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High-resolution imaging of compact high-velocity clouds
Received 4 October 1999 / Accepted 22 December 1999
Six examples of the compact, isolated H I high-velocity clouds (CHVCs) identified by Braun & Burton (1999), 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 70 km s-1, found in the Local Group Standard of Rest. The population is in-falling at 100 km s-1 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 H I line 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 H I , the CNM, with kinetic temperature near 100 K, while the halos appear to represent a shielding column of warm diffuse H I, the WNM, with temperature near 8000 K. We detect a core with one of the narrowest H I emission lines ever observed, with intrinsic FWHM of no more than 2 km s-1 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 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-1 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 (1997) cold dark matter profiles. These kinematic signatures imply a high dark-to-visible mass ratio of 10-50, for Mpc, which scales as . 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 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.
Key words: ISM: clouds Galaxy: kinematics and dynamics galaxies: abundances galaxies: ISM galaxies: Local Group
Send offprint requests to: R. Braun (firstname.lastname@example.org)
Online publication: February 25, 2000