Chemo-dynamics of the stellar component of the Sculptor dwarf galaxy

Aims. Recently, both the presence of multiple stellar chemo-kinematic components and rotation in the Sculptor dwarf spheroidal galaxy have been put into question. Therefore, we re-examine the chemo-kinematic properties of this galaxy, making use of the best spectroscopic dataset available containing both the line-of-sight velocities and metallicities of individual stars. Methods. We carried out a detailed, quantitative analysis on a recent spectroscopic dataset from the literature that contains high precision velocities and metallicities for 1339 members of Sculptor. In particular, we assessed whether Sculptor is best represented by a single stellar population with a negative metallicity gradient or by the super-position of two or more components with a different mean metallicity, spatial distribution, and kinematic properties. For this analysis, we also include the incompleteness of the spectroscopic dataset. Results. We find that Sculptor is better described by a two-population model than by a single-population model with a metallicity gradient. Moreover, given the assumptions of the current modeling, we find evidence of a third population, composed of few stars, that is more extended and metal-poor than the two other populations. This very metal-poor group of stars shows a shift of ∼15 km s⁻¹ in its average line-of-sight velocity (v_los) with respect to the rest of the galaxy. We discuss several possible origins for this new population, finding a minor merger as the most likely one. We also find a v_los gradient of 4.0⁺₋¹_1.⁵₅ km s⁻¹ deg⁻¹ but its statistical evidence is inconclusive and, moreover, its detection is partially driven by the group of stars with off-set velocities.