HVC Structure

Continuous streamer structure is a common phenomenon which appears in the fully sampled HIPASS data. The Magellanic Stream (which trails for 100 behind the Clouds in Figure 3) and Leading Arm, discussed in the previous section, are obvious examples. Another example of a spatially continuous filament was shown in Figure 1; this feature also has a strikingly systematic velocity structure (see Putman & Gibson 1998 or Morras & Bajaja 1983). Figure 1 (and also Figure 4) can be put into further context by examining Figure 5. Here, the filamentary structure of Figure 1 (shown at the top of Figure 5) appears to be an extension of the Leading Arm shown in Figure 4, the entire structure perhaps representing a pseudo-continuous,

-long, leading tidal feature. Such a scenario is supported by the metallicity determination for the dominant filament in Figure 1 from Lu et al. (1998), which matches that of the Magellanic Clouds. The ``kink'' in the Leading Arm above the Galactic Plane, though, appears to be difficult to recover under a tidal scenario, as well tidal models which incorporate a small degree of drag (Gardiner 1999).

Figure 5: Peak intensity map which includes regions from Figures 1 and 3, and velocities

v_lsr=165 - 400 km s^-1.

We should stress that while filamentary HI structure may be common, assigning a tidal origin to all such features would certainly be incorrect. There are many situations where the filamentary HVC actually connects to Galactic emission when it is traced back in position and velocity. This is demonstrated in Figure 6, where the knot of emission in the v_lsr=86 km s^-1 channel corresponds to HVC#350 in the Wakker & van Woerden (1991) compilation. While technically this is an HVC because of its anomalous velocity, Figure 6 makes it readily apparent that it is not an isolated HVC, but an extension of Galactic emission. This example reiterates an earlier point, that clearly not all HVCs are created by the same mechanism - while HVC#350 appears to be associated with the Galaxy, those of Figure 5 are almost certainly (predominantly) associated with tidal stripping of material from the Magellanic System. Figure 6 demonstrates that blind adoption of velocity cut-offs in any HVC catalog construction is a dangerous practice which does not necessarily take into account the ``big picture''.

Figure 6: Channel maps showing the HVC designated #350 in the Wakker & van Woerden (1991) compilation (particularly evident by the knot of emission in the v_lsr=86 km s^-1 channel (v_lsr is labelled in the upper left of each panel.)). This is clearly not an isolated HVC, per se, but one whose emission merges with the neighbouring Galactic emission.

It is tempting to associate stream-like ``HVCs'', such as those shown in Figure 6, with the continuation of the worms defined by Koo, Heiles & Reach (1992), possible supernova break-outs, or the splash-down effects of discrete HVCs. However, as noted already, continuous streams of anomalous HI, such as the Magellanic Stream and its associated Leading Arm, which are clearly separate from Galactic emission, almost certainly reflect the tidal disruption of our neighbours (both present-day and past). Perhaps the overall filamentary structure of the clouds is also related to magnetic fields which extend into the Galactic halo (Kalberla & Kerp 1998)? Dense clumps of emission, or CHVCs in the nomenclature of Braun & Burton (1999),² may represent a distinct category, populating the intergalactic medium of the Local Group, as suggested by Blitz et al. (1999). On the other hand, they often appear to be associated in position and velocity with neighbouring large-scale complexes, and they could simply be small bits of interactive debris, as demonstrated in the hydrodynamical simulations of Li & Thronson (1998). Mapping the H emissivities of these CHVCs, along with their neighbouring complexes, may provide clues as to any differences which might exist in the ionising environments in which they reside.

The complex structure of HVCs defines our need for dense spatial sampling and high resolution observations. As shown here, the Parkes Multibeam HVC Survey has the capability to greatly increase our understanding of HVC distribution and structure, with the Leading Arm feature a perfect example of its success in uncovering new features in the Galactic halo. Categorising clouds according to their spatial and velocity distribution will be a crucial future step for uncovering the origin of various high velocity complexes. We are actively pursuing a program of H emission studies, high-resolution 21cm synthesis work, and FUSE (Far Ultraviolet Spectroscopic Explorer) and GHRS (Goddard High-Resolution Spectrograph) metallicity determinations for a number of HVCs which populate a wide variety of environments. By putting these results into the context of the Parkes Multibeam HVC Survey we should be able to provide key clues to HVCs' mysterious origin(s).

© Copyright Astronomical Society of Australia 1997