ALFALFA Discovery of the Nearby Gas-Rich Dwarf Galaxy Leo~P. III. An
  Extremely Metal Deficient Galaxy
  We present KPNO 4-m and LBT/MODS spectroscopic observations of an HII region
in the nearby dwarf irregular galaxy Leo P discovered recently in the Arecibo
ALFALFA survey. In both observations, we are able to accurately measure the
temperature sensitive [O III] 4363 Angstrom line and determine a "direct"
oxygen abundance of 12 + log(O/H) = 7.17 +/- 0.04. Thus, Leo P is an extremely
metal deficient (XMD) galaxy, and, indeed, one of the most metal deficient
star-forming galaxies ever observed. For its estimated luminosity, Leo P is
consistent with the relationship between luminosity and oxygen abundance seen
in nearby dwarf galaxies. Leo P shows normal alpha element abundance ratios
(Ne/O, S/O, and Ar/O) when compared to other XMD galaxies, but elevated N/O,
consistent with the "delayed release" hypothesis for N/O abundances. We derive
a helium mass fraction of 0.2509 +0.0184 -0.0123 which compares well with the
WMAP + BBN prediction of 0.2483 +/- 0.0002 for the primordial helium abundance.
We suggest that surveys of very low mass galaxies compete well with emission
line galaxy surveys for finding XMD galaxies. It is possible that XMD galaxies
may be divided into two classes: the relatively rare XMD emission line galaxies
which are associated with starbursts triggered by infall of low-metallicity gas
and the more common, relatively quiescent XMD galaxies like Leo P, with very
low chemical abundances due to their intrinsically small masses.
