A 3D view of dwarf galaxies with Gaia and VLT/FLAMES

II. The Sextans dwarf spheroidal^★

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700AV Groningen, The Netherlands
² Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, 38206 La Laguna, Tenerife, Spain
³ Universidad de La Laguna, Avda. Astrofísico Fco. Sánchez, 38205 La Laguna, Tenerife, Spain
⁴ Leiden Observatory, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands
⁵ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁶ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
⁷ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
⁸ Institute of Physics, Laboratory of Astrophysics, Ecole Polytechnique Fédérale de Lausanne (EPFL), 1290 Sauverny, Switzerland
⁹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany

^★★ Corresponding author: etolstoy@astro.rug.nl

Received: 18 February 2025
Accepted: 3 April 2025

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 3–4 kpc), and an extremely low central surface brightness of ∑_V ∼ 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 υ_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 Ca II triplet lines, for 312 of these stars. We look again at the global characteristics of Sextans, deriving a mean line-of-sight velocity of ⟨υ_los⟩ = +227.1 km/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 nine probable members in the outer region of Sextans at higher [Fe/H] than the mean in this region. If these stars were 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, and at the relation between Sextans and the intriguingly nearby globular cluster, Pal 3. 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.