The discovery of the faintest known Milky Way satellite using UNIONS. (arXiv:2311.10147v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Smith_S/0/1/0/all/0/1">Simon E. T. Smith</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cerny_W/0/1/0/all/0/1">William Cerny</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hayes_C/0/1/0/all/0/1">Christian R. Hayes</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sestito_F/0/1/0/all/0/1">Federico Sestito</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jensen_J/0/1/0/all/0/1">Jaclyn Jensen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+McConnachie_A/0/1/0/all/0/1">Alan W. McConnachie</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Geha_M/0/1/0/all/0/1">Marla Geha</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Navarro_J/0/1/0/all/0/1">Julio Navarro</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Li_T/0/1/0/all/0/1">Ting S. Li</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cuillandre_J/0/1/0/all/0/1">Jean-Charles Cuillandre</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Errani_R/0/1/0/all/0/1">Raphaël Errani</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chambers_K/0/1/0/all/0/1">Ken Chambers</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gwyn_S/0/1/0/all/0/1">Stephen Gwyn</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hammer_F/0/1/0/all/0/1">Francois Hammer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hudson_M/0/1/0/all/0/1">Michael J. Hudson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Magnier_E/0/1/0/all/0/1">Eugene Magnier</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Martin_N/0/1/0/all/0/1">Nicolas Martin</a>
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 $+2.2^{+0.4}_{-0.3}$ mag, equivalent to a total stellar mass of
16$^{+6}_{-5}$ M$_{odot}$. 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 ($tau > 11$ Gyr), metal-poor ([Fe/H] $sim -2.2$)
stellar population at a heliocentric distance of $sim$ 10 kpc. Despite being
compact ($r_{text{h}} = 3pm1$ 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 $Gaia$ and are co-moving based on their proper motions.
Based on these 11 radial velocity members, we derive an intrinsic velocity
dispersion of $3.7^{+1.4}_{-1.0}$ km s$^{-1}$ 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
$1.9^{+1.4}_{-1.1}$ km s$^{-1}$, 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 to true nature of this
incredibly faint satellite.
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 $+2.2^{+0.4}_{-0.3}$ mag, equivalent to a total stellar mass of
16$^{+6}_{-5}$ M$_{odot}$. 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 ($tau > 11$ Gyr), metal-poor ([Fe/H] $sim -2.2$)
stellar population at a heliocentric distance of $sim$ 10 kpc. Despite being
compact ($r_{text{h}} = 3pm1$ 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 $Gaia$ and are co-moving based on their proper motions.
Based on these 11 radial velocity members, we derive an intrinsic velocity
dispersion of $3.7^{+1.4}_{-1.0}$ km s$^{-1}$ 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
$1.9^{+1.4}_{-1.1}$ km s$^{-1}$, 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 to true nature of this
incredibly faint satellite.
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