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 HI in Extremely Low-Mass Dwarfs (SHIELD)
project, which is a multi-wavelength study of galaxies with HI masses in the
range of 10$^{6}$-10$^{7.2}$~M$_{\odot}$ discovered by the ALFALFA survey. We
have obtained spectra of the lone HII region in AGC 198691 with the new
high-throughput KPNO Ohio State Multi-Object Spectrograph (KOSMOS) 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 [OIII]$\lambda$4363 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 $\pm$ 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 HI survey; this high yield of XMD
galaxies represents a paradigm shift in the search for extremely metal-poor
galaxies.
