Research
Overview
- 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
The Magellanic Stream
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.
Structure formation in the Sculptor Group
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.
The high-velocity clouds of M31
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.
Compact high-velocity clouds around the Milky Way
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.
An all-sky map of high-velocity clouds
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.
Ca ii and Na i absorbers in the Galactic halo
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.