First LOFAR observations reveal a whirlpool of wonder
Sen—A European team of astronomers has opened up a new window on the Universe by using the international LOFAR radio telescope to observe the famous Whirlpool Galaxy Messier 51 (M51).
Their work has allowed them to obtain the most sensitive image of any galaxy ever taken at frequencies below 1 GHz in the electromagnetic spectrum that extends beyond visible light.
M51 is seen from a distance of approximately 30 million light years and appears almost face-on, displaying a beautiful system of spiral arms.
Optical images show predominantly the visible light from stars, but radio waves reveal two constituents of galaxies that are invisible to optical telescopes: electrons and magnetic fields. The electrons are particles produced in the shock fronts of giant supernova explosions.
Magnetic fields are generated by dynamo processes driven by gas motions. When the electrons spiral around the magnetic field lines, radio waves are emitted. The intensity increases with the number and energy of the electrons and with magnetic field strength.
For many decades, radio astronomy has been unable to explore low frequencies below 300 MHz because the Earth's ionosphere acts as a barrier of low-frequency radio waves (which are completely blocked below about 10 MHz). So spiral galaxies have hardly been studied at these low radio frequencies. The only observations were of poor resolution with no detail.
The Low Frequency Array (LOFAR) is a new radio telescope giving access to very low radio frequencies. It consists of 38 stations in the Netherlands, six stations in Germany and one station each in the UK, France and Sweden. LOFAR observed the galaxy in the frequency range 115-175 MHz, just above the normal commercial FM radio frequency band of 88-108 MHz.
The locations of LOFAR Stations in Europe. Image credit: ASTRON, The Netherlands
With LOFAR's high sensitivity, the disk of M51 could be traced much further out than before. The astronomers were able to detect cosmic electrons and magnetic fields 40,000 light years away from the centre. With LOFAR’s high angular resolution, the spiral arms, where magnetic fields and cosmic rays are densest, are clearly visible.
“Low-frequency radio waves are important as they carry information about electrons of relatively low energies that are able to propagate further away from their places of origin in the star-forming spiral arms and are able to illuminate the magnetic fields in the outer parts of galaxies”, says David Mulcahy of UK's University of Southampton. “We need to know whether magnetic fields are expelled from galaxies and what their strength is out there.”
Colleague Anna Scaife, also from Southampton, says: "This beautiful image, coupled with the important scientific result it represents, illustrates the fantastic advances that can be made at low radio frequencies with the LOFAR telescope.
"Unravelling the mysteries of magnetic fields is crucial to understanding how our Universe works. For too long, many of the big questions about magnetic fields have simply been untestable and this new era of radio astronomy is very exciting."
Rainer Beck, of the Max-Planck Institute For Radio Astronomy, in Germany, says: "This opens a new window to the Universe where we do not know how galaxies will look like. Maybe we will see how galaxies are magnetically connected to intergalactic space.
"This is a key experiment in preparation for the planned Square Kilometre Array (SKA) that should tell us how cosmic magnetic fields are generated."