(Almost) Dark HI Sources in the ALFALFA Survey: The Intriguing Case of
  HI1232+20
  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 Msun/Lsun,
and has an HI mass of 7.2*10^8 Msun. The other two HI sources (with HI masses
2.0*10^8 and 1.2*10^8 Msun) 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 Msun/Lsun. 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.
