2000A&A...354..853B

Six examples of the compact, isolated HIhigh-velocity clouds (CHVCs) identified by Braun & Burton (1999A&A...341..437B), but only marginally resolved in single-dish data, have been imaged with the Westerbork Synthesis Radio Telescope. The 65 confirmed objects in this class define a dynamically cold system, with a global minimum for the velocity dispersion of only 70km/s, found in the Local Group Standard of Rest. The population is in-falling at 100km/s toward the Local Group barycenter. These objects have a characteristic morphology, in which one or more compact cores is embedded in a diffuse halo. The compact cores typically account for 40% of the HIline flux while covering some 15% of the source area. The narrow line width of all core components allows unambiguous identification of these with the cool condensed phase of HI, the CNM, with kinetic temperature near 100 K, while the halos appear to represent a shielding column of warm diffuse HI, the WNM, with temperature near 8000K. We detect a core with one of the narrowest HIemission lines ever observed, with intrinsic FWHM of no more than 2km/s and 75 K brightness. From a comparison of column and volume densities for this feature we derive a distance in the range 0.5 to 1Mpc. We determine a metallicity for this same object of 0.04 to 0.07 solar. Comparably high distances are implied by demanding the stability of objects with multiple cores, which show relative velocities as large as 70km/s on 30 arcmin scales. Many of the compact cores show systematic velocity gradients along the major axis of their elliptical extent which are well-fit by circular rotation in a flattened disk system. Two out of three of the derived rotation curves are well-fit by Navarro, Frenk & White (1997ApJ...490..493N) cold dark matter profiles. These kinematic signatures imply a high dark-to-visible mass ratio of 10-50, for D=0.7Mpc, which scales as 1/D. The implied dark matter halos dominate the mass volume density within the central 2kpc (10 arcmin) of each source, providing a sufficent hydrostatic pressure to allow CNM condensation. The CHVC properties are similar in many respects to those of the Local Group dwarf irregular galaxies, excepting the presence of a high surface brightness stellar population.