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

I. Observed properties

¹ Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, 38206 La Laguna, Santa Cruz de Tenerife, Spain
² Universidad de La Laguna, Avda. Astrofísico Francisco Sánchez, 38205 La Laguna, Santa Cruz de Tenerife, Spain
³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 93/3, 40129 Bologna, Italy
⁴ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700AV Groningen, The Netherlands
⁴ Dipartimento di Fisica e Astronomia “Augusto Righi” – DIFA, Alma Mater Studiorum – Università di Bologna, via Gobetti 93/2, 40129 Bologna, Italy

^★ Corresponding author; jmarroyo@iac.es (IAC)

Received: 13 June 2024
Accepted: 8 November 2024

Abstract

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_−1.5^+1.5 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.