Isochrone mapping. Both panels represent the predicted (g − r) color and μ_r surface brightness for the same old and metal-poor population of Fig. 9, observed as well with a 1 arcsec pixel scale at 15 Mpc. Each point is a different FASTAR realization with the same age, metallicity, IMF and number of stars. In the top panel, FASTAR predictions are shown for a semi-resolved population of 100 stars. Because of the sparse sampling of the IMF and thus of the different stellar evolutionary phases, the color and brightness fluctuations effectively map the underlying isochrone used to construct the model. In the bottom panel, FASTAR predictions are shown for the cases of having 100 000 stars. Measured surface brightnesses are therefore higher than in the upper panel, but exhibiting milder variations driven by red giant stars. Dashed vertical lines in the top panel illustrate the color range probed in the bottom one.