Fundamental Questions & Advantages
Fundamental questions about galactic magnetic fieldsIn spite of significant progress in radio studies of the Milky Way and other galaxies, some fundamental questions still remained unanswered:
- When were the first magnetic fields generated: in young galaxies, or are they relics from the early Universe before the galaxies were formed?
- How were magnetic fields amplified and ordered during the evolution of galaxies?
- Do magnetic fields exist only in the star-forming inner regions of galaxies, Or do they extend out to intergalactic space?
- How important are magnetic fields for the physics of galaxies, like the efficiency to form stars from gas, the formation of spiral arms, or the generation of gas outflows (“galactic winds”)?
Answers to these questions can hardly be expected from present-day radio telescopes, but need much better sensitivity and angular resolution, and the opening of new frequency windows, especially at low frequencies.
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 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 (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 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. Faraday rotation is also caused by the ionised gas in the Earth’s ionosphere which can 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 to open this window.
Why “Key Science”?
The LOFAR project “Cosmic Magnetism” aims to investigate fundamental astrophysical questions on the distribution of magnetic fields in the Universe which may help to understand the origin of cosmic magnetism. The project makes use of the LOFAR capacity to detect linear polarisation and hence widens the horizon of the instrument. Polarisation calibration is an instrumental challenge and requires dedicated software, which is under development by the project team.