Dynamics of stellar and HI streams in the Milky Way halo

TL;DR

This paper demonstrates how radial velocity gradients can be used to determine the orbital dynamics of stellar and HI streams in the Milky Way halo, providing insights into their nature and the galaxy's gravitational potential.

Contribution

It introduces methods to analyze the orbital dynamics of streams using radial velocity gradients, applicable to both stellar and gaseous streams in the Milky Way halo.

Findings

Hercules may be a stellar stream rather than a dwarf galaxy.

High-velocity HI clouds likely form a gaseous stream at ~20kpc.

Radial velocity gradients are effective for orbit determination in incomplete data scenarios.

Abstract

Stellar streams are key players in many aspects of Milky Way studies and, in particular, studying their orbital dynamics is crucial for furthering our understanding of the Milky Way's gravitational potential. Although this is not a trivial task when faced with incomplete dynamical phase-space information, transverse motions of streams can nevertheless be comprehended by harnessing the information contained within their radial velocity gradients. Such methods are not only applicable to stellar streams, but also to HI streams residing in the Milky Way halo. Here, I present the results of two studies that use radial velocity gradients to determine the system's orbit: for Hercules, one of the 'ultra-faint' dwarf galaxies exhibiting a large ellipticity and located at a distance of 140kpc, showing that it may in fact be a stellar stream, and for a string of high-velocity HI clouds belonging to the GCN complex, indicating its likelihood for being a gaseous stream at a distance of approximately 20kpc.