Galactic magnetic fields

Magnetic fields are an essential ingredient of the matter in the Universe. A particular process where they are crucial is star formation because they can provide the transport of angular momentum of a collapsing cloud. Otherwise the further contraction of the cloud and the formation of the protostar would be prevented. However, the formation of magnetic fields in the Universe is not yet clear. There are several processes which can produce weak seed fields which have to be amplified in order to explain the observed magnetic fields in the local Universe. Among the favourite mechanisms is the galactic dynamo which acts by the combined action of turbulence and shear. A strong test for the theory of galactic dynamos are large-scale galactic magnetic fields.

To study the magnetic field structure in nearby galaxies radio continuum polarimetry can be used since the cosmic ray electrons radiate highly polarized synchrotron emission. While the linear polarization signal gives the orientation of the magnetic field in the plane of the sky, the Faraday rotation allows us to determine the magnetic field component along the line-of-sight. For this the Rotation Measure (RM) is used which measures the rotation of the polarization angle as a function of the wavelength. Moreover, the Faraday rotation reveals the direction of the line-of-sight magnetic field component. The analysis of the linear polarization together with the Rotation Measure can hence provide a model for the three-dimensional structure of the large-scale magnetic field in galaxies.

Cosmic rays

Using the synchrotron emission of the relativistic cosmic ray electrons we can also study the distribution of cosmic rays in galaxies. For this the distribution of the radio spectral index can be used which is an indicator for the age of the cosmic ray electrons. Since high energy cosmic rays lose their energy fastest only young accelerated cosmic rays in supernova remnants posses a flat spectral index. The observed spectral index in galaxies is flat in the disk and steep in the halo which shows that cosmic rays in the halo are older. This requires a vertical transport of cosmic rays from the disk into the halo. The steepening of the spectral index with increasing distance from the disk can be used to determine the cosmic ray bulk speed.

Radio continuum polarimetry

Radio telescopes are either single-dish telescope, like the Effelsberg 100-m telescope near Bad Münstereifel (Germany), or interferometric telescopes like the Very Large Array in New Mexico (USA). The interferometric telescopes provide a very high angular resolution because the resolution is determined by the widest spacing of any pair of telescopes. Thus, the VLA has a resolution of 1.4 arcsec while the Effelsberg telescope has only a very coarse resolution of 8 arcmin = 480 arcsec (!) at 20 cm. However, interferometers can not "see" any emission on large angular scales. At 20 cm with the VLA the largest emission is limited to an angular scale below 15 arcmin.

Interferometric radio telescopes detect hence only a fraction of the flux density detected with a single-dish telescope. The difference is called the missing zero-spacing flux density . Hence, it is necessary to observe objects with very extended emission both with an interferometer and a single-dish telescope. The data can be combined in the Fourier domain in order to fill up the missing zero-spacing flux so that the map posses both the high angular resolution of the interferometer and the extended emission of the single-dish telescope. In the case of NGC 253 I combined VLA and Effelsberg observations at 6 cm in order to obtain a map of this galaxy which has a diameter of 25 arcmin.

What you can see in the images

1.Radio continuum map of the nearby starbust galaxy NGC 253. The vectors show the orientation of the large-scale magnetic field with the vector length proportional to the strength of the large-scale magnetic field.

2.Rotation Measure distribution in NGC 253 between 6.2 cm and 3.6 cm calculated from VLA and Effelsberg observations.

3.Radio spectral index of NGC 253 between 20 cm and 6.2 cm calculated from VLA and Effelsberg observations.

4.Australia Telescope Compact Array radio interferometer. The five of the in total six antennas in the image are movable on a railway track.

5.100-m Effelsberg single-dish telescope near Bad Münstereifel (Germany).