The DEEP2 Galaxy Redshift Survey: Discovery of Luminous, Metal-poor Star-forming Galaxies at Redshifts z ~ 0.7

We have discovered a sample of 17 metal-poor, yet luminous, star-forming galaxies at redshifts z ~ 0.7. They were selected from the initial phase of the DEEP2 survey of 3900 galaxies and the Team Keck Redshift Survey (TKRS) of 1536 galaxies as those showing the temperature-sensitive [O III] λ4363 auroral line. These rare galaxies have blue luminosities close to L*, high star formation rates of 5-12 M☉ yr-1, and oxygen abundances of 1/3 to 1/10 solar. They thus lie significantly off the luminosity-metallicity relation found previously for field galaxies with strong emission lines at redshifts z ~ 0.7. The prior surveys relied on indirect, empirical calibrations of the R23 diagnostic and the assumption that luminous galaxies are not metal-poor. Our discovery suggests that this assumption is sometimes invalid. As a class, these newly discovered galaxies are (1) more metal-poor than common classes of bright emission-line galaxies at z ~ 0.7 or at the present epoch; (2) comparable in metallicity to z ~ 3 Lyman break galaxies but less luminous; and (3) comparable in metallicity to local metal-poor extreme blue compact galaxies (XBCGs), but more luminous. Together, the three samples suggest that the most luminous, metal-poor, compact galaxies become fainter over time.