Three Quenched, Faint Dwarf Galaxies in the Direction of NGC 300: New Probes of Reionization and Internal Feedback

We report the discovery of three faint and ultrafaint dwarf galaxies—Sculptor A, Sculptor B, and Sculptor C—in the direction of NGC 300 (D = 2.0 Mpc), a Large Magellanic Cloud-mass galaxy. Deep ground-based imaging with Gemini/GMOS resolves all three dwarf galaxies into stars, each displaying a red giant branch indicative of an old, metal-poor stellar population. No young stars or H i gas are apparent, and the lack of a GALEX UV detection suggests that all three systems are quenched. Sculptor C (D = 2.04 − 0.13 + 0.10 Mpc; M_V = −9.1 ± 0.1 mag or L_V = (3.7 − 0.3 + 0.4 ) × 10⁵L_⊙) is consistent with being a satellite of NGC 300. Sculptor A (D = 1.35 − 0.08 + 0.22 Mpc; M_V = −6.9 ± 0.3 mag or L_V = (5 − 1 + 1 ) × 10⁴L_⊙) is likely in the foreground of NGC 300 and at the extreme edge of the Local Group, analogous to the recently discovered ultrafaint Tucana B in terms of its physical properties and environment. Sculptor B (D = 2.48 − 0.24 + 0.21 Mpc; M_V = −8.1 ± 0.3 mag or L_V = (1.5 − 0.4 + 0.5 ) × 10⁵L_⊙) is likely in the background, but future distance measurements are necessary to solidify this statement. It is also of interest due to its quiescent state and low stellar mass. Both Sculptor A and B are ≳2-4 r_vir from NGC 300 itself. The discovery of three dwarf galaxies in isolated or low-density environments offers an opportunity to study the varying effects of ram-pressure stripping, reionization, and internal feedback in influencing the star formation history of the faintest stellar systems.