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; ] and the 12 log (O/H) ! 7.65 recovery of four previously known or suspected XMPGs (I Zw 18, HS 08223542, HS 08374717, and A1116517) 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 deg 2 on the sky. Our oxygen abundance determinations have an accuracy of ≤0.1 dex and are based on the temperature-sensitive [O iii] l4363 line and on the direct calculation of the electron temperature. We briefly discuss a new method of oxygen abundance determinations using the [O ii] ll7319, 7330 lines that is particularly useful for SDSS emission-line spectra with redshifts ≤0.024 since the [O ii] l3727 emission line falls outside of the SDSS wavelength range. We detect XMPGs with redshifts ranging from 0.0005 to 0.0443 and luminosities from 12.4 to 18.6 mag. Our eight new M g XMPGs increase the number of known metal-deficient galaxies by approximately one-quarter. The estimated surface density of XMPGs is 0.004 deg for mag. 2 r ≤ 17.77