Measuring cosmic magnetic fields

Most of what we know about astrophysical magnetic fields has been detected via radio astronomical observations. Most of the cosmic broad-band ("continuum") radio emission is synchrotron radiation by relativistic electrons which spiral around magnetic field lines. These electrons are probably accelerated in the remnants of supernova explosions, together with other particles of the cosmic ray population. The observed synchrotron luminosity allows us to measure the total field Strength. Synchrotron radiation is linearly polarised, up to 75% in a fully regular magnetic field. The polarisation plane yields the orientation of the regular field in the plane of the sky. The degree of linear polarisation tells us the field's degree of ordering. Moreover, Faraday rotation of the polarisation plane provides information on the field component along the line of sight. Hence, a three-dimensional picture of cosmic magnetic fields can be derived from radio waves.

 

Figure 1 was derived from combined observations of the Prototypical spiral galaxy M51 with the interferometric radio telescope Very Large Array (VLA) near Socorro (New Mexico/USA) and with the 100m single disk radio telescope near Effelsberg (Germany). The contour lines trace the total radio emission at 5 GHz (6 cm wavelength) and the vectors the polarised emission. The vectors indicate that the orientation of the regular magnetic field is spiral and mostly follows the spiral arms, as evident from the background optical image obtained by the Hubble Space Telescope. The magnetic field is strong also between the spiral arms. This was not expected from the idea that the spiral arms are density waves where gas and magnetic fields should be compressed. Instead, the magnetic field must be enhanced between the spiral arms by some other mechanism which is still not understood. Outside of the optical extent of the galaxy, little radio synchrotron emission is detected at high frequencies because the relativistic electrons, responsible for the radio emission and "illuminating" the magnetic field, cannot travel far away from the regions of their origin, supernova remnants. These are embedded into the inner disk of galaxies where the star formation rate is highest. Only in a few cases, radio halos could be detected around galaxies seen edge-on.

 

 

Figure 2 shows the galaxy NGC 891 which is believed to be very similar to our Milky Way. It was observed at 3.6 cm wavelength with the Effelsberg 100m telescope. The background optical image is from the CFHT Observatory. The X-shaped structure of the magnetic fields indicates the action of a galactic wind.

The few spectacular cases where the radio synchrotron emission at high frequencies is not restricted to the optical extent are all related to interactions between galaxies. For example, huge radio lobes were discovered on two sides of the galaxy NGC 4569 which is located in the dense Virgo cluster of galaxies where interactions are frequent. Figure 3 shows the radio map observed at 6 cm wavelength with the Effelsberg telescope.