Antlia B: A Faint Dwarf Galaxy Member of the NGC 3109 Association

We report the discovery of Antlia B, a faint dwarf galaxy at a projected distance of ~72 kpc from NGC 3109 (M_V ~ -15 mag), the primary galaxy of the NGC 3109 dwarf association at the edge of the Local Group. The tip of the red giant branch distance to Antlia B is D = 1.29 ± 0.10 Mpc, which is consistent with the distance to NGC 3109. A qualitative analysis indicates the new dwarf's stellar population has both an old, metal-poor red giant branch (≳10 Gyr, [Fe/H] ~ −2), and a younger blue population with an age of ~200–400 Myr, analogous to the original Antlia dwarf, another likely satellite of NGC 3109. Antlia B has H i gas at a velocity of v_(helio,HI) = 376 km s^(−1), confirming the association with NGC 3109 (v_(helio) = 403 km s^(−1)). The H i gas mass (M_(H i) = 2.8 ± 0.2 × 10^5 M_☉), stellar luminosity (M_V = −9.7 ± 0.6 mag) and half light radius (r_h = 273 ± 29 pc) are all consistent with the properties of dwarf irregular and dwarf spheroidal galaxies in the Local Volume, and is most similar to the Leo P dwarf galaxy. The discovery of Antlia B is the initial result from a Dark Energy Camera survey for halo substructure and faint dwarf companions to NGC 3109 with the goal of comparing observed substructure with expectations from the Λ+Cold Dark Matter model in the sub-Milky Way regime.