  The discovery of a previously unknown 21cm HI line source identified as an
ultra-compact high velocity cloud in the ALFALFA survey is reported. The HI
detection is barely resolved by the Arecibo 305m telescope ~4' beam and has a
narrow HI linewidth (HPFW of 24 km/s). Further HI observations at Arecibo and
with the VLA corroborate the ALFALFA HI detection, provide an estimate of the
HI radius, ~1' at the 5 x 10^19 cm^-2 isophote, and show the cloud to exhibit
rotation with an amplitude of ~9.0 +/- 1.5 km/s. In other papers, Rhode et al.
(2013) show the HI source to have a resolved stellar counterpart and ongoing
star forming activity, while Skillman et al. (2013) reveal it as having
extremely low metallicity: 12 + log(O/H) = 7.16 +/- 0.04. The HI mass to
stellar mass ratio of the object is found to be 2.6. We use the Tully-Fisher
template relation in its baryonic form (McGaugh 2012) to obtain a distance
estimate D = 1.3 (+0.9,-0.5) Mpc. Additional constraints on the distance are
also provided by the optical data of Rhode et al. (2013) and McQuinn et al.
(private communication), both indicating a distance in the range of 1.5 to 2.0
Mpc. The three estimates are compatible within their errors. The object appears
to be located beyond the dynamical boundaries of, but still in close proximity
to the Local Group. Its pristine properties are consistent with the sedate
environment of its location. At a nominal distance of 1.75 Mpc, it would have
an HI mass of ~1.0 x 10^6 Msun, a stellar mass of ~3.6 x 10^5 Msun, and a
dynamical mass within the HI radius of ~1.5 x 10^7 Msun. This discovery
supports the idea that optically faint - or altogether dark - low mass halos
may be detectable through their non-stellar baryons.
