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 similar to 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] lambda 4363 auroral line. These rare galaxies have blue luminosities close to L*, high star formation rates of 5-12 M-circle dot 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 similar to 0.7. The prior surveys relied on indirect, empirical calibrations of the R-23 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 or at the present epoch; (2) comparable in metallicity z similar to 0.7 to Lyman break galaxies but less luminous; and (3) comparable in metallicity to local metal-poor extreme z similar to 3 blue compact galaxies (XBCGs), but more luminous. Together, the three samples suggest that the most luminous, metal-poor, compact galaxies become fainter over time.