Introduction

High-velocity clouds (HVCs) are concentrations of neutral hydrogen which have velocities forbidden by simple models of Galactic rotation. Their velocities, with respect to the Local Standard of Rest, typically fall in the range of

km s^-1, depending on the direction of observation.¹Despite covering 37% of the sky (Murphy, Lockman & Savage 1995), a consensus has yet to be reached on their origin. Several models have been proposed, both Galactic (e.g. supernova shells, warped structure in the disk, Galactic fountain infall/outflow) and extragalactic (e.g. remnants of the formation of Local group, tidally-disrupted material from the Magellanic Clouds or other dwarf galaxy companions), but theory alone has not been able to discriminate between these scenarios (and as we discuss here, ascribing the entire HVC population to one in particular would probably be a mistake). Wakker & van Woerden (1997) provide a timely review of the subject, both from an observational and theoretical point of view. In principle, observational tests can be devised which can discriminate (roughly) between Galactic and extragalactic models, but to date, the implementation of such tests (e.g. direct distance determinations, metallicity, or emission measure properties) has proven difficult. Each of these observational tests depends on an understanding of the spatial and kinematical structure of HVCs, and while such high-quality HI data does exist in the north (e.g. Hulsbosch & Wakker 1988; Hartmann & Burton 1997), the southern sky has not enjoyed the same status.

To date the most complete investigation in the south is due to Bajaja et al. (1985, hereafter B85), which sampled the sky on a

grid with the IAR 30m at Villa Elisa. Next year should see the release of the updated Villa Elisa Southern Sky Survey (VESSS, Morras et al. 1999), the southern analog to the Leiden-Dwingeloo Survey (LDS, Hartmann & Burton 1997), but for now one is restricted to the B85 survey for southern HVCs (and indeed, this was the data source for the southern half of the canonical catalog of HVCs by Wakker & van Woerden 1991). The B85 spatial sampling and beam size is wholly unsatisfactory for unveiling the presence of fine-scale structure and any large population of Compact HVCs (henceforth, CHVCs). In the subsequent sections, we present results from the first complete, fully sampled, southern sky survey for HVCs. Our initial results reveal intricate detail in the spatial structure of southern HVCs, a large population of undiscovered CHVCs, and filamentary connections between previously assumed discrete clumps.

© Copyright Astronomical Society of Australia 1997