ALFALFA Almost Dark galaxies

Results from: Janowiecki et al. 2015, ApJ, 801, 96 (arXiv)

The Arecibo Legacy Fast ALFA (ALFALFA) survey has used the largest radio telescope in the world to thoroughly search for gas-rich galaxies across 7000 square degrees. The vast majority (>99%) of galaxies discovered in this blind Hi survey have substantial stellar populations, but a small minority of objects lack obvious optical counterparts and constitute the (Almost) Dark galaxy sample (Cannon, 2015, AJ, 149, 72). A team of us has been following up with more sensitive and higher resolution optical imaging and Hi interferometery to understand the nature of these unusual objects. The most extreme (Almost) Dark to date, “Coma P,” is shown below (Janowiecki et al. 2015, ApJ, 801, 96). While no genuine dark galaxies have yet been identified, Coma P has a HI-to-stellar mass ratio of >200, and a gas distribution which extends significantly beyond its stellar light. Recent HST imaging has allowed us to fit its red giant branch and measure its distance.