A Ghost in Boötes: The Least-Luminous Disrupted Dwarf Galaxy

We report the discovery of Specter, a disrupted ultrafaint dwarf galaxy revealed by the H3 Spectroscopic Survey. We detected this structure via a pair of comoving metal-poor stars at a distance of 12.5 kpc, and further characterized it with Gaia astrometry and follow-up spectroscopy. Specter is a 25° × 1° stream of stars that is entirely invisible until strict kinematic cuts are applied to remove the Galactic foreground. The spectroscopic members suggest a stellar age τ ≳ 12 Gyr and a mean metallicity $\langle [\mathrm{Fe}/{\rm{H}}]\rangle =-{1.84}_{-0.18}^{+0.16}$ , with a significant intrinsic metallicity dispersion ${\sigma }_{[\mathrm{Fe}/{\rm{H}}]}={0.37}_{-0.13}^{+0.21}$ . We therefore argue that Specter is the disrupted remnant of an ancient dwarf galaxy. With an integrated luminosity M _V ≈ -2.6, Specter is by far the least-luminous dwarf galaxy stream known. We estimate that dozens of similar streams are lurking below the detection threshold of current search techniques, and conclude that spectroscopic surveys offer a novel means to identify extremely low surface brightness structures.