2013AJ....146...15G

The discovery of a previously unknown 21 cm H I line source identified as an ultra-compact high velocity cloud in the ALFALFA survey is reported. The H I detection is barely resolved by the Arecibo 305 m telescope ∼4' beam and has a narrow H I linewidth (half-power full width of 24 km/s). Further H I observations at Arecibo and with the Very Large Array corroborate the ALFALFA H I detection, provide an estimate of the H I radius, ∼1' at the 5x10¹⁹/cm2 isophote, and show the cloud to exhibit a velocity field which, if interpreted as disk rotation, has an amplitude of ≃9.0±1.5 km/s. In other papers, Rhode et al. show the H I source to have a resolved stellar counterpart and ongoing star forming activity, while Skillman et al. reveal it as having extremely low metallicity: 12 + log (O/H) = 7.16±0.04. The H I 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 to obtain a distance estimate D_Mpc = 1.3_–0.5^+0.9. Additional constraints on the distance are also provided by the optical data of Rhode et al. and McQuinn et al., 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 H I mass of ≃ 1.0x10⁶ M_☉, a stellar mass of ≃ 3.6x10⁵ M_☉, and a dynamical mass within the H I radius of ≃ 1.5x10⁷ M_☉. This discovery supports the idea that optically faint–or altogether dark–low mass halos may be detectable through their non-stellar baryons.