Color–magnitude distribution (I vs. (V − I)) for the measured stars in the outer radial range (115′′−155′′) of our ACS field. The superimposed solid lines are fiducial 12-Gyr RGB tracks from the VandenBerg et al. 2000 model library, in steps of roughly 0.1 dex from log (Z / Z_⊙) = −2.0 to −0.1. These are supplemented with empirical tracks for two high-metallicity values at log (Z / Z_⊙) = +0.16 and +0.4 (dashed lines; see Harris et al. 2002).