1999/12/20 by Robert Braun, Róbert Braun, Braun, Robert +2
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics (astro-ph) #Astrophysics and Star Formation Studies #FOS: Physical sciences #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.48550/arxiv.astro-ph/9912417
23 page LaTeX, 15 PS figures. Submitted to A&A
arxiv created 1999/12/20 · openalex publication_date 1999/12/20 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/04
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)