2002ApJ...576..232H

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_H I_ decreases slowly and a larger outer region where N_H I_ declines more rapidly, smoothly, and exponentially, from ∼2x10¹⁹ down to less than 10¹⁸ 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 5x10¹⁸atoms.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 1x10¹⁹atoms.cm^–2, offset by an even larger velocity of ∼98 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_H I_ at the point at which the neutral and ionized column densities are equal, is discussed. With N_1/2∼2x10¹⁹atoms.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.