|Annu. Rev. Astron. Astrophys. 1997. 35:
Copyright © 1997 by . All rights reserved
1.2. Preconception Number 1: Bulges Are Old
The expectation of "old age" arose, as far as we can ascertain, from the interpretation of the observed correlation between stellar kinematics and metallicity for local stars in the Milky Way by Eggen et al (1962). These authors proposed a model of Galaxy formation by collapse of a galaxy-sized density perturbation, generalized to models wherein the spheroidal components of galaxies - including the entire stellar mass of an elliptical galaxy - formed stars prior to the dissipational settling to a disk and so contained the oldest stars (e.g. review of Gott 1977). The high central surface brightnesses of bulges (and of ellipticals), assuming they correspond to high mass densities, also imply a higher redshift of formation, for a fixed collapse factor of the protogalaxy, because at higher redshift the background density was higher (Peebles 1989).
An older component in the central regions of the Milky Way Galaxy clearly exists. The first real work on the bulge (or "nucleus" as it was called at the time) used classical "halo" tracers, such as globular clusters, RR Lyrae, and planetary nebulae. Of course, one must remember that "older" is used here in the sense that the term was used until very recently, which meant much older than the local disk, which contains ongoing star formation. That is, "old" means "there is no obvious AF star population." The Baade-era concept of "old" meant a turnoff in the F-region, which is of course old only for a very metal-poor system (see Sandage 1986, and the Carnegie Atlas for thorough reviews of Baade's Population concept). Furthermore, the very idea of discriminating between ages of 10 Gyr and 15 Gyr is a recent concept, in spite of the large fractional difference between the two.
Constraints on the redshift of formation of bulges can be obtained by direct observations of high-redshift galaxies, for which morphological information may be obtained with the Hubble Space Telescope (HST) (see Section 4). In general, disentangling the effects of age and metallicity on stellar colors is difficult, even when the stars are resolved and color-magnitude diagrams may be examined. The state-of-the-art mean age determinations for lower redshift bulges and disks are discussed in Section 3, and the interpretations of color-magnitude diagrams are discussed in Section 2. Much ambiguity and uncertainty remains.
Implicit in the Eggen et al (1962) scenario was the hypothesis that the Galactic bulge was simply the central region of the stellar halo, traced at the solar neighborhood by the high-velocity subdwarfs. These stars are old by anyone's definition. Stellar haloes can be studied easily only in the Local Group, and we discuss the stellar populations in those galaxies in Section 2 below.