Metallicity distributions

It will be a long time before it is possible to get spectra of individual stars in our closest spiral neighbor, M31. Jablonka et al. (2000), using the MCS deconvolution technique, counted ~ 40 bright RGB stars per arcsec2 1-1.5 kpc from the galaxy's center. Therefore, attempts to derive a metallicity distribution function (MDF) in the bulge of M31 have to be based on high-spatial-resolution images and analyzed with isochrones. Thus far, there have been two studies addressing this issue, one in the optical (Sarajedini & Jablonka 2005) and the other in the infrared (Olsen et al. 2006), both using HST data. Sarajedini & Jablonka analyzed a field located about 1.5 kpc from the nucleus of M31 in V and I bands. The MDF that they derived is presented in Figure 27.2 and compared with the metallicity distribution of the bulge of the Milky Way from Zoccali et al. (2003). Within 0.1-0.2 dex, the range of metallicity covered by the two galaxies' bulges is the same, and so are the peaks of the distributions. Had bulges straightforwardly reflected the differences between the halos of the two galaxies, one would have expected a differential shift of at least 1 dex between their MDFs (Ryan & Norris 1991; Durrell Harris & Pritchet 2001). On the contrary, it seems that the bulge of M31 does not know about the

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Figure 27.2. A comparison between the metallicity distribution of the bulge of M31 (filled points and dashed line) and that of the Milky Way (plain line), in regions at comparable distances from the galaxy centers.

Figure 27.2. A comparison between the metallicity distribution of the bulge of M31 (filled points and dashed line) and that of the Milky Way (plain line), in regions at comparable distances from the galaxy centers.

metal-richness of its halo. Besides, the M31 bulge MDF exhibits a total absence of metal-poor stars, just like the Milky Way bulge does. This is a secure result, since the lowest metallicities identified (—1.5 dex) are well away from the gray zone of the very low metallicities where isochrones are degenerated in colors.

In contrastto Sarajedini and Jablonka, Olsen etal. (2006) left the age of the stellar population as a free parameter in their analysis. Also, instead of trying to reproduce the complete color-magnitude diagrams, they fit model stellar populations to the K luminosity functions of their fields, using a maximum-likelihood method. They found the stellar population mix of their 12 fields to be dominated by old (defined as having ages >6Gyr), nearly Solar-metallicity stars. This old population seems to dominate the star-formation history at all radii, irrespective of the relative contributions of bulge and disk stars. In their Figure 19, Olsen et al. show the population integrated over all their fields. Neglecting the possibly spurious intermediate-age metal-poor component, which possibly arose due to crowding, they measured an MDF that is a bit more sharply peaked than that of Sarajedini and Jablonka, but still in excellent qualitative agreement.

Assembling these observational facts suggests that the first stars in bulges formed from an already pre-enriched gas. It remains unknown whether this resulted from the first stellar generations in the halo or is due to the location of the observed fields. Indeed, as we will see later, bulges do exhibit radial gradients in metallicity and one might not yet have probed the outermost regions of the M31 and Milky Way bulges. In any case, the bulk of the bulge formation must have taken place before the mergers whose traces are witnessed today (Ferguson et al. 2002; Ibata et al. 1994; Yanny et al. 2003) could influence the bulges' evolution. Otherwise, the large difference between the M31 and Milky Way halos should have been reflected in the properties of their bulges.

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