Summary of Recent Progress in Understanding HVCs

The study of high-velocity clouds has progressed much since the appearance of the review article by Wakker & van Woerden (1997), less than two years ago. Much of this progress is described in these workshop proceedings. Here we update the review article, summarizing the topics discussed at the workshop as well as covering the recent literature. We follow the outline of the review, describing HI properties in Sect. 2 and 3, interactions between HVCs and other gas in Sect. 4, observations at wavelengths other than 21-cm in Sects. 5 and 6, absorption-line studies of metallicities and distances in Sects. 7 and 8, extragalactic HVCs in Sect. 9, the Magellanic Stream in Sect. 10, and a discussion of HVC origins in Sect. 11. Key contributions of the past two years include (a) the first determination of a distance bracket for an HVC: d=4–10 kpc for complex A (van Woerden et al. 1999); (b) the first determinations of a true metallicity: ∼0.25 Z⊙ for HVC 287+22+240 (Lu et al. 1998), and ∼0.1 Z⊙ for complex C (Wakker et al. 1999); (c) the recognition of a leading counterpart to the Magellanic Stream (Gardiner & Noguchi 1996, Lu et al. 1998, Putman et al. 1998), indicative of a tidal origin; (d) the quickly growing number of optical emission line observations (primarily Hα) of HVCs, led by Tufte et al. (1998) and Bland-Hawthorn et al. (1998); and (e) the proposal by Blitz et al. (1998) that HVCs are intergalactic remnants of the formation of the Local Group.