Spectroscopic Confirmation of the Milky Way Satellites Boötes V and Leo Minor I and the LMC Satellite DELVE 2: Three Small Ultra-Faint Dwarf Galaxies

We present the first multi-object spectroscopy of Boötes V, Leo Minor I, and DELVE 2 – three small (20 pc $\le r_{1\mathrm{/}2}\le$ 26 pc), ultra-faint ($-3.2\le M_V\le -2.1$) stellar systems in the outer Galactic halo (71 kpc $\le D_{\odot }\le$ 102 kpc) with heretofore ambiguous classifications. Leveraging at least two epochs of Keck/DEIMOS or Magellan/IMACS observations targeting each satellite, we identify 13, 7, and 7 spectroscopic members in the three systems, respectively. We tentatively resolve a ${\sigma }_v=2.1_{-0.9}^{+1.5}$ km s${}^{-1}$ velocity dispersion for DELVE 2 but place upper limits of ${\sigma }_v<2.7$ km s${}^{-1}$ and ${\sigma }_v<7.8$ km s${}^{-1}$ for Boötes V and Leo Minor I. We find that all three systems are very metal-poor, with mean iron abundances $-2.85\le \overline{[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]}\le -2.15$. Both Boötes V and Leo Minor I display unambiguous metallicity spreads (${\sigma }_{[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]}=0.46_{-0.12}^{+0.18}$ dex and ${\sigma }_{[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]}=0.57_{-0.18}^{+0.23}$ dex), while DELVE 2 displays a modestly significant spread (${\sigma }_{[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]}=0.52_{-0.25}^{+0.29}$ dex) across three stars. Based primarily on these metallicity measurements, we confidently classify Boötes V and Leo Minor I as dwarf galaxies and determine that DELVE 2 is most likely a dwarf galaxy as well. We explore the orbit of each satellite in a joint Milky Way + LMC potential, confirming for the first time that DELVE 2 is a long-term bound satellite of the LMC. Finally, we review the empirical Milky Way satellite classification landscape and find that nearly every spectroscopically characterized satellite with azimuthally averaged half-light radius $r_{1\mathrm{/}2}>15$ pc and surface brightness ${\mu }_{V,1\mathrm{/}2}>26.5$ mag arcsec${}^{-2}$ is likely a dwarf galaxy. This result can guide initial probabilistic classifications for satellites discovered by $\mathit{\text{Rubin}}$, $\mathit{\text{Roman}}$, and $\mathit{\text{Euclid}}$.