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The Magellanic System's Interactive Formations
M.E. Putman, PASA, 17 (1), 1.

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Is it all Magellanic debris?

The ability of HIPASS to uncover the Leading Arm leads to speculation about the origin of the other HI clouds about the Magellanic System. An important class of HVC that HIPASS is successful in revealing is the compact HVCs (CHVCs) which surround the Magellanic Stream and Leading Arm (see Figures 2 & 3). Are these objects additional interactive debris or possibly distant remnants of the Local Group (Blitz et al. 1999; Braun & Burton 1999)? The majority of the compact HVCs follow the Magellanic complexes in position and velocity, suggesting they are related to the Stream or Leading Arm; however, between declinations $-30^{\circ}$ to $-40^{\circ}$ the CHVCs have a much larger velocity dispersion ( $\sim 500$ km ${\rm s}^{-1}$ ; from v

$_{lsr} \approx -250$ to 250 km ${\rm s}^{-1}$ ) than the narrow dispersion of the Stream material ( $\sim 80$ km ${\rm s}^{-1}$ ). This declination range also encompasses the southern and near side (at $\sim 2$ Mpc) of the Sculptor Group of galaxies. Figure 4 shows the complexity of this region, with the positive velocity CHVCs mixed in amongst the galaxies while the Magellanic Stream continues southwards.

These results are particularly interesting when one considers the recent distance determination work of Jerjen, Freeman and Binggeli (1998) which indicates that the Sculptor Group is not actually a separate group, but an extension of the Local Group, forming a 6 x 2 Mpc filament of galaxies with M31 and the Milky Way at one end². The compact HVCs along this sightline do not fit into the normal distribution of Magellanic Stream material, and could lie anywhere along this filament of galaxies. At a distance of 2 Mpc most of the clouds would have HI masses of $\sim 10^7$ M $_{\odot}$ and diameters of $\sim 20$ kpc. The clouds may represent leftover galactic building blocks and the local equivalent of the Ly-limit absorber systems (e.g. Stocke et al. 1995). They also fit nicely into hierarchical structure formation simulations which predict that there should be more satellites associated with the Local Group galaxies than is currently observed (e.g. Moore et al. 1999).

The Sculptor Group sightline may be a nearby tracer of the cosmic web and should be investigated more closely. H $_\alpha$ observations are in progress and will provide significant insight into the origin of the compact HVCs. If the CHVCs are within a 100 kpc radius from the Galactic Plane they should be detectable, as ionizing photons from the Milky Way can easily reach this radius (Bland-Hawthorn & Maloney 1999a). This is shown through the H $_\alpha$ detections of the Magellanic Stream and other HVCs which have been detected at the 0.2 - 0.4 Rayleigh level (Weiner & Williams 1996; Tufte, Reynolds and Haffner 1998). If the population of compact HVCs shown here are truly extragalactic objects at Mpc distances they should not be detectable in H $_\alpha$ .

**Figure 4:** The Magellanic Stream meets the Sculptor Group. A peak HI intensity map including velocities from 73 - 450 km ${\rm s}^{-1}$ . Several galaxies of the Sculptor Group are labeled, as well as the positions of two nearby dwarf galaxies. The bifurcation of the Magellanic Stream and the clumps which follow the Stream are also depicted. The contours are 0.4, 0.8, 1.6, 3.2 and 6.4 K.
$\begin{figure} \centerline{\psfig{figure=scul_lab.ps,height=6in,width=6in,bbllx=10pt,bburx= 650pt,bblly=200pt,bbury=850pt,clip=}} \end{figure}$

Next Section: Conclusions
Title/Abstract Page: The Magellanic System's Interactive
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Contents Page: Volume 17, Number 1

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