Supermetallicity in stellar populations

The major paradigm in the chemical evolution of stellar populations is that the chemical enrichment due to massive-star SNe relative to Type Ia SNe reflects the rate of star formation; the emergence of Solar abundance ratios is a reflection of the point at which the Type Ia SNe begin to produce substantial iron (Wheeler et al. 1989; McWilliam 1997).

Metal-rich populations have turned up in unexpected locations, not only in the Galactic bulge or elliptical galaxies. In Baade's original population model, Population II was thought to be older and more globular-cluster-like, and therefore more metal-poor. These ideas persisted well into the 1950s, aided by the

Figure 1.2. Color-magnitude diagrams of the field population in the halo of M31, derived from the field populations of the globular clusters indicated, obtained using WFPC2 on the HST (Bellazini et al. 2003). The blue HB ridgeline is that for M68 while the RGB ridges are for G11 ([Fe/H = -2) and G58 ([Fe/H] = -0.6). Notice the large numbers of fainter, redder stars that must clearly be more metal-rich. These populations may reflect either an extension of the spheroid or the debris of merger events.

Figure 1.2. Color-magnitude diagrams of the field population in the halo of M31, derived from the field populations of the globular clusters indicated, obtained using WFPC2 on the HST (Bellazini et al. 2003). The blue HB ridgeline is that for M68 while the RGB ridges are for G11 ([Fe/H = -2) and G58 ([Fe/H] = -0.6). Notice the large numbers of fainter, redder stars that must clearly be more metal-rich. These populations may reflect either an extension of the spheroid or the debris of merger events.

difficulties of actually making quantitative measurements in the Galactic bulge. At present, metal-rich stars are found in the old (10-11 Gyr) disk (Castro et al. 1997; Pompeia et al. 2002, 2003) and in NGC 6791, as mentioned earlier. In fact, the metallicities of these populations reach extremes as high as those found in the Galactic bulge. One important difference, however, is that these disk populations generally have scaled Solar abundances for the light elements, in contrast with the bulge, for which the levels of light elements (especially Mg) remain enhanced to

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Figure 1.3. Correlation between the photometric metallicity (from the red giant-branch color) of halo populations roughly 10 kpc distant from the plane, and luminosity, for spiral and S0 galaxies (Mouhcine et al. 2005a). At this time, it is not clear whether we are observing a trend, or a bimodal distribution, where the presence of metal-rich stars arises from the extension of massive bulges. In some of the most luminous galaxies, it appears that the halo is indeed an outer extension of the spheroid. However, the distant-field population in M31 is polluted by material from merger remnants; this process likely also populates the halo with metal-rich stars.

metallicities even above the Solar value. The galaxy NGC 6791 has, in fact, subsolar abundances.

There is now evidence for resolved metal-rich populations in the halo and disk of M31. Bellazzini et al. (2003) find descending metal-rich giant branches in the M31 halo (Figure 1.2), and numerous studies since the pioneering work of Mould & Kristian (1986) find a metal-rich halo extended to 30 kpc (e.g. Durrell et al. 2004). Brown et al. (2003,2006) have undertaken deep HST/ACS imaging of M31 halo fields and find evidence for suprasolar metallicities both in the disk (32 kpc distant from the nucleus) and in the spheroid (12 kpc from the nucleus). In the case of the M31 fields, the case for the super-metal-rich populations is based on the modeling of the main-sequence turnoff. Keck spectroscopy of the stars in these populations (Koch et al., work in preparation) now in progress will further test whether super-metal-rich stars are present in these low-density environments.

The galaxy M31 is not alone in possessing an extended population of metal-rich stars. Mouhcine et al. (2005a) find a trend of halo metallicity with parent-galaxy luminosity (Figure 1.3), with the field populations spanning 1.5 dex in metallicity. However, M31 does remain as having among the most metal-rich halos in our sample of nine galaxies. Is the complex interaction history of M31 an anomaly, or does it suggest a mechanism for populating the stellar content of the halos of massive galaxies? Since it is difficult to form metal-rich stars in the very-low-density environments of halos, one might instead suspect that these stars form in dense star clusters or in bulges and inner disks. It is more likely that the presence of these metal-rich populations at great distances is the result of their ejection via one or more significant mergers; it will be interesting to model this mechanism in detail.

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