Constraining the shape of Milky Way satellites with distance gradients

TL;DR

This study uses combined photometry data to analyze the 3-D structure of several Milky Way dwarf galaxies, revealing significant distance gradients in Sextans and Sculptor that suggest oblate or triaxial shapes with high ellipticity.

Contribution

It introduces a new method for measuring distance gradients within dwarf galaxies using BHB stars and models their 3-D shapes based on these gradients.

Findings

Significant distance gradients detected only in Sextans and Sculptor.

Gradients are incompatible with prolate shapes, suggesting oblate or triaxial forms.

Oblate models require high intrinsic ellipticity (>0.46) to explain observations.

Abstract

We combine the Dark Energy Camera Legacy Survey (DECaLS) DR8 photometry with Gaia photometry to study the 3-D structure of Bootes I, Draco, Ursa Minor, Sextans and Sculptor dwarf galaxies using blue horizontal branch (BHB) stars as distance indicators. We construct a new colour-absolute magnitude of BHB stars that we use to measure the distance gradients within the body of the dwarf galaxies. We detect a statistically significant non-zero gradient only in Sextans and Sculptor. Through modeling of the gradient and 2-D density of the systems by triaxial Plummer models we find that the distance gradients in both dwarf galaxies are inconsistent with prolate shape, but compatible with oblate or triaxial shapes. In order to explain the observed gradients, oblate models of Sextans and Sculptor need to have a significant intrinsic ellipticity larger than $0.47$ for Sextans and $0.46$ for Sculptor. The flattened oblate shape may imply a significant anisotropy in velocity distribution in order to be consistent with the lack of significant velocity gradients in these systems.