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.