We present KPNO 4-m and LBT/MODS spectroscopic observations of an H ii 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 ] \lambda 4363 line and determine a “ direct ” oxygen abundance of 12 + log ( O/H ) = 7.17 \pm 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 \pm 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 .