Hα Dots: Direct-Method Metal Abundances of Low-Luminosity Star-Forming Systems

We present the results of a detailed spectroscopic study of twenty-six dwarf star-forming galaxies discovered in the Hα Dots survey. Observations were carried out with the KPNO 4m telescope and employed the KOSMOS spectrograph. For our full sample of targets, detection of the temperature-sensitive [O III]λ4363 auroral emission line has permitted determination of robust, “direct”-method metal abundances. We measure gas-phase oxygen abundances in the range of 12+log(O/H) = 7.00 to 8.17, and also present elemental abundances for nitrogen, neon, sulfur, and argon. The selection method of the Hα Dots survey preferentially detects compact, low-luminosity galaxies, providing an effective new avenue for identifying star-forming systems probing to very low luminosities, stellar masses, and metallicities. We compare these properties of our sample with previous studies, helping to improve the census of the lowest luminosity and most metal poor star-forming sources. Examining the metallicity characteristics and scaling relations for these low-luminosity sources, we show that the chemical-enrichment properties for star-forming dwarf systems differ markedly from galaxies of higher stellar mass. Ongoing efforts to acquire additional abundance-quality Hα Dot spectra will continue to enhance our understanding of this regime, and are likely to yield additional examples of extremely metal-poor (XMP) galaxies, which may represent accessible high-redshift analogs to actively star-forming dwarf systems found in the early universe.