A Population of Metal-Poor Galaxies with ~L* Luminosities at Intermediate Redshifts

We present new spectroscopy and metallicity estimates for a sample of 15 star-forming galaxies with redshifts in the range 0.29 - 0.42. These objects were selected in the KPNO International Spectroscopic Survey via their strong emission lines seen in red objective-prism spectra. Originally thought to be intermediate-redshift Seyfert 2 galaxies, our new spectroscopy in the far red has revealed these objects to be metal-poor star-forming galaxies. These galaxies follow a luminosity-metallicity (L-Z) relation that parallels the one defined by low-redshift galaxies, but is offset by a factor of more than ten to lower abundances. The amount of chemical and/or luminosity evolution required to place these galaxies on the local L-Z relation is extreme, suggesting that these galaxies are in a very special stage of their evolution. They may be late-forming massive systems, which would challenge the current paradigm of galaxy formation. Alternatively, they may represent intense starbursts in dwarf-dwarf mergers or a major infall episode of pristine gas into a pre-existing galaxy. In any case, these objects represent an extreme stage of galaxy evolution taking place at relatively low redshift.