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

Abstract

We 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 Zw18, 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. © 2012 ESO.