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Paper   IPM / Astronomy / 17234
School of Astronomy
  Title:   Diffusion of cosmic-ray electrons in M 51 observed with LOFAR at 54 MHz
1.  V. Heesen
2.  F. de Gasperin
3.  S. Schulz
4.  A. Basu
5.  R. Beck
6.  M. Bruggen
7.  R.-J. Dettmar
8.  M. Stein
9.  L. Gajovic
10.  F. S. Tabatabaei
11.  P. Reichherzer
  Status:   Published
  Journal: Astronomy & Astrophysics
  Vol.:  672
  Year:  2023
  Supported by:            ipm IPM
Context. The details of cosmic-ray transport have a strong impact on galaxy evolution. The peak of the cosmic-ray energy distribution is observable in the radio continuum using the electrons as proxy. Aims. We measure the length that the cosmic-ray electrons (CRE) are transported during their lifetime in the nearby galaxy M 51 across one order of magnitude in cosmic-ray energy (approximately 1-10 GeV). To this end we use new ultra-low frequency observations from the LOw Frequency ARay (LOFAR) at 54 MHz and ancillary data between 144 and 8350 MHz. Methods. As the the CRE originate from supernova remnants, the radio maps are smoothed in comparison to the distribution of the star formation. By convolving the map of the star-formation rate (SFR) surface density with a Gaussian kernel, we can linearise the radio-SFR relation. The best-fitting convolution kernel is then our estimate of the CRE transport length. Results. We find that the CRE transport length increases at low frequencies, as expected since the CRE have longer lifetimes. The CRE transport length is lCRE=4Dtsynâ??â??â??â??â??â??, where D is the isotropic diffusion coefficient and tsyn is the CRE lifetime as given by synchrotron and inverse Compton losses. We find that the data can be well fitted by diffusion, where D=(2.14±0.13)Ã?1028 cm2sâ??1. With Dâ?�E0.001±0.185, the diffusion coefficient is independent of the CRE energy E in the range considered. Conclusions. Our results suggest that the transport of GeV-cosmic ray electrons in the star-forming discs of galaxies is governed by energy-independent diffusion.

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