|Annu. Rev. Astron. Astrophys. 1997. 35:
Copyright © 1997 by . All rights reserved
In the Local Group, all spiral galaxies, and probably all disk galaxies, have an old metal-poor spatially extended stellar population that we define to be a stellar halo. These seem to be the first stars formed in what would later become the galactic potential, though the possibility of later accretion of a minor fraction remains viable. The bulges of Local Group spiral galaxies are more diverse in properties, ranging from the very luminous, intermediate metallicity and very spatially extended bulge of M 31 through the intermediate luminosity, centrally concentrated bulge of the Milky Way, to no firm detection of a bulge in M 33.
In general, well-studied bulges are reasonably old, have a near-solar mean abundance, though with a very wide abundance distribution function, which is of importance, and are consistent with isotropic oblate rotator models for their kinematics, in which the basic support is provided by random motions and the flattening is consistent with additional rotational effects. Given these properties, bulges are most simply seen as the more dissipated descendents of their haloes.
However, diversity is apparent. All bulges of disk galaxies are not old, super-metal-rich, and simply small elliptical galaxies. This is not to say that such systems do not exist, but rather that bulges are heterogeneous. Higher luminosity bulges seem to have a closer affinity to ellipticals, whereas lower luminosity bulges prefer disks. But even this statement does not apply to all the properties of the stellar populations of bulges.
This diversity, together with the surprisingly limited database available concerning the photometric, structural, and kinematic properties of bulges, precludes firm conclusions. Much new and much needed data are about to become available, with the advent of 6- to 10-m class telescopes, with their exceptionally efficient spectrographs, and wide field array imaging systems on smaller telescopes. It will be interesting to see if the next review on bulges will be entitled "Disks and Ellipticals."
RFGW and GG thank the North Atlantic Treaty Organization for a collaborative grant. RFGW acknowledges the support of the NASA Astrophysics Theory Program and the Seaver Institution, and she thanks the UC Berkeley Astronomy Department and Center for Particle Astrophysics, and the Institute of Astronomy, Cambridge, for hospitality during some of the writing of this review.