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We present 6.5-meter MMT and 3.5m APO spectrophotometry of 69 H II regions in42 low-metallicity emission-line galaxies, selected from the Data Release 7 ofthe Sloan Digital Sky Survey to have mostly [O III]4959/Hbeta < 1 and [NII]6583/Hbeta < 0.1. The electron temperature-sensitive emission line [O III]4363 is detected in 53 H II regions allowing a direct abundance determination.The oxygen abundance in the remaining 16 H II regions is derived using asemi-empirical method. The oxygen abundance of the galaxies in our sampleranges from 12 + log O/H ~ 7.1 to ~ 7.9, with 14 H II regions in 7 galaxieswith 12 +log O/H < 7.35. In 5 of the latter galaxies, the oxygen abundance isderived here for the first time. Including other known extremelymetal-deficient emission-line galaxies from the literature, e.g. SBS 0335-052W,SBS 0335-052E and I Zw 18, we have compiled a sample of the 17 mostmetal-deficient (with 12 +log O/H < 7.35) emission-line galaxies known in thelocal universe. There appears to be a metallicity floor at 12 +log O/H ~ 6.9,suggesting that the matter from which dwarf emission-line galaxies formed waspre-enriched to that level by e.g. Population III stars.

Hunting for extremely metal-poor emission-line galaxies in the Sloan Digital Sky Survey: MMT and 3.5m APO observations