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

International audienceWe present 6.5-m MMT and 3.5 m 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]λ4959/Hβ ≲ 1 and [N ii]λ6583/Hβ ≲ 0.1. The electron temperature-sensitive emission line [O iii] λ4363 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. Based on observations with the Multiple Mirror telescope (MMT) and the 3.5 m Apache Point Observatory (APO). The MMT is operated by the MMT Observatory (MMTO), a joint venture of the Smithsonian Institution and the University of Arizona. The Apache Point Observatory 3.5-m telescope is owned and operated by the Astrophysical Research Consortium.Figures 1-3 and Tables 2-8 are available in electronic form at http://www.aanda.org