The discovery of the faintest known Milky Way satellite using UNIONS
Simon E. T. Smith, William Cerny, Christian R. Hayes, Federico, Sestito, Jaclyn Jensen, Alan W. McConnachie, Marla Geha, Julio Navarro, Ting, S. Li, Jean-Charles Cuillandre, Rapha\"el Errani, Ken Chambers, Stephen Gwyn,, Francois Hammer, Michael J. Hudson, Eugene Magnier

TL;DR
This paper reports the discovery of the faintest known Milky Way satellite, Ursa Major III/UNIONS 1, using deep wide-field survey data, and provides initial spectroscopic and astrometric characterization of its properties.
Contribution
It introduces a new ultra-faint satellite of the Milky Way discovered through UNIONS survey data, with detailed follow-up spectroscopy and astrometry analysis.
Findings
Confirmed the satellite's reality with spectroscopy
Derived a tentative velocity dispersion of 3.7 km/s
Identified the satellite as extremely faint and compact
Abstract
We present the discovery of Ursa Major III/UNIONS 1, the least luminous known satellite of the Milky Way, which is estimated to have an absolute V-band magnitude of mag, equivalent to a total stellar mass of 16 M. Ursa Major III/UNIONS 1 was uncovered in the deep, wide-field Ultraviolet Near Infrared Optical Northern Survey (UNIONS) and is consistent with an old ( Gyr), metal-poor ([Fe/H] ) stellar population at a heliocentric distance of 10 kpc. Despite being compact ( pc) and composed of so few stars, we confirm the reality of Ursa Major III/UNIONS 1 with Keck II/DEIMOS follow-up spectroscopy and identify 11 radial velocity members, 8 of which have full astrometric data from and are co-moving based on their proper motions. Based on these 11 radial velocity members, we derive an…
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Taxonomy
TopicsStellar, planetary, and galactic studies · Astronomy and Astrophysical Research · Astrophysics and Star Formation Studies
