The Whirlpool galaxy and its surrounding: LOFAR radio map of the Whirlpool galaxy M51 and its neighbourhood at a frequency of 150 MHz. The field covers 4 by 2.6 degrees, the observations were performed with the Dutch LOFAR high-band antennas. The map shows the distribution of relativistic electrons in M51 and also a large number of background galaxies. The inset shows an enlarged view of M51 in the frequency range 115 - 175 MHz (white contour lines) overlayed onto an optical image of M51 from the Digital Sky Survey (DSS). For more details, see Mulcahy et al., 2014, A&A, 568, A74.
Radio observations provide a new view on galaxies. Whereas optical images show predominantly the visible light from stars, the radio waves unravel two constituents of galaxies that are invisible to optical telescopes: electrons, almost as fast as light, and magnetic fields. Their role for the stability and evolution of galaxies is increasingly under discussion.
The electrons are "cosmic ray" 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, a process called synchrotron emission. Its 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 ionosphere (similar to scientillation of starlight) distorts the propagation of low-frequency radio waves (which are completely blocked below about 10 MHz). Sophisticated methods of data processing and superfast computers are needed to recover the emission. Due to these technical challenges, spiral galaxies have hardly been studied before at these very low radio frequencies. The only observations were of poor resolution and no details could be made out.
NGC 5775 as seen by LOFAR (orange colours) and optical SDSS (false-colour). This edge-on galaxy has a substantial star formation rate and an impressive radio halo that extends more than 10 kpc above the star forming disk. Our LOFAR data are complemented by new broadband multifrequency VLA data (from the CHANG-ES survey); collectively they will allow strong constraints to be placed on the cosmic ray propagation into and through the disk-halo interface, and perhaps to the boundary with the intergalactic medium.