1999A&A...341..437B

We have identified a class of high-velocity clouds which are compact and apparently isolated. The clouds are compact in that they have angular sizes less than 2 degrees FWHM. They are isolated in that they are separated from neighboring emission by expanses where no emission is seen to the detection limit of the available data. Candidates for inclusion in this class were extracted from the Leiden/Dwingeloo Hisurvey of Hartmann & Burton (1997, Atlas of Galactic Hydrogen) and from the Wakker & van Woerden (1991A&A...250..499W) catalogue of high-velocity clouds identified in the surveys of Hulsbosch & Wakker (1988A&AS...75..191H) and of Bajaja et al. (1985ApJS...58..143B) The candidates were subject to independent confirmation using either the 25-meter telescope in Dwingeloo or the 140-foot telescope in Green Bank. We argue that the resulting list, even if incomplete, is sufficiently representative of the ensemble of compact, isolated HVCs - CHVCs - that the characteristics of their disposition on the sky, and of their kinematics, are revealing of some physical aspects of the class. The sample is more likely to be representative of a single phenomenon than would a sample which included the major HVC complexes. We consider the deployment of the ensemble of CHVCs in terms used by others to ascertain membership in the Local Group, and show that the positional and kinematic characteristics of the compact HVCs are similar in many regards to those of the Local Group galaxies. The velocity dispersion of the ensemble is minimized in a reference frame consistent with the Local Group Standard of Rest. The CHVCs have a mean infall velocity of 100km/s in the Local Group reference frame. These properties are strongly suggestive of a population which has as yet had little interaction with the more massive Local Group members. At a typical distance of about 1Mpc these objects would have sizes of about 15 kpc and gas masses, M_HI, of a few times 10⁷M_☉, corresponding to those of (sub-)dwarf galaxies.