For the most galaxies in the Local Group it is possible to take spectra of large samples of individual RGB stars at intermediate resolution. This allows the observation of well calibrated, simple to use metallicity indicators, such as the Ca II triplet (e.g., Starkenburg et al. 2010, Battaglia et al. 2008b). These measurements allow a detailed measurement of the metallicity distribution function from many hundreds and sometimes even thousands of individual stars. The kinematic properties of galaxies, can also be disentangled with these spectra (e.g., Battaglia et al. 2008a), as well as any connection between distinct kinematic components and metallicity. This leads to accurate mass modelling of individual galaxies and also to the discovery of distinct kinematic components, even in small dwarf galaxies, and sometimes also rotation (e.g., Lewis et al. 2007, Fraternali et al. 2009).
In the most nearby systems (ie., mostly dwarf galaxies, but also the Magellanic Clouds) it is possible to take high resolution spectra of individual RGB stars. This allows us to measure detailed abundances of numerous chemical elements. The most commonly observed are alpha elements (e.g., O, Ca, Mg, Ti), but also heavy elements, such as r-process elements (e.g., Eu), Iron-peak elements (e.g., Mn, Cr, Fe, Ni) and also s-process elements (e.g., Ba). The abundances of these elements in RGB stars allow us to probe their levels over the entire star formation history that occurred > 1 Gyr ago. This allows us to follow which enrichment processes dominate at different epochs in the galaxy, and thus their time scale, and how they effect and are effected by the presence or absence of other elements.
The most important elements for tracing the effect of AGB stars and their pollution of the ISM out of which subsequent generations of stars are made are s-process elements. Fig. 5 shows the detailed abundances of Barium compared to Iron, [Ba/Fe], based on high resolution spectroscopic observations of individual RGB stars in the Sculptor dSph, the Fornax dSph and the Large Magellanic Cloud, compared to RGB stars in the Galactic disk and halo. Barium is of particular interest because at these [Fe/H] values it is produced almost entirely by the s-process. This also makes it a good indicator of how many potential s-process sources there have been and when they were most productive. Fig. 5 shows that both the LMC and Fornax have significantly enhanced [Ba/Fe] compared the Galaxy at [Fe/H] > -1. It seems that this enhancement only starts at [Fe/H] ~ -1. Sculptor thus does not show the same effect, presumably because it never reached a high enough metallicity before all star formation stopped. It might also be because Sculptor stopped forming stars before the feedback of s-process elements from AGB stars became important to the chemical enrichment.
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Figure 5. Here are the high resolution abundances of individual red giant branch stars in Sculptor dSph (green solid circles, Hill et al. (2011) in prep, FLAMES high resolution; open circles, Shetrone et al. 2003, UVES); Fornax dSph (blue solid circles, Letarte et al. 2010, FLAMES high resolution; open circles, Shetrone et al. 2003, UVES); Large Magellanic Cloud (red circles, Pompéia et al. 2008, FLAMES high resolution). The small black squares are Galactic observations (from compilation, Venn et al. 2004). |
In Fig. 6 we consider the evolution of [Fe/H] in the same galaxies shown in Figure 4, i.e., Sculptor dSph, Fornax dSph and the Large Magellanic Cloud. In Fig. 4 we show colour-colour diagrams coming from 2MASS data for each of the galaxies. The physical region sampled is the same for Sculptor & Fornax (1 deg; which is about the distance to the tidal radius). This region is a smaller fraction of the whole galaxy for the LMC. Clearly the LMC is a larger, more luminous (with a higher peak star formation rate) galaxy than the other two, and the LMC also contains many more AGB stars.
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Figure 6. For the same galaxies shown in Fig. 5 the metallicity-age relations are shown, as are the colour-colour diagrams which clearly show the numbers of AGB (C-stars) present. The age-metallicity relations come from de Boer et al., in prep for Sculptor; Battaglia et al. (2006) for Fornax and Pagel & Tautvaisiene (1998), Hill et al. (2000) for LMC. The Infra-red data are all selected from 2MASS (only those stars with AAA quality flags), in a region that corresponds to the tidal radius of Scl and Fnx, and within the central 1 degree of the LMC. |
The variation in the number of AGB stars seen in these nearby galaxies may be due to the different masses, sizes and/or luminosities of the systems, but there is also likely to be a significant effect due to metallicity. It can be seen that the galaxy that never forms stars with [Fe/H] > -1 (e.g., Sculptor, see Fig. 6) also appears to contain no AGB (C-stars) and no sign of enrichment by these stars during its star formation history (e.g., Fig. 5). Of course Sculptor also stopped forming stars around 6 Gyrs ago, and for several Gyr before this it formed stars at a very low rate (see de Boer et al., in prep), thus it might be a case of low number statistics. But Leo A is a galaxy with a similar luminosity to Sculptor, and a current metallicity (from H II region spectroscopy) which is similar to the average metallicity found in Sculptor. Leo A also formed most of its stars over the last 5 Gyrs and yet there are very few, if any, AGB in Leo A (see Fig. 4).