Hunting for extremely metal-poor emission-line galaxies in the Sloan
  Digital Sky Survey: MMT and 3.5m APO observations
  We present 6.5-meter MMT and 3.5m APO spectrophotometry of 69 H II regions in
42 low-metallicity emission-line galaxies, selected from the Data Release 7 of
the Sloan Digital Sky Survey to have mostly [O III]4959/Hbeta < 1 and [N
II]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 a
semi-empirical method. The oxygen abundance of the galaxies in our sample
ranges from 12 + log O/H ~ 7.1 to ~ 7.9, with 14 H II regions in 7 galaxies
with 12 +log O/H < 7.35. In 5 of the latter galaxies, the oxygen abundance is
derived here for the first time. Including other known extremely
metal-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 most
metal-deficient (with 12 +log O/H < 7.35) emission-line galaxies known in the
local 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 was
pre-enriched to that level by e.g. Population III stars.
