High Velocity Cloud Edges and Mini-HVCs

Abstract

Arecibo mapping is reported of the neutral hydrogen distribution along selected directions out from the centers of two small High Velocity Clouds (HVC), W486 and W491. Both HVCs have a small inner region where the neutral hydrogen column density N_HI decreases slowly and a larger outer region where N_HI declines more rapidly, smoothly and exponentially from ~ 2 X 10^19 atoms cm^-2 down to < 10^18 atoms cm^-2. Line widths, and presumably temperature and turbulence, do not increase in the outermost regions. Therefore pressure decreases smoothly, making confinement by dark matter gravity more likely than confinement by external pressure. The more extended HVC, W491, has a superimposed small cloud (which we dub a ``mini-HVC''), offset by 66 km s^-1 in velocity along the line of sight with peak column density about 5 X 10^18 atoms cm^-2. Preliminary data toward future mapping of two more HVCs reveals two more mini-HVCs of similarly small size and central column density a bit less than 1 X 10^19 atoms cm^-2. We suggest that these three mini-HVCs are not physically associated with the HVCs on which they are superimposed, but are either very small outlyers of the extended Magellanic Stream HVC complex or more distant and/or smaller isolated CHVCs. The value of N_HI at the point where the neutral and ionized column densities are equal is ~ 2 X 10^19 atoms cm^-2 for the two mapped HVCs. Therefore the angular scalelength of the total hydrogen is appreciably larger than the observed HI scalelength. Previous distance estimates, related to absolute size and mass of the total hydrogen cloud, may have to be scaled down because of the undetected, more extended ionized hydrogen.

24 pages, 8 figures, ApJ in press

References in corpus (11)

• Thermal Conduction in Clusters of Galaxies
• HIPASS High-Velocity Clouds: Properties of the Compact and Extended Populations
• The Diversity of High- and Intermediate-Velocity Clouds: Complex C versus IV Arch
• A Very Sensitive 21cm Survey for Galactic High-Velocity HI
• A Targeted Survey for HI Clouds in Galaxy Groups
• Lyman Continuum Escape from Inhomogeneous ISM
• All-Stokes Parameterization of the Main Beam and First Sidelobe for the Arecibo Radio Telescope
• An HI survey of the Centaurus and Sculptor Groups - Constraints on the space density of low mass galaxies
• Arecibo imaging of compact high-velocity clouds
• The effects of an ionizing background on the HI column density distribution in the local Universe
• The Fine Structure and Outskirts of DDO 154

Cited by in corpus (3)

• Neutral Hydrogen Mapping of Virgo Cluster Blue Compact Dwarf Galaxies
• Mini High Velocity Clouds
• The HI Detection of Low Column Density Clouds and Galaxies