- The Magellanic Stream
- Structure formation in the Sculptor Group
- The high-velocity clouds of M31
- Compact high-velocity clouds around the Milky Way
- An all-sky map of high-velocity clouds
- Ca ii and Na i absorbers in the Galactic halo
Spanning over 100° across the sky, the Magellanic Stream is one of the most prominent H i structures in the vicinity of the Milky Way. It is believed to be the result of tidal interaction between the Magellanic Clouds and the Milky Way, but ram-pressure stripping may also have played an important role in forming and shaping the stream. The Magellanic Stream consists of two parts: a trailing stream, which is known as the original Magellanic Stream, and a leading stream, the so-called Leading Arm.
The aim of this project is to study the nearest spiral galaxies in the nearby Sculptor Group in neutral hydrogen with the ATCA to search for high-velocity clouds (HVCs) and extra-planar gas in their environment. HVCs are thought to be either primordial, dark-matter-dominated satellites or the remnants of tidal stripping in the vicinity of larger galaxies. Therefore, we want to use them as tracers of the formation and evolution of spiral galaxies.
If high-velocity clouds are a phenomenon occuring naturally during the formation and evolution of large spiral galaxies we would expect to find HVCs also around other galaxies. The closest large spiral galaxy comparable to our own is the Andromeda Galaxy (M31), making it an ideal target for a systematic search for HVCs. With the 100-m radio telescope at Effelsberg we mapped a large area around M31 in H i and detected several HVCs within about 50 kpc projected distance of the centre of M31. Many of them were studied in follow-up synthesis observations with the WSRT.
High-velocity clouds are gas clouds with high radial velocities being incompatible with a participation in the regular rotation of our Milky Way. Since their discovery in 1963 many hypotheses about their origin and distribution have been discussed. With the 100-m radio telescope at Effelsberg we studied several compact high-velocity clouds in the 21-cm line emission of H i to study their morphology and properties in more detail. Our results demonstrate that most CHVCs have a rather complex morphology probably related to ram-pressure interaction of the clouds with an ambient medium.
High-velocity clouds are not homogeneously distributed across the sky. Instead, they are forming large complexes, some of which extend over tens of degrees. The most famous and extended complexes in the northern hemisphere are complex A, C, and M, the first two of which were among the first HVCs ever detected by Muller, Oort, and Raimond (1963) with the 25-m radio telescope near Dwingeloo. The southern hemisphere is dominated by the H i emission of the Magellanic Clouds, the Magellanic Stream, and the Leading Arm.
During the last decades, absorption and emission line measurements have demonstrated that the Milky Way is surrounded by a complex, multi-phase gaseous halo. Embedded in a corona of million-degree gas, neutral and ionised gas clouds move with high radial velocities through the Milky Way halo, giving rise to the population of IVCs and HVCs. Using data taken with the UVES spectrograph at the VLT, we detected high-velocity Ca ii and Na ii absorption lines towards 8 out of 13 quasars, suggesting that the Milky Way halo contains a population of low column density neutral gas clumps.