Searching for Optical Counterparts to Ultra-compact High Velocity
  Clouds: Possible Detection of a Counterpart to AGC 198606
  We report on initial results from a campaign to obtain optical imaging of a
sample of Ultra Compact High Velocity Clouds (UCHVCs) discovered by the ALFALFA
neutral hydrogen (HI) survey. UCHVCs are sources with velocities and sizes
consistent with their being low-mass dwarf galaxies in the Local Volume, but
without optical counterparts in existing catalogs. We are using the WIYN 3.5-m
telescope and pODI camera to image these objects and search for an associated
stellar population. In this paper, we present our observational strategy and
method for searching for resolved stellar counterparts to the UCHVCs. We
combine careful photometric measurements, a color-magnitude filter, and spatial
smoothing techniques to search for stellar overdensities in the g- and i-band
images. We also run statistical tests to quantify the likelihood that whatever
overdensities we find are real and not chance superpositions of sources. We
demonstrate the method by applying it to two data sets: WIYN imaging of Leo P,
a UCHVC discovered by ALFALFA and subsequently shown to be a low-mass
star-forming dwarf galaxy in the Local Volume, and WIYN imaging of AGC198606,
an HI cloud identified by ALFALFA that is near in position and velocity to the
Local Group dwarf Leo T. Applying the search method to the Leo P data yields an
unambiguous detection (>99% confidence) of the galaxy's stellar population.
Applying our method to the AGC198606 imaging yields a possible detection (92%
confidence) of an optical counterpart located ~2.5 arc minutes away from the
centroid of AGC198606's HI distribution and within the HI disk. We use the
optical data to estimate a distance to the stellar counterpart between 373 and
393 kpc, with an absolute magnitude M_i = -4.67+/-0.09. Combining the WIYN data
with our previous estimate of the HI mass of AGC198606 from WSRT imaging yields
an HI-to-stellar mass ratio of ~45-110.
