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The Andromeda Galaxy (also known as "M31" in Messier's catalogue of nebulae and clusters) is a stellar system consisting of many hundred billion stars, similar to our own Milky Way. M31 is the nearest spiral galaxy in a distance of "only" 2.5 million light-years,. The angular diameter in the sky is nearly 3 degrees. With the naked eye, however, only the innermost part can be seen. M31 is an ideal object for the investigation of galactic radio radiation and can help us to understand our own galaxy much better.

Most of the radio radiation originates from a ring-like structure in approximately 30,000 light-years distance from the center of M31. There is a large number of remnants from supernova explosions, sources of "cosmic radiation" (charged particles, accelerated to almost the speed of light). The magnetic field in this ring is created by a giant dynamo, fed from the energy of the rotation of the galaxy. The electrons of the cosmic radiation gyrate on spiral orbits around magnet field lines, thereby creating radiation in radio frequencies (so-called "synchrotron radiation"). The radio map thus shows the distribution of the magnetic fields and the cosmic radiation.

In 1974, M31 was observed with the Effelsberg 100m telescope at 11cm wavelength for the first time. The angular resolution of approximately 5 arc minutes was still unsatisfactory. Highly resolved images require observations at shorter wavelength. At such wavelengths, however, the radiation becomes weaker, and bad weather conditions (clouds in the sky) strongly disturb the observations.

There is (status: 1996!) a new receiver at 6 cm wavelength available for the Effelsberg 100m telescope. Modern HEMT transistors make it the most sensitive receiver built at the Max-Planck-Institut for Radio Astronomy so far. Two horn antennas in the focal plane enable the elimination of disturbances by atmospheric clouds with a "differential" method: A cloud is seen almost simultaneous by both horns, the target galaxy, however, only by one horn of the receiver.

At the end of August 1996 we could complete the most sensitive radio map of the Andromeda Galaxy to date in only 25 hours observing time. An area of 2.5 degrees x 1.2 degrees in the sky was observed with the 100m radio telescope twelve times, and all twelve coverages were summed up in the computer in order to create a very sensitive image. The angular resolution of 3 arc minutes corresponds to about 2000 light years in the distance of M31. The colors show different intensities: Blue for very weak, red for the strongest radio radiation. The weakest structures in the image correspond to only one millionth nano-Watt (or 10^-15 Watt)!

The circular objects in the image do not show stars, but radio emission from quasars in very large distance, which are not related to M31. The limited angular resolution of the radio telescope shows them "smeared out", in reality they have only diameters of a few arc seconds far beyond the angular resolution of the telescope at this wavelength. In the "ring" of M31 there are many bright spots, mainly magnetic field concentrations, which refer to areas of active star formation. The central area of M31 is also a source of strong radio radiation, probably caused by violent explosions close to the center, which we can investigate in more detail now.

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Last modified (and translated) by njn on Tuesday, May 4th, 2004.
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