Uncovering fossils of the distant Milky Way with UNIONS: NGC 5466 and its stellar stream

TL;DR

This study uncovers and characterizes the extended stellar stream of NGC 5466 using CFIS and Gaia data, revealing its complex structure, distance gradient, and recent interaction with the Milky Way's disk, providing insights into Galactic dynamics.

Contribution

First detailed identification and analysis of NGC 5466's stellar stream using combined CFIS and Gaia data, including its spatial, kinematic, and distance properties.

Findings

Discovered extended tidal tails of NGC 5466 with a strong distance gradient.

Stream does not follow previously known path, indicating complex dynamics.

Models broadly reproduce the stream's overall path and kinematics.

Abstract

We examine the spatial clustering of blue horizontal branch (BHB) stars from the $\textit{u}$-band of the Canada-France Imaging Survey (CFIS, a component of the Ultraviolet Near-Infrared Optical Northern Survey, or UNIONS). All major groupings of stars are associated with previously known satellites, and among these is NGC 5466, a distant (16 kpc) globular cluster. NGC 5466 reportedly possesses a long stellar stream, although no individual members of the stream have previously been identified. Using both BHBs and more numerous red giant branch stars cross-matched to $\textit{Gaia}$ Data Release 2, we identify extended tidal tails from NGC 5466 that are both spatially and kinematically coherent. Interestingly, we find that this stream does not follow the same path as the previous detection at large distances from the cluster. We trace the stream across 31$^{\circ}$ of sky and show that it exhibits a very strong distance gradient ranging from 10 $<$ R$_{helio}$ $<$ 30 kpc. We compare our observations to simple dynamical models of the stream and find that they are able to broadly reproduce the overall path and kinematics. The fact that NGC 5466 is so distant, traces a wide range of Galactic distances, has an identified progenitor, and appears to have recently had an interaction with the Galaxy's disk, makes it a unique test-case for dynamical modelling of the Milky Way.