Small-scale stellar haloes: detecting low surface brightness features in the outskirts of Milky Way dwarf satellites

TL;DR

This study develops an improved method to detect low surface brightness features in the outskirts of Milky Way dwarf satellites using Gaia data, revealing extended stellar populations and potential tidal debris.

Contribution

The paper introduces an updated algorithm for identifying dwarf galaxy members that accounts for outer components, enabling detection of extended stellar halos or streams.

Findings

Nine dwarf galaxies show evidence of extended low-density outer profiles.

Many member stars are located beyond 5 half-light radii, indicating possible stellar halos or tidal streams.

The method aligns well with spectroscopic data despite not requiring radial velocities.

Abstract

Dwarf galaxies are valuable laboratories for dynamical studies related to dark matter and galaxy evolution, yet it is currently unknown just how physically extended their stellar components are. Satellites orbiting the Galaxy's potential may undergo tidal stripping by the host, or alternatively, may themselves have accreted smaller systems whose debris populates the dwarf's own stellar halo. Evidence of these past interactions, if present, is best searched for in the outskirts of the satellite. However, foreground contamination dominates the signal at these large radial distances, making observation of stars in these regions difficult. In this work, we introduce an updated algorithm for application to Gaia data that identifies candidate member stars of dwarf galaxies, based on spatial, color-magnitude and proper motion information, and which allows for an outer component to the stellar distribution. Our method shows excellent consistency with spectroscopically confirmed members from the literature despite having no requirement for radial velocity information. We apply the algorithm to all $\sim$60 Milky Way dwarf galaxy satellites, and we find 9 dwarfs (Bo\"otes 1, Bo\"otes 3, Draco 2, Grus 2, Segue 1, Sculptor, Tucana 2, Tucana 3, and Ursa Minor) that exhibit evidence for a secondary, low-density outer profile. We identify many member stars which are located beyond 5 half-light radii (and in some cases, beyond 10). We argue these distant stars are likely tracers of dwarf stellar haloes or tidal streams, though ongoing spectroscopic follow-up will be required to determine the origin of these extended stellar populations.