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Paper   IPM / Astronomy / 17007
School of Astronomy
  Title:   Nearby galaxies in the LOFAR Two-metre Sky Survey I. Insights into the non-linearity of the radio SFR relation?
1.  V. Heesen
2.  M. Staffehl
3.  A. Basu
4.  R. Beck
5.  M. Stein
6.  F. S. Tabatabaei
7.  M. J. Hardcastle
8.  K. T. Chyzy
9.  T. W. Shimwell
10.  B. Adebahr
11.  R. Beswick
12.  D. J. Bomans
13.  A. Botteon
14.  E. Brinks
15.  M. Bruggen
16.  R.-J. Dettmar
17.  A. Drabent
18.  F. de Gasperin
19.  G. Gurkan
20.  G. H. Heald
21.  C. Horellou
22.  B. Nikiel-Wroczynski
23.  R. Paladino
24.  J. Piotrowska
25.  H. J. A. Rottgering
26.  D. J. B. Smith
27.  C. Tasse
  Status:   Published
  Journal: Astronomy & Astrophysics
  Vol.:  664
  Year:  2022
  Supported by:            ipm IPM
Context. Cosmic rays and magnetic fields are key ingredients in galaxy evolution, regulating both stellar feedback and star formation. Their properties can be studied with low-frequency radio continuum observations, free from thermal contamination. Aims. We define a sample of 76 nearby (< 30 Mpc) galaxies, with rich ancillary data in the radio continuum and infrared from the CHANG-ES and KINGFISH surveys, which will be observed with the LOFAR Two-metre Sky Survey (LoTSS) at 144 MHz. Methods. We present maps for 45 of them as part of the LoTSS data release 2 (LoTSS-DR2), where we measure integrated flux densities and study integrated and spatially resolved radio spectral indices. We investigate the radio-SFR relation, using star-formation rates (SFR) from total infrared and H �± + 24-�¼m emission. Results. The radio-SFR relation at 144 MHz is clearly super-linear with L144â?�SFR1.4â??1.5. The mean integrated radio spectral index between 144 and â??1400 MHz is â?¨�±â?©=â??0.56�±0.14, in agreement with the injection spectral index for cosmic ray electrons (CRE). However, the radio spectral index maps show a variation of spectral indices with flatter spectra associated with star-forming regions and steeper spectra in galaxy outskirts and, in particular, in extra-planar regions. We found that galaxies with high star-formation rates (SFR) have steeper radio spectra; we find similar correlations with galaxy size, mass, and rotation speed. Conclusions. Galaxies that are larger and more massive are better electron calorimeters, meaning that the CRE lose a higher fraction of their energy within the galaxies. This explains the super-linear radio-SFR relation, with more massive, star-forming galaxies being radio bright. We propose a semi-calorimetric radio-SFR relation, which employs the galaxy mass as a proxy for the calorimetric efficiency.

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