A 6-d View of Stellar Shells

TL;DR

This paper introduces a six-dimensional analysis of stellar shells formed during galaxy mergers, using action-angle coordinates to better understand their formation, evolution, and potential for constraining galaxy properties.

Contribution

It presents a novel 6D framework for analyzing stellar shells and a method to infer galaxy potential and merger history even with contaminated data.

Findings

Action-angle coordinates simplify shell dynamics.

Different stripping episodes influence shell properties.

Method allows potential and epoch constraints from complex data.

Abstract

Stellar shells are low surface brightness features, created during nearly head-on galaxy mergers from the debris of the tidally disrupted satellite. Here, we investigate the formation and evolution mechanism of shells in six dimensions (3d positions and velocities). We propose a new description in action-angle coordinates which condenses the seemingly complex behaviour of an expanding shell system into a simple picture, and stresses the crucial role of the existence of different stripping episodes in the properties of shells. Based on our findings, we construct a method for constraining the potential of the host galaxy and the average epoch of stripping. The method is applicable even if the shells cannot be identified or isolated from the data, or if the data are heavily contaminated with additional foreground stars. These results open up a new possibility to study the ancient merger that built the Milky Way Galaxy's stellar halo.