Diffusion of cosmic-ray electrons in M 51 observed with LOFAR at 54MHz

Vol. 672
4. Extragalactic astronomy

Diffusion of cosmic-ray electrons in M 51 observed with LOFAR at 54MHz

Cosmic rays are accelerated to relativistic energies in supernova remnants. Cosmic ray electrons (CREs) then emit synchrotron radiation while traveling and spiraling along magnetic field lines. The synchrotron radio continuum is therefore a tracer of star formation in galaxy disks, as confirmed by its tight correlation with far-infrared flux. In this work, the authors exploit this correlation to estimate the length that the CREs are transported during their lifetime in the nearby galaxy M 51, as well as for cosmic-ray energy between 1 and 10 GeV. They use new ultra-low frequency observations from the LOw Frequency ARay (LOFAR) at 54 MHz and ancillary data between 144 and 8350 MHz. Because of the CRE diffusion length, the radio maps are smoothed in comparison to the distribution of the star formation. The authors then convolve the map of the star formation rate (SFR) surface density to fit the radio map. The best-fitting convolution length yields a measure of the CRE transport length. They find that the CRE transport length increases at low frequencies, as is expected since the CREs have longer lifetimes. The phenomenon is similar to a diffusion, with a coefficient independent of the CRE energy in the range considered.