MMT extremely metal-poor galaxy survey I. An efficient technique for identifying metal-poor galaxies

We demonstrate a successful strategy for identifying extremely metal-poor galaxies. Our preliminary survey of 24 candidates contains 10 metal-poor galaxies of which 4 have log(O/H)+12 < 7.65, some of the lowest-metallicity blue compact galaxies known to date. Interestingly, our sample of metal-poor galaxies have systematically lower metallicity for their luminosity than comparable samples of blue compact galaxies, dwarf irregulars, and normal star-forming galaxies. Our metal-poor galaxies share very similar properties, however, with the host galaxies of nearby long-duration gamma-ray bursts (GRBs), including similar metallicity, stellar ages, and star formation rates. We use H beta to measure the number of OB stars present in our galaxies and estimate a core-collapse supernova rate of similar to 10(-3) yr(-1). A larger sample of metal-poor galaxies may provide new clues about the environment where GRBs form and may provide a list of potential GRB hosts.