The Discovery of the Faintest Known Milky Way Satellite Using UNIONS

• DOI: 10.3847/1538-4357/ad0d9f
• Publication Date: 2024-01-17T09:22:56Z
• Authors: Simon E. T. Smith; William Cerny; Christian R. Hayes; Federico Sestito; Jaclyn Jensen; Alan W. McConnachie; Marla Geha; Julio F. Navarro; Ting S. Li; Jean-Charles Cuil...; Raphaël Errani; Ken Chambers; Stephen Gwyn; Francois Hammer; Michael J. Hudson; Eugene Magnier; Nicolas Martin
• Keywords: Milky Way stellar halo; Broad band photometry; FOS: Physical sciences; Astrophysics; Astrophysics - Astrophysics of Galaxies; 01 natural sciences; 520; [SDU] Sciences of the Universe [physics]; QB460-466; [SDU]Sciences of the Universe [physics]; Astrophysics of Galaxies (astro-ph.GA); 0103 physical sciences; Local Group; Stellar dynamics

Abstract We present the discovery of Ursa Major III/UNIONS 1, least luminous known satellite Milky Way, which is estimated to have an absolute V -band magnitude <?CDATA $+{2.2}_{-0.3}^{+0.4}$?> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>+</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>2.2</mml:mn> </mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.3</mml:mn> <mml:mn>0.4</mml:mn> </mml:msubsup> </mml:math> mag, equivalent a total stellar mass ${16}_{-5}^{+6}$?> <mml:mn>16</mml:mn> <mml:mn>5</mml:mn> <mml:mn>6</mml:mn> M ⊙ . 1 was uncovered in deep, wide-field Ultraviolet Near Infrared Optical Northern Survey (UNIONS) and consistent with old ( τ &gt; 11 Gyr), metal-poor ([Fe/H] ∼ −2.2) population at heliocentric distance ∼10 kpc. Despite its being compact r h = 3 ± pc) composed few stars, we confirm reality Keck II/DEIMOS follow-up spectroscopy identify radial velocity members, eight full astrometric data from Gaia are co-moving based on their proper motions. Based these derive intrinsic dispersion ${3.7}_{-1.0}^{+1.4}$?> <mml:mn>3.7</mml:mn> <mml:mn>1.0</mml:mn> <mml:mn>1.4</mml:mn> km s −1 but some caveats preclude this value interpreted as direct indicator underlying gravitational potential time. Primarily, exclusion largest outlier member list drops ${1.9}_{-1.1}^{+1.4}$?> <mml:mn>1.9</mml:mn> <mml:mn>1.1</mml:mn> , subsequent removal additional star produces unresolved dispersion. While presence binary stars may be inflating measurement, possibility significant makes high-priority candidate for multi-epoch spectroscopic follow-ups deduce true nature incredibly faint satellite.