A Note on the Theoretical Isochrones Adopted

As discussed in Section 4.4.1, it has become standard in the literature to assume that not only magnesium, but all other $\alpha$-elements, including oxygen, are equally enhanced in massive early-type galaxies. However, there is mounting evidence that this might not be the case. For instance, calcium seems at least not to be as enhanced as magnesium (e.g., Vazdekis et al. 1997, Worthey 1998, Trager et al. 1998, Henry & Worthey 1999, Thomas et al. 2003, Prochaska et al. 2005, Section 6.2.4). Likewise, oxygen has been found not to track magnesium in Galactic bulge metal-rich stars (Fulbright et al. 2005, Cunha & Smith 2006). To our knowledge, there has been no compelling determination of oxygen abundances in the stellar populations of early-type galaxies to this date, so that it is fair to say that the abundance of that element in early-type galaxies is unknown. This is unfortunate because, as discussed in Section 4.3.1, of all $\alpha$ elements, oxygen is the most relevant for the interior structure and evolution of stars. In view of this uncertainty and the problems with the $\alpha$-enhanced theoretical isochrones adopted in this paper (Section 4.3.1), we feel justified in adopting solar-scaled theoretical isochrones in Figure 27 and in the remaining of this paper. We refer the reader interested in assessing the effect on our final results of adopting $\alpha$-enhanced isochrones to the discussion in Section 4.3.1, where it was shown that this effect is minor.