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

Abstract

We present results from deep HI and optical imaging of AGC 229101, an unusual HI source detected at v${}_{\mathrm{helio}}$ = 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 $10^{9.31\pm 0.05}$ 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 $10^{7.32\pm 0.33}$ solar masses, yielding an overall neutral gas-to-stellar mass ratio of M${}_{\mathrm{HI}}$/M${}_{\ast }=$~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.

18 pages, 8 figures, accepted by AJ

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