ALFALFA Discovery of the most metal-poor gas-rich galaxy known: AGC 198691

Image may be NSFW.
Clik here to view.© Alec Hirschauer (Indiana University)

Metals refer to elements other than hydrogen and helium in astronomy; these elements are formed by stars. Metal-poor galaxies lack these elements compared to other galaxies and are one of the best testbeds for understanding conditions early in the Universe. The gas out of which metal-poor galaxies are forming stars has a similar metal content as the gas forming stars in the early Universe. In addition, the amount of helium found in extremely metal-poor galaxies can be compared to predictions for the primordial abundance from Big Bang nucleosynthesis.

The above images show AGC 198681, also known as Leoncino, or "little lion", as it is found in the constellation Leo Minor. This galaxy was recently identified as being the most metal-poor gas-rich galaxy known (Hirschauer+ 2016). Leoncino was originally found by the ALFALFA HI survey and identified as a low mass dwarf galaxy. Subsequent observations of its only HII region revealed it to have an extremely low oxygen abundance, making it the lowest-abundance star-forming galaxy known in the Local Universe. The left image above shows an HST image of this source with neutral hydrogen (HI) contours from Westerbork overlaid; the right image is a zoom-in on the optical galaxy. The WSRT observations localize the ALFALFA HI detection to this galaxy. They also hint at signs of interaction in this system, although further observations are needed to confirm this.

Interestingly, two of the five lowest-abundance star-forming galaxies known have been discovered by the ALFALFA HI survey. This highlights an important parameter space for HI surveys, including the upcoming surveys with Apertif at ASTRON.

More information can be found in the paper on Leoncino: Hirschauer et al. 2016, ApJ, 822, 108.