A Ghost in Bo\"otes: The Least Luminous Disrupted Dwarf Galaxy

TL;DR

The paper reports the discovery of Specter, the least luminous disrupted dwarf galaxy stream, identified through spectroscopic and astrometric data, revealing insights into ancient, ultra-faint galactic remnants.

Contribution

This work introduces Specter, the faintest known dwarf galaxy stream, and demonstrates the effectiveness of spectroscopic surveys in detecting ultra-low surface brightness structures.

Findings

Specter is an ultra-faint dwarf galaxy remnant at 12.5 kpc.

Specter has a stellar age > 12 Gyr and metallicity around -1.84.

Dozens of similar faint streams likely remain undetected.

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 $25^\circ \times 1^\circ$ 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 $\tau \gtrsim 12$ Gyr and a mean metallicity $\langle\text{[Fe/H]}\rangle = -1.84_{-0.18}^{+0.16}$, with a significant intrinsic metallicity dispersion $\sigma_{ \text{[Fe/H]}} = 0.37_{-0.13}^{+0.21}$. We therefore argue that Specter is the disrupted remnant of an ancient dwarf galaxy. With an integrated luminosity $M_{\text{V}} \approx -2.6$, 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.