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

Abstract

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_solar 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.

Based on observations obtained with the KECK2 Telescope at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. This work also uses data obtained with the NASA/ESA Hubble Space Telescope through the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555.