Appearances can be deceiving: clear signs of accretion in the seemingly ordinary Sextans dSph

TL;DR

This study uncovers evidence of past accretion events in the Sextans dwarf spheroidal galaxy through analysis of its stellar distribution, kinematics, and metallicity, revealing complex structures and anomalies indicative of merger history.

Contribution

It provides the first clear observational signs of accretion in a very low-mass galaxy, using combined spatial, kinematic, and metallicity data.

Findings

Distinct spatial distributions for different stellar populations.

Detection of a kinematic 'ring-like' feature with unique velocity and metallicity.

Identification of a shell-like overdensity indicating past merger activity.

Abstract

We report the discovery of clear observational signs of past accretion/merger events in one of the Milky Way satellite galaxies, the Sextans dwarf spheroidal (dSph). These were uncovered in the spatial distribution, internal kinematics and metallicity properties of Sextans stars using literature CTIO/DECam photometric and Magellan/MMFS spectroscopic catalogues. We find the spatial distribution of stars to vary as a function of the colour/metallicity, being rather regular and round for the blue (metal-poor) red giant branch and main-sequence turn-off stars but much more elliptical and irregularly shaped for the red (metal-rich) ones, with a distinct "shell-like" overdensity in the northeast side. We also detect kinematic anomalies, in the form of a "ring-like" feature with a considerably larger systemic line-of-sight velocity and lower metallicity than the rest of stars; even the photometrically selected component with a regular looking spatial distribution displays complex kinematics. With a stellar mass of just $\sim5\times10^{5} M_{\odot}$, Sextans becomes the smallest galaxy presenting clear observational signs of accretion to date.