Blue horizontal-branch stars in the Sagittarius dwarf spheroidal galaxy

We report on the discovery of a blue horizontal-branch (BHB) population belonging to the Sagittarius dwarf spheroidal galaxy. The sequence is clearly identified in the (V, V-I) color-magnitude diagram obtained for about 500,000 stars in the region of the globular cluster M54. The BHB morphology is similar to the analogous sequence in M54, but it is unambiguously associated with Sgr since (1) it is detected well outside the main body of the cluster, up to more than five tidal radii from the cluster center and (2) the BHB stars follow the radial distribution of the other stellar populations of Sgr. This finding finally demonstrates that the Sgr galaxy hosts a significant (of the order of ∼10%) old and metal-poor stellar population ([Fe/H] ≲-1.3; age ≳ 10 Gyr), similar to that of its oldest clusters (M54, Ter 8). We also show that the Sgr BHB sequence found here is the counterpart of the analogous feature observed by Newberg et al. in the Sgr stream, in a field more than 80° away from the center of the galaxy.