Hunting for extremely metal-poor emission-line galaxies in the Sloan Digital Sky Survey: MMT and 3.5 m APO observations ⋆,⋆⋆
Max-Planck-Institut für Radioastronomie,
Auf dem Hügel 69, 53121
2 Main Astronomical Observatory, Ukrainian National Academy of Sciences, Zabolotnoho 27, Kyiv 03680, Ukraine
3 Institut d’Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France
4 Astronomy Department, University of Virginia, PO Box 400325, Charlottesville, VA 22904, USA
Received: 1 June 2012
Accepted: 24 July 2012
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 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.
Key words: galaxies: abundances / galaxies: irregular / galaxies: evolution / galaxies: ISM / HII regions / ISM: abundances
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
© ESO, 2012