| Abstract | We have obtained deep CCD photometry of the recently discovered Sextans dwarf spheroidal (dSph) galaxy.The resulting deep color-magnitude (CM) diagram of Sextans reveals the main-sequence turnoff at V>_~_ 23.3, a blue straggler sequence, an extended but predominantly red horizontal branch (HB) at V=20.35+/-0.2, and a well-defined red giant branch. The instability strip is well populated and we predict that Sextans may contain as many as 300 RR Lyr and anomalous Cepheid variables. From an analysis of the CM diagram of the galaxy, we conclude that [Fe/H]_sextans_=-1.5+/-0.2, and (m - M)_0_, Sextans_=19.7+/-0.3 for E(B-V)=0.02+/-0.02. Based on comparisons with the theoretical isochrones of VandenBerg and Bell [ApJS, 58, 561(1985)] and the CM diagram of the Galactic globular cluster NGC 6752, we conclude that there is no significant population of stars in Sextans younger than about 12 Gyr, and possibly none younger than the age of NGC 6752, or about 16 Gyr. The central V surface brightness and integrated apparent V magnitude of Sextans are 17.2+/-0.5 mag arcmin^-2^ and 10.3+/-0.7 mag, respectively. The luminosity function (LF) of Sextans is similar to those of the intermediate metallicity clusters M5 and M13 below the level of the horizontal branch. Above the horizontal branch, the Sextans LF more closely resembles that of M5 which has a redder horizontal branch than M13. Sextans appears to be overabundant by 0.4-0.7 dex compared to the luminosity-metallicity relation defined by the other dSph galaxies of the Milky Way. We discuss a number of possible explanations for this and comment on the relationship of Sextans with some of the other systems of the outer Galactic halo. |
