The ALFALFA Almost Dark Galaxy AGC 229101: A 2 Billion Solar Mass HI Cloud with a Very Low Surface Brightness Optical Counterpart

We present results from deep H I and optical imaging of AGC 229101, an unusual H I source detected at nu(helio) =7116 km s(-1) in the Arecibo Legacy Fast ALFA (ALFALFA) blind H I survey. Initially classified as a candidate "dark" source because it lacks a clear optical counterpart in Sloan Digital Sky Survey (SDSS) or Digitized Sky Survey 2 (DSS2) imaging, AGC 229101 has 10(9.31 +/- 0.05) M-circle dot of H I, but an H I line width of only 43 +/- 9 km s(-1). Low-resolution Westerbork Synthesis Radio Telescope (WSRT) imaging and higher-resolution Very Large Array (VLA) B-array imaging show that the source is significantly elongated, stretching over a projected length of similar to 80 kpc. The H I imaging resolves the source into two parts of roughly equal mass. WIYN partially populated One Degree Imager (pODI) optical imaging reveals a faint, blue optical counterpart coincident with the northern portion of the H I. The peak surface brightness of the optical source is only mu(g) similar to 26.6 mag arcsec(-2), well below the typical cutoff that defines the isophotal edge of a galaxy, and its estimated stellar mass is only 10(7.32 +/- 0.33) M-circle dot, yielding an overall neutral gas-to-stellar mass ratio of M/M-* = 98(-52)(+111). We demonstrate the extreme nature of this object by comparing its properties with those of other H I-rich sources in ALFALFA and the literature. We also explore potential scenarios that might explain the existence of AGC 229101, including a tidal encounter with neighboring objects and a merger of two dark H I clouds.