bulge of our galaxy and of other galaxies is in fact metal-rich, and may even contain some young stars.
An averaged projected Robbins-Monro algorithm for estimating the parameters of a truncated spherical distribution
