|Annu. Rev. Astron. Astrophys. 1997. 35:
Copyright © 1997 by . All rights reserved
1.3. Preconception Number 2: The Galactic Bulge Is Super-Solar Metallicity
This belief was strongly supported by study of late M-giants in Baade's Window (cf Frogel 1988), motivated by the Whitford (1978) paper that compared the spectrum of the Milky Way bulge to that of the integrated light of the central regions of external bulges and giant elliptical galaxies (see Whitford 1986 for a personal interpretation of his research). Whitford's investigation aimed to determine whether or not the bulge of our galaxy was "normal," i.e. the same as others. Whitford was apparently influenced, as were most people at that time, by the interpretation of the color-magnitude relation of Faber (1973) to assume that bulges and ellipticals were differentiated only by luminosity, which determined the metallicity, and that ages were invariant and old, with a turnoff mass of ~ 1 M (Faber 1973), at least for the dominant population. In this case, the most metal-rich stars in a lower luminosity bulge, like that of the Milky Way, could be used as a template for the typical star in a giant elliptical.
Whitford (1978) concluded from his data that indeed "the strengths of the spectral features in the sampled areas of the nuclear bulge of the Galaxy are very close to those expected from measures on similar areas of comparable galaxies." However, Whitford's data were, by current standards, of low spectral resolution and were limited to the following: spectra, with a resolution of 32 Å in the blue and 64 Å in the red, for three regions in Baade's Window and for the central regions of five edge-on spirals of type Sa to Sb; lower spectral resolution data for the central regions of M 31; partial data - blue wavelengths only - for one elliptical (NGC3379, E1); and full wavelength coverage spectra for one other elliptical (NGC4976, E4), which he emphasized did not match the Milky Way and was anomalous. Furthermore, the data for Baade's Window in the blue wavelength region - where direct comparison with a "normal" elliptical galaxy was possible - were emphasized to be very uncertain, owing to the large corrections for reddening and foreground (disk) emission. Thus, while the Whitford paper was deservedly influential in motivating comparison between stars in the Milky Way bulge and the integrated population of external galaxies, its detailed conclusions rest on rather poor foundations.
The results of Rich (1988), based on his low-resolution spectra, that the mean metallicity of K/M giants in Baade's Window was twice the solar value, was very influential and widely accepted; however, it is now apparent that line-blending and elemental abundance variations contributed to a calibration error. We discuss below the current status of the metallicity-luminosity relation for bulges and for ellipticals and the detailed chemical abundance distribution for stars in the bulge of the Milky Way. Although super-metal-rich stars clearly exist in the bulge of the Milky Way, they are a minority, and their relationship to the majority population (are they the same age?) remains unknown.