ALFALFA Discovery of the Most Metal-Poor Gas-Rich Galaxy Known: AGC 198691

TL;DR

This paper reports the discovery of AGC 198691, the most metal-poor gas-rich galaxy known locally, identified through spectroscopic analysis as part of the ALFALFA survey, highlighting a new approach to finding extremely metal-deficient galaxies.

Contribution

It presents the first direct oxygen abundance measurement of AGC 198691, establishing it as the most metal-poor star-forming galaxy in the local universe, and demonstrates the effectiveness of HI surveys in discovering such galaxies.

Findings

AGC 198691 has an oxygen abundance of 12+log(O/H) = 7.02.

ALFALFA survey has a high yield of extremely metal-deficient galaxies.

Discovery suggests a paradigm shift in searching for metal-poor galaxies.

Abstract

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.