Chemo-dynamics of the stellar component of the Sculptor dwarf galaxy I: observed properties

TL;DR

This study re-examines the chemo-kinematic properties of the Sculptor dwarf galaxy using high-precision spectroscopic data, revealing multiple stellar populations with distinct metallicities and kinematics, and suggesting a possible minor merger origin.

Contribution

It provides the first detailed analysis supporting multiple stellar populations in Sculptor, including a potential third, metal-poor, extended component, based on comprehensive spectroscopic data.

Findings

Sculptor is better modeled by multiple stellar populations than a single metallicity gradient.

Evidence suggests a third, metal-poor, extended population with distinct velocity properties.

A possible minor merger scenario is proposed for the origin of the metal-poor population.

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 data-set available containing both line-of-sight velocities and metallicities of individual stars. Methods. We carry out a detailed, quantitative analysis on the recent spectroscopic data-set from Tolstoy et al. (2023) that contains high precision velocities and metallicities for 1339 members of Sculptor. In particular, we assess 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 different mean metallicity, spatial distribution and kinematic properties. For this analysis, we also include the incompleteness of the spectroscopic data-set. Results. We find that Sculptor is better described by a two-populations 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 around 15 km/s in its average l.o.s. velocity (vlos) 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 vlos 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.