Electron vortex in the vortex galaxy

High-definition recording in the radio area indicates magnetic fields

LOFAR radio map of the vortex galaxy M51 and its surroundings at a frequency of 115 to 175 MHz (right) and overlay with optical image (bottom left). © David Mulcahy et al., Astronomy & Astrophysics
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Enormous progress in radio astronomy: Astronomers have now succeeded in recording the spectacular vortex galaxy with the LOFAR radio telescope. Magnetic fields and fast cosmic electrons can be detected down to the outer areas of the galaxy - and these in turn provide answers to numerous astronomical questions.

The whirlpool galaxy is almost perfectly in line with the image of a spiral galaxy: From the earth, the gaze falls almost exactly on the plane of the galaxy, whose spiral arms can already be observed with an amateur telescope. The star cluster with the systematic name Messier 51 (M51) is located about 30 million light-years away in the constellation Hounds, near the Big Dipper.

However, as with many cosmic objects, the outstanding features of M51 do not appear in the visible light of the stars, but at other wavelengths. Especially in the radio sector, many components of galaxies that can not be detected by optical telescopes, including magnetic fields and electron currents accelerated almost to the speed of light, are hidden.

Specialized in low radio frequencies

The role these components play in the stability and evolution of galaxies is increasingly becoming a focus of astronomers. For many decades, however, it was impossible to evaluate radio emissions at frequencies below 300 megahertz. The earth's ionosphere weakens these low-frequency radio waves too much - below ten megahertz nothing even reaches it to the earth's surface.

The vortex galaxy Messier 51, taken by the Hubble Space Telescope in visible light, in January 2005. © NASA

However, the Low Frequency Array (LOFAR) radio telescope specializes in radio frequencies below 240 megahertz. Thus, researchers around David Mulcahy from the Max Planck Institute for Radio Astronomy (MPIFR) in Bonn managed a spectacular shot of the Whirlpool galaxy M51, on which the magnetic fields are visible in the radio area - more clearly than ever before. display

Electrons and magnetic fields

"Low-frequency radio waves are so important because they contain information about electrons at relatively low energies, " says Mulcahy. These electrons can reach much longer distances from their origins within the spiral arms up to 40, 000 light-years from the center of the galaxy. "We would like to know if magnetic fields are being emitted by the galaxies and how strong they still are in the outer regions of the galaxies, " says the astronomer. These magnetic fields at the edge of the galaxy are literally illuminated by the electrons.

The required electrons are particles of the cosmic radiation, which arise in the shockwaves of gigantic supernova explosions. The origin of the magnetic fields, in turn, are dynamo processes that are driven by the movement of the gas in the galaxy. When the electrons hit a magnetic field, they move around the field lines on spiral tracks, radiating radio waves. The stronger the magnetic field and the faster the electrons, the more intense the radio radiation.

"This great picture of M51, along with the new insights it brings, shows the tremendous advances that can be achieved with low-frequency LOFAR telescopes, " adds co-author Anna Scaife of the University of Southampton The Revelation The secrets of magnetic fields are crucial to understanding how our universe works. "For too long, the big questions about the magnetic fields simply could not be verified by observations. This new epoch of radio astronomy is very exciting

LOFAR stations in Europe ASTRON, The Netherlands

Another window to the universe

LOFAR is a completely new kind of radio telescope: it consists of a large number of small antennas of simple design without any moving parts, distributed over 38 measurement stations in the Netherlands, six stations in Germany and one each in the UK, France and Sweden. The novel principle consists of the online connection of signals from all stations in a high-performance computer cluster in the Dutch University of Groningen. This is able to correct the disturbances of the ionosphere.

With LOFAR astronomers have measured the vortex galaxy at radio frequencies of 30 to 80 MHz. "This opens up another new window to the universe, and we do not yet know what the galaxies look like in this frequency range, " concludes Rainer Beck from the MPIFR. "Maybe we can even detect a galaxy's magnetic connection to intergalactic space. This would be a key experiment in preparation for the planned Square Kilometer Array, which could show us where and how cosmic magnetic fields are generated. "

(Astronomy & Astrophysics, 2014; doi: 10.1051 / 0004-6361 / 201424187)

(Max Planck Institute for Radio Astronomy, 21.08.2014 - AKR)