Leonessa: An Extremely Metal-poor Galaxy Undergoing Secular Chemical Evolution^*

Abstract Extremely metal-poor (XMP) galaxies are systems with gas-phase oxygen abundances below ∼5% solar metallicity (12+log(O/H) ≤ 7.35). These galaxies populate the metal-poor end of the mass–metallicity and luminosity–metallicity relations (MZR and LZR, respectively). Recent studies have found XMP galaxies in the nearby Universe to be outliers on the LZR, where they show enhanced luminosities relative to other galaxies of similar gas-phase oxygen abundance. Here, we present a study of the recently discovered XMP galaxy Leonessa and characterize the system’s properties using new imaging from the Hubble Space Telescope and spectra from the Green Bank Telescope and Hobby–Eberly Telescope. We use these observations to measure a tip of the red giant branch distance (15.86 ± 0.78 Mpc) to Leonessa, the H i gas mass, the gas-phase oxygen abundance, and N/O ratio. We find Leonessa is an isolated, gas-rich (gas fraction μ = 0.69), low-mass (log( M ⋆ / M ⊙ ) = 6.12 ± 0.08), XMP (12+log(O/H) = 7.32 ± 0.04), star-forming galaxy at a distance of 15.86 ± 0.78 Mpc. Our measurements show that Leonessa agrees with the MZR, but disagrees with the LZR; we conclude the LZR offset is due to recent star formation enhancing the system’s luminosity. To investigate possible chemical evolution pathways for nearby XMP galaxies, we also compile a comparison sample of 150 dwarf galaxies (53 XMP systems) taken from the literature with gas-phase metallicity measurements based on the direct method. We find evidence for an anticorrelation between gas-phase oxygen abundance and H i gas-to-stellar mass ratios. We posit Leonessa is undergoing a chemical evolution pathway typical of field dwarf galaxies.