SPECTROSCOPY OF GIANTS IN THE SEXTANS DWARF SPHEROIDAL GALAXY

We have obtained spectra in the region of the Ca II infrared triplet at 8500 A for 80 stars in the direction of the Sextans dwarf spheroidal galaxy. We have found 43 radial velocity members of the Sextans galaxy and 36 field stars. The mean velocity of the Sextans galaxy is 227.9 +/- 1.8 km s-1, in agreement with the previously determined value of 230 +/- 6 km s-1 from Da Costa et al. (1991). We find that the intrinsic velocity dispersion of Sextans is 6.2 +/- 0.9 km s-1 based on 33 stars. The two standard methods for estimating the total mass-to-light ratio yield (M/L)tot = 54(-24)+44 and 30(-13)+20, implying significant amounts of dark matter are present, although we cannot rule out binary contamination as the cause of the large observed velocity dispersion with only a single epoch of observations. The mean metallicity of Sextans is <[Fe/H]> = -2.05 +/- 0.04, with an intrinsic star-to-star metallicity dispersion of 0.19 +/- 0.02 dex in [Fe/H]. The abundance measurements based on the Ca II line strengths are found to be correlated with the color of the star, supporting the existence of a real metallicity dispersion. The mean metallicity is in excellent agreement with the general trend of metallicity-absolute galaxian magnitude for the dwarf spheroidal galaxies that are satellites to the Milky Way galaxy. The horizontal branch of Sextans is quite red for the derived mean metallicity implying that Sextans, like Draco, is a ''second parameter'' object that may be a few Gyr younger than the typical Galactic globular cluster.