|Annu. Rev. Astron. Astrophys. 1997. 35:
Copyright © 1997 by . All rights reserved
4.3. The Bulge
Measurement of the metallicity of Galactic bulge stars has been somewhat controversial in the last 15 years. Early bulge metallicity studies focused on stars in Baade's window, at Galactic latitude -4°. Initial low-resolution studies of 21 bulge giants by Whitford & Rich (1983) suggested that most of the bulge stars are super metal-rich.
Frogel & Whitford (1987) amassed photometric and spectral-type data for a large number of bulge giants. They found that bulge M giants have stronger TiO and CO bands than solar neighborhood M giants consistent with a metal-rich bulge.
Rich (1988) measured low-resolution indices of strong lines (Mg b and Fraunhofer Fe I lines) in 88 bulge giants in Baade's window and several bright standards. Calibration of the indices suggested a range of [Fe/H], from -1.0 to +0.8 dex for the bulge, with a mean value twice the solar value.
Terndrup et al (1991) found a mean bulge metallicity of +0.3 dex for M giant stars in Baade's Window, based on R = 1000 spectrophotometry, which confirmed earlier results.
Geisler & Friel (1992) used Washington photometry to measure the metallicity of 314 red giants in the Galactic bulge, through Baade's window. They found the mean [Fe/H] = +0.17 ± 0.15 dex, in good agreement with (Rich 1988). They also found a high frequency of metal-poor stars, consistent with that expected from a simple closed box model, as found by Rich (1990).
Rich (1990) showed that the Galactic bulge contains a higher frequency of metal-poor stars than the solar neighborhood. In fact, the bulge metallicity function does not exhibit the G-dwarf problem. This is perhaps somewhat surprising because the bulge must be the final repository of infalling material (for example, the Sagittarius dwarf found by Ibata et al 1994). It may be that most of the infall occurred very rapidly, or that material that fell into the bulge, such as a dwarf galaxy, was stripped of its gas before reaching the bulge.
With the apparent convergence of different methods used to measure the bulge metallicity, it was a surprise that the first high-dispersion model atmosphere abundance analysis of bulge stars (McWilliam & Rich 1994) found that the bulge is slightly iron-poor relative to the solar neighborhood. McWilliam & Rich (1994) computed [Fe/H] for 11 bulge red giants, covering the full metallicity range, which had previously been measured by Rich (1988). A correlation of [Fe/H] values of McWilliam & Rich (1994) with those of Rich (1988) showed that Rich (1988) systematically overestimated the [Fe/H] of the most metal-rich stars. A regression relation between McWilliam & Rich's (1994), Rich's (1988) [Fe/H] results was used to compute corrected [Fe/H] Rich (1988) for the full sample of 88 stars. Rich's (1988) [Fe/H] values corrected in this way have a mean of -0.25 dex, slightly below the mean value of -0.17 dex for solar neighborhood red giants (McWilliam 1990). The corrected bulge metallicity function still shows the excess of metal-poor stars relative to the solar neighborhood noted by Rich (1990). McWilliam & Rich (1994) also found unusually high [Mg/Fe] and [Ti/Fe] ratios in the bulge stars, which might explain why previous investigators found high average metallicities.
Subsequent model atmosphere abundance analyses of two stars in the McWilliam & Rich (1994) sample (Castro et al 1996; A McWilliam, RC Peterson, DM Terndrup & RM Rich, in preparation) confirmed the McWilliam & Rich (1994) [Fe/H] results. The low [Fe/H] of bulge stars in Baade's window found by McWilliam & Rich (1994) was supported by later low-resolution studies; for example, the analysis of low-resolution spectra of 400 bulge giants by Terndrup et al (1995), Sadler et al (1996) found a low mean [Fe/H]~ -0.1 dex.