SIMBAD references

We present 6.5-m MMT and 3.5m APO spectrophotometry of 69 HII regions in 42 low-metallicity emission-line galaxies, selected from the data release 7 of the Sloan Digital Sky Survey to have mostly [OIII]λ4959/Hβ≲ 1 and [NII]λ6583/Hβ≲0.1. The electron temperature-sensitive emission line [OIII]λ4363 is detected in 53 HII regions allowing a direct abundance determination. The oxygen abundance in the remaining 16 HII regions is derived using a semi-empirical method. The oxygen abundance of the galaxies in our sample ranges from 12+logO/H∼7.1 to ∼7.9, with 14 HII regions in 7 galaxies with 12+logO/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 IZw18, we have compiled a sample of the 17 most metal-deficient (with 12+logO/H≤7.35) emission-line galaxies known in the local universe. There appears to be a metallicity floor at 12+logO/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.