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

TL;DR

This study identifies and analyzes extremely metal-poor emission-line galaxies using spectrophotometry from SDSS, MMT, and APO, revealing a potential metallicity floor and providing new abundance measurements for several galaxies.

Contribution

It presents new spectroscopic data for low-metallicity galaxies, expanding the sample of known extremely metal-deficient galaxies and suggesting a primordial metallicity floor in the universe.

Findings

Detection of [O III] 4363 in 53 H II regions enabling direct abundance measurements

Identification of 14 H II regions with 12 + log O/H < 7.35, including first measurements for some

Evidence for a metallicity floor at 12 + log O/H ~ 6.9

Abstract

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.