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

Abstract

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 〈 [ Fe / H ] 〉 = − 1.84 − 0.18 + 0.16 , with a significant intrinsic metallicity dispersion σ [ 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 ≈ −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.

What Is Missing from the Local Stellar Halo?

“…We note that several lower-mass (<10 6 M e ) dwarf galaxies (e.g., Shipp et al 2018;Ji et al 2020;Bonaca et al 2021;Chandra et al 2022;Tenachi et al 2022;Dodd et al 2023) are predicted (e.g., Robertson et al 2005;Deason et al 2016;Fattahi et al 2019) andobserved (e.g., Helmi 2020;Naidu et al 2020;An & Beers 2021) to contribute a small minority of the stellar halo. While we do not include such systems in our composite Milky Way model, they are captured in the BJ05 simulated halos we analyze, and the general trends we report apply to them.…”