2004AJ....128.2932H

Neutral hydrogen mapping of several small, very low column density high-velocity clouds (HVCs), using the Arecibo telescope, is reported. Some were found serendipitously superposed at distinct velocities on larger HVCs; others were found to comprise the four most isolated low column density sources in the observations of Lockman et al. but proved to be individually much smaller than the Green Bank 140 Foot (43. m) telescope beam. We call these clouds ``mini-HVCs'' to distinguish them from the larger and denser compact high-velocity clouds (CHVCs). Peak column densities are typically a few times 10¹⁸/cm2 averaged over the Arecibo 3'.2 beam, and diameters to our detection limit ∼10¹⁸/cm2 range from 9' to 35'. These column densities and angular diameters overlap with those for CHVCs but are typically smaller. We consider three possibilities: (1) that most mini-HVCs are related to the Magellanic Stream, (2) that most mini-HVCs are part of M31's retinue of CHVCs, and (3) that the mini-HVC are simply the low column density tail of the distribution of CHVCs. None of these possibilities can be rejected as yet, given the selection biases in our sample. We also discuss controversies about the amount of ionized hydrogen in CHVCs and mini-HVCs, which may be mainly ionized, and the implications of these small clouds for Lyman limit systems.