The spheroidal components of galaxies - elliptical and lenticular (S0) galaxies and the bulges of spirals - contain at least half of the present-day stellar mass in the local Universe (Schechter & Dressler 1987; Fukugita et al. 1998). An understanding of the formation and evolution of these objects is therefore necessary in order to understand the dominant galaxy types in the local Universe.
The abundances of stars in these objects give us direct handles on their formation and evolution. The gross metallicity of a galaxy tells us about its overall chemical evolution. Abundances of specific elements are much more useful, since these give direct evidence regarding the nucleosynthetic processes that occurred during the formation of that star (and, by extension, of the stellar population and even galaxy).
The Metal-rich Universe, eds. G. Israelian and G. Meynet. Published by Cambridge University Press. © Cambridge University Press 2008.
As an example, the ratio [a/Fe] is crudely an indicator of the ratio of SNe II, resulting from the explosion of massive stars and producing the a-elements, to SNe Ia, resulting from the explosion of low- to intermediate-mass stars and producing most of the Fe-peak elements. This leads to [a/Fe] being commonly used as a tracer of the timescale of star formation, since it will decrease with increasing duration of star formation as SNe Ia become more important in the nucleosynthesis in a stellar population (e.g. Worthey et al. 1992; Greggio 1997; Trager et al. 2000b; Thomas et al. 2005). This is an oversimplification, of course, as can be seen in Chapters 28 and 44 in this volume.
Unfortunately, the Universe has made measuring stellar abundances in these objects difficult for us. Spheroids of galaxies are very dense and the crowding makes it impossible to make accurate photometric measurements of individual stars closer than one effective radius away from their centres in all spheroidal objects but our own Galactic bulge (and in that case dust presents a formidable barrier), even with the Hubble Space Telescope (HST). Attempting to determine abundances of individual stars from high-resolution spectra, as we do in the Milky Way, is accordingly impossible beyond the Local Group with the current generation of 8-10-m ground-based telescopes and the 2.5-m HST. The best we can hope for is to determine the colour-magnitude diagrams (CMDs) of the outer regions of these galaxies and infer their abundances from the distribution of stars on the giant branches, attempt to determine abundances from planetary nebulae - which stand out from the crowd as emission-line objects - and finally infer abundances from the integrated light of the galaxies. Each of these methods will be discussed below.
In the following, I refer to as 'direct' methods those techniques that determine abundances from resolved stellar populations. Colour-magnitude diagrams fall into this category, even though abundances of individual stars may be poorly determined, as do abundances from analyses of planetary-nebula spectra. I refer to as 'indirect' methods those techniques that determine abundances from unresolved stellar populations, in particular analysis of absorption-line strengths. I discuss direct abundance measurements in Section 2 and indirect abundance measurements in Section 3. I summarise the results in Section 4.
To conclude this section, I mention that there are other methods for determining the abundances of early-type galaxies, including the analysis of their globular-cluster systems and determinations of the abundance of X-ray-emitting gas. However, for reasons of space and to do with complications in relating these determinations to the stellar population of the host galaxy, I will not discuss them here. For the former, the interested reader can refer to Trager (2004) and Brodie & Strader (2006); for the latter, see e.g. Humphrey & Buote (2006). Finally, Jablonka has covered the subject of bulges of spiral galaxies in Chapter 27 in this volume.
Figure 16.1. The giant branches of old (12-Gyr) stellar populations from the BaSTI (Pietrinferni et al. 2004, 2006) isochrone collection, in the optical (left) and near-infrared (right). These isochrones include the horizontal-branch and AGB phases. Metallicities, from left to right in both panels, are [Fe/H] = -2.27, -1.49, -0.96, -0.35, 0.06 and 0.40.
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