Blue Horizontal-Branch Stars in the Sagittarius Dwarf Spheroidal Galaxy

We report on the recovery of a Blue Horizontal Branch (BHB) populationbelonging to the Sagittarius Dwarf Spheroidal Galaxy (Sgr). The sequence isclearly identified in the (V, V-I) Color Magnitude Diagram (CMD) obtained forabout 500,000 stars in the region of the globular cluster M~54. The BHBmorphology is similar to the analogous sequence in M~54, but it isunambiguously associated with Sgr since {\it(i)} it is detected well outsidethe main body of the cluster, up to more than 5 tidal radii from the clustercenter and {\it(ii)} the BHB stars follow the radial distribution of the otherstellar populations of Sgr. This finding finally demonstrates that the Sgrgalaxy hosts a significant (of the order of $\sim$10%) old and metal-poorstellar population ([Fe/H]\ltsima -1.3; age \gtsima 10 Gyr), similar to that ofits oldest clusters (M~54, Ter~8). We also show that the Sgr BHB sequence foundhere is the counterpart of the analogous feature observed by Newberg et al.(2002) in the Sgr Stream, in a field more than 80$\degr$ away from the centerof the galaxy.Comment: 12 pages, 4 figures. Accepted for publication in ApJ Letter