The Discovery of the Faintest Known Milky Way Satellite Using UNIONS

<jats:p>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 <jats:italic>V</jats:italic>-band magnitude of <jats:inline-formula> <jats:tex-math> <? CDATA $+{2.2}_{-0.3}^{+0.4}$? > </jats:tex-math> <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:mrow> <mml:mo>−</mml:mo> <mml:mn>0.3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.4</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3. org/1999/xlink" xlink:href="apjad0d9fieqn1.gif" xlink:type="simple" /> </jats:inline-formula> mag, equivalent to a total stellar mass of <jats:inline-formula> <jats:tex-math> <? CDATA ${16}_{-5}^{+6}$? > </jats:tex-math> <mml:math xmlns:mml="http://www.w3. org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>16</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>6</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3. org/1999/xlink" xlink:href="apjad0d9fieqn2.gif" xlink:type="simple" /> </jats:inline-formula> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. 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 (<jats:italic>τ</jats:italic> > 11 Gyr), metal-poor ([Fe/H] ∼ −2.2) stellar population at a heliocentric distance of ∼10 kpc. Despite its being compact (<jats:italic>r</jats:italic> <jats:sub>h</jats:sub> = 3 ± 1 pc) and composed of 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, eight of which have full astrometric data from Gaia and are co-moving based on their proper motions. Based on these 11 radial velocity members, we derive an intrinsic velocity dispersion of <jats:inline-formula> <jats:tex-math> <? CDATA ${3.7}_{-1.0}^{+1.4}$? > </jats:tex-math> <mml:math xmlns:mml="http://www.w3. org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>3.7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.4</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3. org/1999/xlink" xlink:href="apjad0d9fieqn3.gif" xlink:type="simple" /> </jats:inline-formula> km s<jats:sup>−1</jats:sup> but some caveats preclude this value from being interpreted as a direct indicator of the underlying gravitational potential at this time. Primarily, the exclusion of the largest velocity outlier from the member list drops the velocity dispersion to <jats:inline-formula> <jats:tex-math> <? CDATA ${1.9}_{-1.1}^{+1.4}$? > </jats:tex-math> <mml:math xmlns:mml="http://www.w3. org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>1.9</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.4</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3. org/1999/xlink" xlink:href="apjad0d9fieqn4.gif" xlink:type="simple" /> </jats:inline-formula> km s<jats:sup>−1</jats:sup>, and the subsequent removal of an additional outlier star produces an unresolved velocity dispersion. While the presence of binary stars may be inflating the measurement, the possibility of a significant velocity dispersion makes Ursa Major III/UNIONS 1 a high-priority candidate for multi-epoch spectroscopic follow-ups to deduce the true nature of this incredibly faint satellite. </jats:p>