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

Abstract

We examine the spatial clustering of blue horizontal branch (BHB) stars from the $\text{u}$-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 $\text{Gaia}$ 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.

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Cited by in corpus (9)

• The Global Dynamical Atlas of the Milky Way mergers: Constraints from Gaia EDR3 based orbits of globular clusters, stellar streams and satellite galaxies
• galstreams: A Library of Milky Way Stellar Stream Footprints and Tracks
• Discovery of a new Local Group Dwarf Galaxy Candidate in UNIONS: Boötes V
• The e-TidalGCs Project: Modeling the extra-tidal features generated by Galactic globular clusters
• The oblateness of the Milky Way dark matter halo from the stellar streams of NGC 3201, M68, and Palomar 5
• ShapePipe: A modular weak-lensing processing and analysis pipeline
• Stellar streams and dark substructure: the diffusion regime
• A Pristine-UNIONS view on the Galaxy: Kinematics of the distant spur feature of the Sagittarius stream traced by Blue Horizontal Branch stars
• Revisit NGC 5466 Tidal Stream with $Gaia$, SDSS/SEGUE and LAMOST