ar X iv : a st ro - p h / 02 05 40 4 v 1 2 3 M ay 2 00 2 ApJ in press High Velocity Cloud Edges and Mini - HVCs

Arecibo mapping is reported of the neutral hydrogen distribution along selected directions out from the centers of two examples of small High Velocity Clouds (HVC). One HVC (W486) is selected from the class of Compact HVCs (CHVCs) thought by some researchers to be good candidates for having distances characteristic of the Local Group; the other (W491) is a bit more extended and possibly nearer. Both HVCs have a small inner region where the neutral hydrogen column density NHI decreases slowly and a larger outer region where NHI declines more rapidly, smoothly and exponentially from ∼ 2 × 10 atoms cm down to < 10 atoms cm. 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 in velocity along the line of sight. The peak column density of the mini-HVC is about 5×10 atoms cm. Preliminary data toward future mapping of two more HVCs reveals two more mini-HVCs of