The Milky Way

The 21 cm line emission of the neutral hydrogen was predicted in 1945 and detected in 1951, soon after the technology became available for its observation. Radio waves allow to see through the dust. Moreover, the narrow 21 cm line allows an accutare kinematical analysis of the emitting region. Since hydrogen is the most abundant element this detection had a major impact to Astronomy.

In 1956 the 25 m Stockert radiotelescope was built and the radioastronomical institute of the university of Bonn was founded. Since then galactic radioastronomy was one of the major research interests of our institute. Even today a significant fraction of our scientific interest is still in this field of research. The HI group focuses on
- the multiphase composition of the interstellar medium (ISM),
- the distribution of the ISM phases within the Milky Way,
the Magellanic System, and nearby galaxies,
- the physics of the gaseous Galacic halo and the disk-halo interaction,
- large scale mass distribution and gravitational potential of the Milky Way using gas as a tracer element.

OBSERVATIONS

To approch these scientific topics it is of utmost importance to have reliable data bases. The 21 cm line emission from the Milky Way is observable anywhere on the sky. Since a radio telescope receives a considerable fraction of stray radiation outside its main beam it is mandatory for high precision observations to determine this stray radiation and remove it from the observations. Since 25 years we know how to do this.

Most of our research is based on the Effelsberg 100 m telescope but within the last years a significant fraction of our effort was spent in observations and data processing for an all sky survey. Combining observations from the Dwingeloo 25 m telescope and the Argentinian 30 m telescope at Villa Elisa, we were able to produce an all sky survey of the Galactic 21 cm line emission with an unprecendented accuracy. Observations and data processing in total took almost a decade, the final release of the survey is soon to come. Below we proudly present a channelmap at a velocity of -37.5 km/s and for comparison an antenna temperature map without removal of the stray radiation from the telescope. The patchy structures are due to stray radiation.

2 plots .... Fig: Galactic 21 cm line emission at V = -37.5 km/s for Galactic latitudes |b| < 80 degrees. The upper plot shows observations after calbration and removal of interferences, the lower one the final result after correcting for stray radiation.

THE GASEOUS GALACTTIC HALO AND THE DARK MATTER DISTRIBUTION IN THE MILKY WAY

Since long it is known that most of the HI gas in the Milky Way belongs to a phase which is called warm neutral medium. This gas has temperatures of a few hundred to a few thousand K and is widely distributed in the Milky Way with scale heights of a few hundred pc. Since long, research in this field has been one of the major activities of our institute.

Very sensitive HI observations which became available only recently show weak but extended wings in the emission profiles. Such faint wings are observable at nearly all directions. In consequence, this part of the HI emission has to be interpreted as an even more extended gaseous phase, tentatively associated with the Milky Way halo.

To verify the significance of the faint profile wings a deep knowledge of instrumental effects was necessary. However, to understand these emission features also further observations were needed. Analysing the ROSAT data base, our group found evidence for a soft X-ray background due to a plasma with a temperature of 1.5 million K. The fact that the observed HI emission is anti-correlated with the soft X-ray background can be explained only if the soft X-ray emitting plasma is located in the Galactic halo with scale heights of 4 kpc. This is just the right scale height which is needed to explain the HI profile wings. Our assumption was therefore that the observed HI wings are caused by HI condensations within a diffuse gaseous plasma, both phases associated with the Galactic halo. Only recently, the clumpy nature of the faint wings in the HI profiles was confirmed by sensitive observations with the new Green Bank Radio Telescope (GBT).

The observational evidence for a gaseous halo was rather unexpected. The gas in the halo needs to withstand the Galactic gravitational potential which tends to pull the material down to the Galactic disk. To explain the observations we needed to assume that both, the large scale mass distribution of the Milky Way, as well as the gaseous halo are in a steady state. In turn, a hydrostic approach can be used to model the gaseous halo and the gas distribution can be understood as a tracer of the gravitational potential. At this point dark matter comes into play. The Bonn model is based on a mass distribution which appears to be described best by an isothermal dark matter disk. All kind of observational constraints were found to be consistent with such a model, in particular the scaleheights of the flaring gas layer. Leonidas Dedes is busy with an analysis of the HI scaleheights using the new HI all sky survey.

Selected Publications

Co-Investigators

Acknowledgements