(ALMOST) Dark HI sources in the alfalfa survey: The intriguing case of HI1232+20

© 2015. The American Astronomical Society. All rights reserved. We report the discovery and follow-up observations of a system of three objects identified by the ALFALFA extragalactic HI survey, cataloged as (almost) dark extragalactic sources, i.e., extragalactic HI detections with no discernible counterpart in publicly available, wide-field, imaging surveys. We have obtained deep optical imaging with WIYN pODI and HI synthesis maps with WSRT of the HI1232+20 system. The source with the highest HI flux has a newly discovered ultra-low surface brightness (LSB) optical counterpart associated with it, while the other two sources have no detected optical counterparts in our images. Our optical observations show that the detected LSB optical counterpart has a peak surface brightness of ∼26.4 mag arcsec-2 in g′, which is exceptionally faint. This source (AGC 229385) has the largest accurately measured HI mass-to-light ratio of an isolated object: MHI Lg = 46 M⊙ L⊙, and has an HI mass of 7.2 × 108 M⊙. The other two HI sources (with HI masses 2.0 × 108 and 1.2 × 108 M⊙) without optical counterparts have upper limit surface brightnesses of 27.9 and 27.8 mag arcsec-2 in g′, and lower limits on their gas mass-to-light ratio of MHI/Lg> 57 and > 31 M⊙ L⊙ . This system lies relatively close in projection to the Virgo Cluster, but velocity flow models indicate that it is located at 25 Mpc, substantially beyond Virgo. The system appears to be quite isolated, with no known object closer than ∼500 kpc. These HI sources may represent both sides of the threshold between "dark" star-less galaxies and galaxies with stellar populations. We discuss a variety of possible formation scenarios for the HI1232+20 system.