Magnetic fields in nearby galaxies: Low-frequency radio observations

• Applicants: Dr. Rainer Beck; Professor Dr. Ralf-Jürgen Dettmar
• Subject Area: Astrophysics and Astronomy
• Term: from 2010 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 108727618

• Final Report Year: 2017

Final Report Abstract

The Project achieved major progress in the observation of nearby galaxies with LOFAR and other radio telescopes. Large radio continuum halos were detected with WSRT in total intensity. Faint diffuse polarised emission around 350 MHz could only be detected from M 31 with WSRT. The LOFAR images of M 51 and NGC 891 are the most sensitive low-frequency radio continuum images obtained so far. The scale lengths of the disks of M 51 at 151 MHz is larger than that at 1.4 GHz which can be explained by diffusion of cosmic-ray electrons, leading to a longer propagation length of the lower-energy electrons emitting at lower frequencies. Diffuse polarised emission from nearby galaxies could not be detected with LOFAR because Faraday depolarisation turned out to be stronger than predicted. Polarised background sources were detected in each galaxy field observed with LOFAR. The JVLA and Effelsberg telescopes were used to obtain images of several galaxies at higher frequencies. In NGC 628, an azimuthal periodic pattern in Faraday depth is a signature of Parker instabilities. The magnetic pitch angle is systematically larger than the morphological pitch angle of the polarisation arms which gives evidence for the action of a large-scale dynamo where the regular magnetic field is not coupled to the gas flow. Huge envelopes of radio emission were discovered around the starburst dwarf galaxy IC 10 and within the compact galaxy group HCG 92. The high resolution total intensity distributions for galaxies at low frequencies provide new means to constrain the spectral index distribution and thus help to advance the discussion of cosmic-ray transport in galaxies. The progress of this project is of high importance for commissioning of the forthcoming Square Kilometre Array (SKA) and its precursor telescopes Karoo Array Telescope (MeerKAT) and Australia Square Kilometre Array Project (ASKAP).

Publications

• Magnetic Fields in Galaxies, in Planets, Stars and Stellar Systems Volume 5. Springer Science. ISBN 978-94-007-5612-0. Dordrecht: Springer-Verlag, 2013, p. 641
  Beck, R., Wielebinski, R.
  (See online at https://doi.org/10.1007/978-94-007-5612-0_13)
• 2014, A&A, 567, 134
  Jurusik, W., Beck, R., Klein, U. et al.
  (See online at https://doi.org/10.1051/0004-6361/201323060)
• Magnetic Fields in Galaxies, in Magnetic Fields in Diffuse Media. Astrophysics and Space Science Library, Volume 407. ISBN 978-3-662-44624-9. Berlin: Springer-Verlag, 2015, p. 507
  Beck, R.
  (See online at https://doi.org/10.1007/978-3-662-44625-6_18)
• 2016, A&A, 592, 123
  Mulcahy, D.D., Fletcher, A., Beck, R. et al.
  (See online at https://doi.org/10.1051/0004-6361/201628446)
• 2016, A&ARv, 24, 4
  Beck, R.
  (See online at https://doi.org/10.1007/s00159-015-0084-4)
• 2016, ApJ, 819, 39
  Chyzy, K.T., Drzazga, R.T., Beck, R., Bomans, D.J. et al.
  (See online at https://doi.org/10.3847/0004-637X/819/1/39)
• 2016, ApJ, 824, 30
  Damas-Segovia, A., Beck, R., Dettmar, R.-J. et al.
  (See online at https://doi.org/10.3847/0004-637X/824/1/30)
• 2016, MNRAS, 456, 1723
  Li, J.-T., Beck, R., Dettmar, R.J. et al.
  (See online at https://doi.org/10.1093/mnras/stv2757)
• 2016, MNRAS, 458, 332
  Heesen, V., Dettmar, R.-J., Krause, M., Beck, R., Stein, Y.
  (See online at https://doi.org/10.1093/mnras/stw360)
• 2017, A&A, 600, 6
  Mulcahy, D.D., Beck, R., Heald, G.H.
  (See online at https://doi.org/10.1051/0004-6361/201629907)