ALFALFA DISCOVERY OF THE NEARBY GAS-RICH DWARF GALAXY LEO P. III. AN EXTREMELY METAL DEFICIENT GALAXY

Abstract

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