Signatures of Kinematic Substructure in the Galactic Stellar Halo

TL;DR

This paper investigates purely kinematic substructures in the Galactic stellar halo using simulations, revealing their potential to inform dark matter properties and upcoming Gaia data analysis.

Contribution

It demonstrates that kinematic substructure is a common feature of tidal debris in simulated halos, with implications for understanding dark matter and stellar halo dynamics.

Findings

Kinematic substructure is prevalent in simulated stellar halos.

Such substructure can explain observed velocity features in the Milky Way.

Halo kinematics can inform dark matter velocity distributions.

Abstract

Tidal debris from infalling satellites can leave observable structure in the phase-space distribution of the Galactic halo. Such substructure can be manifest in the spatial and/or velocity distributions of the stars in the halo. This paper focuses on a class of substructure that is purely kinematic in nature, with no accompanying spatial features. To study its properties, we use a simulated stellar halo created by dynamically populating the Via Lactea II high-resolution N-body simulation with stars. A significant fraction of the stars in the inner halo of Via Lactea share a common speed and metallicity, despite the fact that they are spatially diffuse. We argue that this kinematic substructure is a generic feature of tidal debris from older mergers and may explain the detection of radial-velocity substructure in the inner halo made by the Sloan Extension for Galactic Understanding and Exploration. The GAIA satellite, which will provide the proper motions of an unprecedented number of stars, should further characterize the kinematic substructure in the inner halo. Our study of the Via Lactea simulation suggests that the stellar halo can be used to map the speed distribution of the local dark-matter halo, which has important consequences for dark-matter direct-detection experiments.