The ALFALFA Almost Dark Galaxy AGC 229101: A 2 Billion Solar Mass H i 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 v 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 109.31?0.05 M o˙ 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 ∼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 μ g ∼ 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 107.32?0.33 M o˙, 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.