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 \sim 26.4 mag/arcsec ^ { 2 } in g ^ { \prime } , which is exceptionally faint .
This source ( AGC 229385 ) has the largest accurately measured HI mass-to-light ratio of an isolated object : M _ { HI } / L _ { g ^ { \prime } } = 46 M _ { \odot } / L _ { \odot } , and has an HI mass of 7.2 \times 10 ^ { 8 } M _ { \odot } .
The other two HI sources ( with HI masses 2.0 \times 10 ^ { 8 } and 1.2 \times 10 ^ { 8 } M _ { \odot } ) without optical counterparts have upper limit surface brightnesses of 27.9 and 27.8 mag/arcsec ^ { 2 } in g ^ { \prime } , and lower limits on their gas mass-to-light ratio of M _ { HI } / L _ { g ^ { \prime } } > 57 and > 31 M _ { \odot } / L _ { \odot } .
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 \sim 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 .