Ultra-compact high velocity clouds from ALFALFA

The ALFALFA UCHVCs have median flux densities of 1.34Jy.km/s, median angular diameters of 10', and median velocity widths of 23km/s. We show that the full UCHVC population cannot easily be associated with known populations of high velocity clouds.

Of the 59 clouds presented here, only 11 are also present in the compact cloud catalog extracted from the commensal GALFA-HI survey, demonstrating the utility of this separate dataset and analysis. Based on their sky distribution and observed properties, we infer that the ALFALFA UCHVCs are consistent with the hypothesis that they may be very low mass galaxies within the Local Volume. In that case, most of their baryons would be in the form of gas, and because of their low stellar content, they remain unidentified by extant optical surveys.