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,{{H}} {{I}}} = 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 × 10⁵ {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.