THE ACS LCID PROJECT. IV. DETECTION OF THE RED GIANT BRANCH BUMP IN ISOLATED GALAXIES OF THE LOCAL GROUP

We report the detection and analysis of the red giant branch (RGB) luminosity function bump in a sample of isolated dwarf galaxies in the Local Group. We have designed a new analysis approach comparing the observed color-magnitude diagrams (CMDs) with theoretical best-fit CMDs derived from precise estimates of the star formation histories of each galaxy. This analysis is based on studying the difference between the V magnitude of the RGB bump and the horizontal branch at the level of the RR Lyrae instability strip (Delta V-HB(bump)) and we discuss here a technique for reliably measuring this quantity in complex stellar systems. By using this approach, we find that the difference between the observed and predicted values of Delta V-HB(bump) is +0.13 +/- 0.14 mag. This is smaller, by about a factor of 2, than the well-known discrepancy between theory and observation at low metallicity commonly derived for Galactic globular clusters (GCs). This result is confirmed by a comparison between the adopted theoretical framework and empirical estimates of the V-HB(bump) parameter for both a large database of Galactic GCs and for four other dwarf spheroidal galaxies for which this estimate is available in the literature. We also investigate the strength of the RGB bump feature (R-bump), and find very good agreement between the observed and theoretically predicted R-bump values. This agreement supports the reliability of the evolutionary lifetimes predicted by theoretical models of the evolution of low-mass stars.