Recent 21 cm Arecibo observations have revealed a population of isolated Ultra-Compact High-Velocity HI Clouds (UCHVCs) (Giovanelli et al. 2010; Adams et al. 2013; ALFALFA survey). One of the UCHVCs was recently discovered to be an optically ultra-faint galaxy, now dubbed Leo P (Giovanelli et al 2013; Rhode et al 2013). Leo P is similar in its properties to another dwarf galaxy - Leo T, a satellite of the Milky Way, at a distance of 420 kpc.

We explore the idea that many of the UCHVCs consitute a population of ultra-faint but gas-rich dwarf galaxies in the Local Group, as yet undetected optically. We model the UCHVCs as clouds confined in low-mass dark matter (DM) minihalos. We assume flat-core DM profiles and show analytically that this naturally reproduces the common mass scale observed in (Sloan) low-luminosity Milky Way satellites (Strigari et al. 2008) and also resolves the “Too-Big-To-Fail” problem (Boylan-Kolchin et al. 2011).
We model the HI gas distributions and show how high-resolution 21 cm observations can be used to estimate (presently unknown) distances to the UCHVCs. Alternatively, if the distances to the UCHVCs are measured by some other method, HI column density profiles can be used to constrain the DM halo properties. We use 21 cm observations of Leo T as a test case for our method. The discovery of Leo P supports our theory.