High-velocity cloud edges and mini high-velocity clouds

Arecibo mapping is reported of the neutral hydrogen distribution along selected directions out from the centers of two examples of small high-velocity clouds (HVCs). 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 N(HI) decreases slowly and a larger outer region where N(HI) declines more rapidly, smoothly, and exponentially, from similar to2 x 10(19) down to less than 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. The peak column density of the mini-HVC is about 5 x 10(18) atoms cm(-2). Preliminary data toward future mapping of two more HVCs reveal two more mini-HVCs of similarly small size and central column density a bit less than 1 x 10(19) atoms cm(-2), offset by an even larger velocity of similar to98 km s(-1). We suggest that these three mini-HVCs are not physically associated with the HVCs on which they are superimposed but are either very small outliers of the extended Magellanic Stream HVC complex or more distant and/or smaller isolated CHVCs. The importance of the edge column density N(1/2), the value of N(HI) at the point at which the neutral and ionized column densities are equal, is discussed. With N(1/2) similar to 2 x 10(19) atoms cm(-2) for the two mapped HVCs, the angular scale length of the total hydrogen is appreciably larger than the observed H I scale length. 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.