ALFALFA DISCOVERY of the MOST METAL-POOR GAS-RICH GALAXY KNOWN: AGC 198691

We present spectroscopic observations of the nearby dwarf galaxy AGC 198691. This object is part of the Survey of H I in Extremely Low-Mass Dwarfs project, which is a multi-wavelength study of galaxies with H I masses in the range of 10⁶-10^7.2 M_⊙, discovered by the Arecibo Legacy Fast ALFA (ALFALFA) survey. We have obtained spectra of the lone H II region in AGC 198691 with the new high-throughput KPNO Ohio State Multi-Object Spectrograph on the Mayall 4 m, as well as with the Blue Channel spectrograph on the MMT 6.5 m telescope. These observations enable the measurement of the temperature-sensitive [O III]λ4363 line and hence the determination of a "direct" oxygen abundance for AGC 198691. We find this system to be an extremely metal-deficient (XMD) system with an oxygen abundance of 12+log(O/H) = 7.02 ± 0.03, making AGC 198691 the lowest-abundance star-forming galaxy known in the local universe. Two of the five lowest-abundance galaxies known have been discovered by the ALFALFA blind H I survey; this high yield of XMD galaxies represents a paradigm shift in the search for extremely metal-poor galaxies.