First Detection of the Red Giant Branch Bump in the Sagittarius Dwarf Spheroidal Galaxy

We present V , I photometry of the Sagittarius (Sgr) dwarf spheroidal galaxy for a region of ~1° × 1°, centered on the globular cluster M54. This catalog is the largest database of stars (~500,000) ever obtained for this galaxy. The wide area covered allows us to measure for the first time the position of the red giant branch (RGB) bump, a feature that has been identified in most Galactic globular clusters and only recently in a few galaxies of the Local Group. The presence of a single-peaked bump in the RGB differential luminosity function confirms that there is a dominant population in Sgr (Pop A). The photometric properties of the Pop A RGB and the position of the RGB bump have been used to constrain the range of possible ages and metallicities of this population. The most likely solution lies in the range -0.6 < [M/H] ≤ -0.4 and 4 Gyr ≤ age ≤ 8 Gyr.