Extremely metal-poor ( XMP ) galaxies are low-mass , star-forming galaxies with gas-phase oxygen abundances below 12 + log ( O/H ) = 7.35 ( \sim \frac { 1 } { 20 } Z _ { \odot } ) .
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 % Z _ { \odot } , 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 .