Discovery of Eight New Extremely Metal-poor Galaxies in the Sloan Digital Sky Survey

Abstract

We report the discovery of eight new extremely metal-poor galaxies [XMPGs; 12 + log(O/H) < 7.65] and the recovery of four previously known or suspected XMPGs (I Zw 18, HS 0822+3542, HS 0837+4717, and A1116+517) using Sloan Digital Sky Survey (SDSS) spectroscopy. These new objects were identified after an analysis of 250,000 galaxy spectra within an area of ~3000 deg2 on the sky. Our oxygen abundance determinations have an accuracy of ≤0.1 dex and are based on the temperature-sensitive [O III] λ4363 line and on the direct calculation of the electron temperature. We briefly discuss a new method of oxygen abundance determinations using the [O II] λλ7319, 7330 lines that is particularly useful for SDSS emission-line spectra with redshifts ≤0.024 since the [O II] λ3727 emission line falls outside of the SDSS wavelength range. We detect XMPGs with redshifts ranging from 0.0005 to 0.0443 and Mg luminosities from -12.4 to -18.6 mag. Our eight new XMPGs increase the number of known metal-deficient galaxies by approximately one-quarter. The estimated surface density of XMPGs is 0.004 deg-2 for r ≤ 17.77 mag.