ALFALFA Discovery of the Nearby Gas-Rich Dwarf Galaxy Leo~P. III. An Extremely Metal Deficient Galaxy

TL;DR

This paper reports the discovery and detailed spectroscopic analysis of Leo P, an extremely metal-deficient dwarf galaxy, providing insights into its chemical composition, star formation, and implications for galaxy evolution.

Contribution

It presents the first direct oxygen abundance measurement of Leo P, establishing it as one of the most metal-deficient star-forming galaxies and discusses its significance in galaxy evolution studies.

Findings

Leo P has an oxygen abundance of 12 + log(O/H) = 7.17.

Leo P's alpha element ratios are normal, but N/O is elevated.

The helium mass fraction aligns with primordial predictions.

Abstract

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.