Metallicity as a function of horizontal branch morphology (HB index) for all 51 clusters of our sample, where HB index is from Mackey & van den Bergh (2005). Three isochrones from Rey et al. (2001) are overplotted, where t0 is the mean age of inner halo clusters as defined by Rey et al. as RGC< 8 kpc. Ages from VandenBerg et al. (2013) are available only for 17 out of 51 clusters in our sample, and are shown as red, purple, and blue circles for young, intermediate-age, and old clusters. The hatched triangle shows the region of the Oosterhoff gap as defined by Catelan (2009).
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