Five Gas-rich Ultra-faint Dwarf Galaxy Candidates Discovered in WIYN
  Imaging of ALFALFA Sources
  We present results from the analysis of WIYN pODI imaging of 23 ultra-compact
high-velocity clouds (UCHVCs), which were identified in the ALFALFA HI survey
as possible dwarf galaxies in or near the Local Group. To search for a resolved
stellar population associated with the HI gas in these objects, we carried out
a series of steps designed to identify stellar overdensities in our optical
images. We identify five objects that are likely stellar counterparts to the
UCHVCs, at distances of $\sim 350$ kpc to $\sim 1.6$ Mpc. Two of the
counterparts were already described in Janesh et al. (2015) and Janesh et al.
(2017); the estimated distance and detection significance for one of them
changed in the final analysis of the full pODI data set. At their estimated
distances, the detected objects have HI masses from $2 \times 10^4$ to $3
\times 10^6$ Msun, $M_V$ from -1.4 to -7.1, and stellar masses from $4 \times
10^2$ to $4 \times 10^5$ Msun. None of the objects shows evidence of a young
stellar population. Their properties would make the UCHVCs some of the most
extreme objects in and around the Local Group, comparable to ultra faint dwarf
galaxies in their stellar populations, but with significant gas content. Such
objects probe the extreme end of the galaxy mass function, and provide a
testbed for theories regarding the baryonic feedback processes that impact star
formation and galaxy evolution in this low-mass regime.
