A 3D view of dwarf galaxies with Gaia and VLT/FLAMES II. The Sextans dwarf spheroidal

Abstract

The Sextans dwarf spheroidal galaxy has been challenging to study in a comprehensive way as it is highly extended on the sky, with an uncertain but large tidal radius of between 80-160 arcminutes (or 3-4kpc), and an extremely low central surface brightness of SigmaV = 26.2 mag/arcsec2. Here we present a new homogeneous survey of 41 VLT/FLAMES multi-fibre spectroscopic pointings that contain 2108 individual spectra, and combined with Gaia DR3 photometry and astrometry we present v-los measurements for 333 individual Red Giant Branch stars that are consistent with membership in the Sextans dwarf spheroidal galaxy. In addition, we provide the metallicity, [Fe/H], determined from the two strongest CaII triplet lines, for 312 of these stars. We look again at the global characteristics of Sextans, deriving a mean line-of-sight velocity of <v-los> = +227.1km/s and a mean metallicity of <[Fe/H]> = -2.37. The metallicity distribution is clearly double peaked, with the highest peak at [Fe/H]= -2.81 and another broader peak at [Fe/H]= -2.09. Thus it appears that Sextans hosts two populations and the superposition leads to a radial variation in the mean metallicity, with the more metal rich population being centrally concentrated. In addition there is an intriguing group of 9 probable members in the outer region of Sextans at higher [Fe/H] than the mean in this region. If this group could be confirmed as members they would eliminate the metallicity gradient. We also look again at the Colour-Magnitude Diagram of the resolved stellar population in Sextans. We also look again at the relation between Sextans and the intriguingly nearby globular cluster, Pal3. The global properties of Sextans have not changed significantly compared to previous studies, but they are now more precise, and the sample of known members in the outer regions is now more complete.

19 pages, accepted for publication in A\&A

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