Spectroscopic Confirmation of the Milky Way Satellites Bo"otes 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 r 1/2 26 pc), ultra-faint (-3.2 M V-2.1) stellar systems in the outer Galactic halo (71 kpc D 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 σ v=2.1+1.5-0.9 km s-1 velocity dispersion for DELVE 2 but place upper limits of σ v<2.7 km s-1 and σ 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-2.85-2.15. Both Boötes V and Leo Minor I display unambiguous metallicity spreads (σ[Fe/H]=0.46+0.18-0.12 dex and σ[Fe/H]=0.57+0.23-0.18 dex), while DELVE 2 displays a modestly significant spread (σ[Fe/H]=0.52+0.29-0.25 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 1/2>15 pc and surface brightness μ V,1/2>26.5 mag arcsec-2 is likely a dwarf galaxy. This result can guide initial probabilistic classifications for satellites discovered by Rubin, Roman, and Euclid.