Hubble Space Telescope Imaging of Antlia B: Star Formation History and a New Tip of the Red Giant Branch Distance

A census of the satellite population around dwarf galaxy primary hosts in environments outside the Local Group is essential to understanding ΛCDM galaxy formation and evolution on the smallest scales. We present deep optical Hubble Space Telescope imaging of the gas-rich, faint dwarf galaxy Antlia B (M_V = −9.4) -- a likely satellite of NGC 3109 (D=1.3 Mpc) -- discovered as part of our ongoing survey of primary host galaxies similar to the Magellanic Clouds. We derive a new tip of the red giant branch (TRGB) distance of D=1.35±0.06 Mpc (m−M=25.65±0.10), consistent with membership in the nearby NGC 3109 dwarf association. The color-magnitude diagram shows both a prominent old, metal-poor stellar component and confirms a small population of young, blue stars with ages ≲1 Gyr. We use the color-magnitude diagram fitting algorithm MATCH to derive the star formation history and find that it is consistent with the typical dwarf irregular or transitional dwarf galaxy (dTrans) in the Local Group. Antlia B shows relatively constant stellar mass growth for the first ∼10−11 Gyr and almost no growth in the last ∼2−3 Gyr. Despite being gas-rich, Antlia B shows no evidence of active star formation (i.e., no Hα emission) and should therefore be classified as a dTrans dwarf. Both Antlia B and the Antlia dwarf (dTrans) are likely satellites of NGC 3109 suggesting that the cessation of ongoing star formation in these galaxies may be environmentally driven. Future work studying the gas kinematics and distribution in Antlia B will explore this scenario in greater detail. Our work highlights the fact that detailed studies of nearby dwarf galaxies in a variety of environments may continue to shed light on the processes that drive the star formation history and evolution of dwarf galaxies more generally.