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

Jaclyn Jensen*, Guillaume Thomas, Alan W. McConnachie, Else Starkenburg, Khyati Malhan, Julio Navarro, Nicolas Martin, Benoit Famaey, Rodrigo Ibata, Scott Chapman, Jean Charles Cuillandre, Stephen Gwyn

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

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We examine the spatial clustering of blue horizontal branch (BHB) stars from the 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 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° of sky and show that it exhibits a very strong distance gradient in the range 10 < Rhelio < 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 disc makes it a unique test-case for dynamical modelling of the Milky Way.

Original languageEnglish
Pages (from-to)1923-1936
Number of pages14
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Publication statusPublished - 1-Oct-2021


  • Galaxy: halo
  • Galaxy: kinematics and dynamics
  • Galaxy: structure
  • globular clusters: individual: NGC 5466

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