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Abstract 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 10 9.31±0.05 M ⊙ 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 10 7.32±0.33 M ⊙ , 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. 

The ALFALFA blind HI survey has populated the low-mass end of the HI mass function for the first time, allowing an unprecedented opportunity to explore the physical properties of the galaxies that inhabit this extreme portion of parameter space. Using the now-complete ALFALFA dataset, we have constructed the "Survey of HI in Extremely Low-mass Dwarfs" ("SHIELD"), a complete sample of 82 galaxies that provides a unique opportunity to further our understanding of these cosmologically important systems. Here we present new VLA imaging of the final 3 of the 82 sources to be observed at low angular resolution in the HI spectral line: AGC215284, AGC731448, and AGC732041 (program VLA/18A-177). We compare the HI images to ground-based H-alpha imaging in order to examine the star formation properties of each system.