|Annu. Rev. Astron. Astrophys. 1997. 35:
Copyright © 1997 by . All rights reserved
In his introduction to the report of IAU Symposium #1, Coordination of Galactic Research, held near Groningen, June 1953, Blaauw noted, "In the discussion the terms 'halo', 'nucleus' and 'disk' are used to indicate different parts of the Galaxy. These general regions are not defined more precisely. Their introduction proved very useful, and one might rather say that their more exact description is one of the problems of galactic research." This statement provides an excellent example of the limitations of terminology and of the term galactic bulge in that this component continues to lack a clear definition (nucleus? halo?) of either its structure or its relationship to the other stellar components of the Galaxy. This is compounded by the difficulty of observing bulges even once one has decided which part of the galaxy that is.
The common usage of "bulge," for example in the term bulge-to-disk ratio, allocates all "non-disk" light in any galaxy that has a "disk" into the bulge. That is, the bulge contains any light that is in excess of an inward extrapolation of a constant scale-length exponential disk. Sandage (Sandage & Bedke 1994, Carnegie Atlas of Galaxies; panel S11 and p. 45) emphasizes that "one of the three classification criteria along the spiral sequence is the size of the central amorphous bulge, compared with the size of the disk. The bulge size, seen best in nearly edge-on galaxies, decreases progressively, while the current star formation rate and the geometrical entropy of the arm pattern increases, from early Sa to Sd, Sm and Im types." This is the clearest convenient description of a bulge, namely a centrally concentrated stellar distribution with an amorphous - smooth - appearance. Note that this implicitly excludes gas, dust, and continuing recent star formation by definition, ascribing all such phenomena in the central parts of a galaxy to the central disk, not to the bulge with which it cohabits. Furthermore, for a bulge to be identified at all it must, by selection, have a central stellar surface density that is at least comparable to that of the disk, and/or it must have a (vertical) scale height that is at least not very much smaller than that of the disk. The fact that this working definition can be applied successfully to the extensive classifications in the Carnegie Atlas of Galaxies illustrates some fundamental correctness. Bulges are also clearly very much a defining component whose properties underly the Hubble sequence, and hence the reason why we care - understanding how bulges form and evolve is integral to the questions of galaxy formation and evolution.
This review considers the current widespread beliefs and preconceptions about galaxian bulges - for example, that they are old, metal-rich, and related to elliptical galaxies - in the light of modern data. Our aim is to provide an overview of interesting and topical questions and to emphasize recent and future observations that pertain to the understanding of the formation and evolutionary status of bulges. We begin by considering some common preconceptions.