The ALFALFA Almost-Dark Galaxy AGC~229101: A Two Billion Solar Mass HI
  Cloud with a Very Low Surface Brightness Optical Counterpart
  We present results from deep HI and optical imaging of AGC 229101, an unusual
HI source detected at v$_{\rm helio}$ = 7116 km/s in the ALFALFA survey.
Initially classified as a candidate "dark" source because it lacks a clear
optical counterpart in SDSS or DSS2 imaging, AGC 229101 has $10^{9.31\pm0.05}$
solar masses of HI, but an HI line width of only 43$\pm$9 km/s. Low resolution
WSRT imaging and higher resolution VLA B-array imaging show that the source is
significantly elongated, stretching over a projected length of ~80 kpc. The HI
imaging resolves the source into two parts of roughly equal mass. WIYN pODI
optical imaging reveals a faint, blue optical counterpart coincident with the
northern portion of the HI. The peak surface brightness of the optical source
is only $\mu_{g}$ = 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
$10^{7.32\pm0.33}$ solar masses, yielding an overall neutral gas-to-stellar
mass ratio of M$_{\rm HI}$/M$_*=$~98$_{+111}\atop^{-52}$. We demonstrate the
extreme nature of this object by comparing its properties to those of other
HI-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 HI clouds.
