  Six examples of the compact, isolated high-velocity HI clouds (CHVCs)
identified by Braun and Burton (1999) have been imaged with the WSRT. The 65
confirmed objects in this class define a dynamically cold system, with a global
minimum for the velocity dispersion of only 70 km/s, found in the Local Group
Standard of Rest, while in-falling at 100 km/s toward the LG barycenter. These
objects have a characteristic morphology, in which several compact cores are
embedded in a diffuse halo. The compact cores typically account for 40% of the
HI line flux while covering some 15% of the source area. The cores are the cool
condensed phase of HI, the CNM, with temp. near 100 K, while the halos appear
to be a shielding column of warm diffuse HI, the WNM, with temp. near 8000 K.
We detect a core with one of the narrowest HI emission lines ever observed,
with intrinsic FWHM of 2 km/s and 75 K brightness. From a comparison of column
and volume densities we derive a distance in the range 0.5 to 1 Mpc. 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 70 km/s on 30 arcmin scales.
Many compact cores show systematic velocity gradients along the major axis of
their elliptical extent which are consistent with circular rotation. Several of
the derived rotation curves are well-fit by Navarro, Frenk, and White (1997)
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 2
kpc (10 arcmin) of each source, providing a sufficent hydrostatic pressure to
allow local CNM condensation. (abridged)
