(Almost) Dark Galaxies in the ALFALFA Survey: Isolated HI Bearing Ultra
  Diffuse Galaxies
  We present a sample of 115 very low optical surface brightness, highly
extended, HI-rich galaxies carefully selected from the ALFALFA survey that have
similar optical absolute magnitudes, surface brightnesses, and radii to
recently discovered "ultra-diffuse" galaxies (UDGs). However, these systems are
bluer and have more irregular morphologies than other UDGs, are isolated, and
contain significant reservoirs of HI. We find that while these sources have
normal star formation rates for HI selected galaxies of similar stellar mass,
they have very low star formation efficiencies. We further present deep optical
and HI synthesis follow up imaging of three of these HI-bearing ultra-diffuse
sources. We measure HI diameters extending to ~40 kpc, but note that while all
three sources have large HI diameters for their stellar mass, they are
consistent with the HI mass - HI radius relation. We further analyze the HI
velocity widths and rotation velocities for the unresolved and resolved sources
respectively, and find that the sources appear to inhabit halos of dwarf
galaxies. We estimate spin parameters, and suggest that these sources may exist
in high spin parameter halos, and as such may be potential HI-rich progenitors
to the ultra-diffuse galaxies observed in cluster environments.
