A Ghost in Bootes: 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 degrees x 1 degrees 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 tau greater than or similar to 12 Gyr and a mean metallicity <[Fe/H]> = -1.84(-0.18)(+0.16), with a significant intrinsic metallicity dispersion sigma([Fe/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 approximate to -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.

Automated summary

We report the discovery of Specter, a disrupted ultrafaint dwarf galaxy. It is a stream of stars that can only be detected through strict kinematic cuts. Spectroscopic analysis suggests that it is the remnant of an ancient dwarf galaxy. Specter has an extremely low luminosity and there may be similar streams yet to be discovered.