Compact, Isolated High-velocity Clouds

Abstract

We consider the class of compact, isolated, high-velocity Hi clouds, CHVCs, which are sharply bounded in angular extent, with no kinematic or spatial connection to other Hi features down to a limiting column density of 1.5×1018 cm−2. We describe the automated search algorithm developed by de Heij et al. (2002b) and applied by them to the Leiden-Dwingeloo Survey north of δ=−28° and by Putman et al. (2002) to the Parkes HIPASS data south of δ=0°, resulting in an all-sky catalog numbering 246 CHVCs. We argue that these objects are more likely to represent a single phenomenon in a similar evolutionary state than would a sample that included any of the major HVC complexes. Five principal observables are defined for the CHVC population: (1) the spatial deployment of the objects on the sky, (2) the kinematic distribution, (3) the number distribution of observed Hi column densities, (4) the number distribution of angular sizes, and (5) the number distribution of line widths. We show that the spatial and kinematic deployments of the ensemble of CHVCs contain various clues regarding their characteristic distance. These clues are not compatible with a location of the ensemble within the Galaxy proper. The deployments resemble in several regards those of the Local Group galaxies.