Advantages of low-frequency observations

Observing at low frequencies has a number of important advantages. Synchrotron radio emission from many astronomical objects, including supernova remnants and pulsars in the Milky Way, the magnetised interstellar medium in the Milky Way and other galaxies, the medium between the galaxies of a galaxy cluster, and lobes and jets of radio galaxies driven by central black holes, has a “steep” spectrum, which means that its intensity increases strongly towards low frequencies (long wavelengths). Furthermore,
the observable extent of radio emitters is often limited by the propagation speed of the emitting cosmic-ray (relativistic) electrons away from their sources. At the high radio frequencies (typically 1-10 GHz) of most present-day radio telescopes, the extent of radio emission is restricted by energy losses of the electrons (mostly synchrotron emission and the Inverse Compton effect with background photons) to about 1 kiloparsecs (about 3300 light-years) from the supernova remnants in star-forming regions. Low-frequency radio emission, on the other hand, is emitted by electrons with lower energies which suffer less from energy losses and hence can propagate farther away from their origins into regions with weak magnetic fields, into the outer disks of galaxies and into galaxy halos. For example, a relativistic electron radiating at 50 MHz can travel up to 200 kiloparsecs (about
650 000 light-years) in a magnetic field of about 3 μG (micro-Gauss) or 0.3 nT (nano-Tesla). In regular magnetic fields the travel distance is even longer. Galaxies are expected to be HUGE at low frequencies!

Another important tool to measure cosmic magnetic fields is the effect of Faraday rotation. It is proportional to the average strength of the regular magnetic field along the line of sight and to the density of the ionised gas (plasma). Measuring the Faraday rotation of emission from an astronomical source gives astronomers information about the physical conditions within that source. Faraday rotation is also proportional to the square of the wavelength or to the inverse square of the frequency, so that weak fields and/or low plasma densities, as expected e.g. in galaxy halos, can be measured with much higher precision at low frequencies / long wavelengths. Faraday rotation is also caused by the ionised gas in the Earth’s ionosphere which has to be subtracted.

The precision of a Faraday rotation measurement also depends on the signal-to-noise ratio of the polarised synchrotron emission which can be rather low in regions of weak magnetic fields. A grid of bright, polarised background sources helps here. Their emission is Faraday-rotated when passing through a foreground galaxy.

In summary, measuring polarised radio waves at low frequencies offers a new window to study cosmic magnetism, and LOFAR is the first radio telescope of sufficient sensitivity to open this window.