Properties of the Lowest Metallicity Galaxies Over the Redshift Range z = 0.2 to z = 1

Abstract

Low-metallicity galaxies may provide key insights into the evolutionary history of galaxies. Galaxies with strong emission lines and high equivalent widths (rest-frame EW(H-beta) > 30 A) are ideal candidates for the lowest metallicity galaxies to z ~ 1. Using a Keck/DEIMOS spectral database of about 18,000 galaxies between z = 0.2 and z = 1, we search for such extreme emission-line galaxies with the goal of determining their metallicities. Using the robust direct Te method, we identify 8 new extremely metal-poor galaxies (XMPGs) with 12 + log O/H < 7.65, including one at 6.949 +/- 0.091, making it the lowest metallicity galaxy reported to date at these redshifts. We also improve upon the metallicities for two other XMPGs from previous work. We investigate the evolution of H-beta using both instantaneous and continuous starburst models, finding that XMPGs are best characterized by continuous starburst models. Finally, we study the dependence on age of the build-up of metals and the emission-line strength.