The Galactic bulge

The metal-rich populations in NGC 6791 and the old disk have Solar or subsolar scaled alpha abundances, suggesting that those populations have enriched over timescales longer than 1 Gyr, so that Type Ia SNe stars had time to contribute substantial iron. Bulges form more rapidly. The Galactic bulge's formation timescale is likely ~1 Gyr or less (Ortolani et al. 1995; Zoccali et al. 2003). When disk stars in the foreground of the bulge are excluded by proper motion (Kuijken & Rich 2002) the remaining bulge population bears a strong resemblance to the main-sequence turnoff of an old globular cluster; the constraint on the numbers of stars brighter than the turnoff (even blue stragglers) is remarkable. In short, there appears to be very little room for an extended star-formation history in the bulge, arguing both from the standpoint of the main-sequence turnoff and from the chemical-enrichment perspective (see below).

In the case of the bulge, the Blancos produced an R, I color-magnitude diagram that for the first time revealed a clear red giant branch. Armed with the new pulse-counting detectors developed by Shectman at Las Campanas, Whitford and I took the first digital spectra of bulge giants in the 1980s (Whitford & Rich 1983). At the telescope, the bulge giants looked remarkable, especially their strong Na D and Mg lines, which exceeded dramatically anything from the standard stars. My original abundance scale was high (another pang in the supermetallicity controversy). Two factors likely contributed to this. First, I derived an abundance scale based on iron, but using the Mg b 5170 (Mg2) index (and not accounting for selective Mg enhancement). A second factor is more subtle: the standard stars observed in Rich (1988) and Rich (1990) were very bright and were observed behind heavy neutral-density filters. Even so, they frequently came close to or exceeded the coincidence count limits, diminishing the measured depth of the Mg index in the standard stars. Nonetheless, the Rich (1990) abundance scale was only 0.3 dex higher than the McWilliam & Rich (1994) scale based on high-resolution echelle spectra. The present-day iron-abundance scale of Fulbright et al. (2006a), derived from R = 67,000, SNR > 50 Keck echelle spectra, is very close to the original McWilliam &

Figure 1.4. Left panel: [O/Fe] versus [Fe/H] for Galactic-bulge giants (filled symbols) and the thin/thick-disk population (Fulbright et al. 2006b). Notice that the bulge [O/Fe] is only mildly elevated relative to the disk. Right panel: the trend of [Mg/Fe] versus [Fe/H] for the bulge stars (filled symbols), also from Fulbright et al. (2006b). Notice the very strong enhancement of Mg that is carried through to the highest metallicities. This is not characteristic of the SMR disk population. The enhancement in Mg is also seen in the integrated light of elliptical galaxies (Worthey et al. 1992).

Figure 1.4. Left panel: [O/Fe] versus [Fe/H] for Galactic-bulge giants (filled symbols) and the thin/thick-disk population (Fulbright et al. 2006b). Notice that the bulge [O/Fe] is only mildly elevated relative to the disk. Right panel: the trend of [Mg/Fe] versus [Fe/H] for the bulge stars (filled symbols), also from Fulbright et al. (2006b). Notice the very strong enhancement of Mg that is carried through to the highest metallicities. This is not characteristic of the SMR disk population. The enhancement in Mg is also seen in the integrated light of elliptical galaxies (Worthey et al. 1992).

Rich (1994) scale. The mean [Fe/H] is slightly subsolar, extending to [Fe/H] = +0.5 (which is also, coincidentally, the upper limit of the old disk stars). With the problem of the bulge iron-abundance scale settled (Fulbright et al. 2006a) we may turn to the determination of the alpha abundances (Fulbright etal. 2006b). However, we should emphasize that the bulge is not extremely metal-rich; rather, {[Fe/H]) is somewhat subsolar. It is in the alpha elements than one can observe striking differences in composition relative to the disk. Figure 1.4 shows a fundamental result that has been established since McWilliam & Rich (1994): that Mg remains elevated in the bulge to [Fe/H] = +0.5. It has been known for more than a decade that Mg is also elevated in massive elliptical galaxies (Worthey et al. 1992). However, oxygen, which is also believed to be formed in hydrostatic burning, follows a trend very similar to that of the disk, with only marginal elevation relative to the Solar vicinity. Because O and Mg are believed to be produced in the hydrostatic burning envelopes of massive stars, the disconnect between these two elements is not understood. Fortunately, both our results and that of Zoccali et al. (2006), derived on a different sample of bulge giants, find this trend for oxygen. It is possible that the early generations of massive stars underwent substantial mass loss via a Wolf-Rayet phase; much of the outer envelope was lost to the interstellar medium before the nucleosynthesis of substantial oxygen (McWilliam & Rich 2004).

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Figure 1.5. Left panel: [(SiCaTiI)/Fe] for the Fulbright etal. (2006b) bulge giants (filled circles) compared with the halo stars of Fulbright (2000) (plus signs) and Cayrel et al. (2004) (filled squares). Notice that the bulge stars occupy a locus that defines an upper envelope relative to the halo; the bulge locus is also tighter than that of the halo (from Fulbright et al. 2006b). Right panel: the trend of [O/Fe] versus [Fe/H] for bulge fields close to the Galactic Center, the well-studied Baade's Window field at b = -4°, and for Galactic-bulge globular clusters (Origlia & Rich 2007). The open symbols correspond to four disk giants in the Solar vicinity. See Rich & Origlia (2005) for details. The oxygen is measured from the 1.6-^m OH lines in the infrared. Notice the absence of metal-rich stars in the bulge fields; the wider abundance range toward the Galactic center continues to be a topic of investigation.

Finally, considering the run of the explosive alphas (those light elements synthesized during the explosion rather than in hydrostatic burning, [ (SiCaTil) /Fe] versus [Fe/H]), the bulge locus defines an envelope that is always more alpha-enhanced than the halo (Figure 1.5). Earlier notions that the bulge is an extension of the halo abundance distribution must be revised; even those bulge stars with [Fe/H] overlapping that of the halo define an upper envelope in the explosive alphas that is not seen in the halo. In the scheme of metal-rich populations, the bulge has clearly experienced the chemical fingerprinting of its unique enrichment process.

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