The Leoncino Dwarf Galaxy: Exploring the Low-Metallicity End of the
  Luminosity-Metallicity and Mass-Metallcity Relations
  Extremely metal-poor (XMP) galaxies are low-mass, star-forming galaxies with
gas-phase oxygen abundances below 12+log(O/H) = 7.35 (~1/20 Zsun). Galaxy
evolution scenarios suggest three pathways to form an XMP: (1) secular
evolution at low galaxy masses, (2) slow evolution in voids, or (3) dilution of
measured abundances from infall of pristine gas. The recently discovered XMP
galaxy Leoncino, with an oxygen abundance below 3% Zsun, provides an
opportunity to explore these different scenarios. Using Hubble Space Telescope
imaging of the resolved stellar populations of Leoncino, we measure the
distance to the galaxy to be D=12.1 (+1.7/-3.4) Mpc and find that Leoncino is
located in an under-dense environment. Leoncino has a compact morphology, hosts
a population of young, massive stars, has a high gas-to-star mass ratio, and
shows signs of interaction with a galaxy nearby on the sky, UGC 5186. Similar
to nearly all XMP galaxies known in the nearby universe, Leoncino is offset
from the Luminosity-Metallicity (LZ) relation. Yet, Leoncino is consistent with
the stellar Mass-Metallicity (MZ) relation defined by Local Volume galaxies.
Thus, our results suggest that the offset from the LZ relation is due to higher
recent star formation, likely triggered by a minor interaction, while the low
oxygen abundance is consistent with the expectation that low-mass galaxies will
undergo secular evolution marked by inefficient star formation and metal-loss
via galactic winds. This is in contrast to XMP galaxies that are outliers in
both the LZ and MZ relations; in such cases, the low oxygen abundances are best
explained by dilution due to the infall of pristine gas. We also discuss why
quiescent XMP galaxies are underrepresented in current surveys.
