Nearby galaxies in the LOFAR Two-metre Sky Survey

Context . Radio continuum emission has the potential to be useful as an extinction-free tracer of star formation. However, the relation between the radio continuum luminosity and star formation rate (SFR) is potentially limited by various effects such as cosmic-ray transport, free-free absorption, and cosmic-ray electron energy losses. Aims . We aim to calibrate the radio-SFR relation in a sample of nearby galaxies ranging from dwarf to spiral galaxies, covering nearly five orders of magnitude in terms of the SFR range. We include both global and local measurements from individual and spatially resolved, kiloparsec-sized galaxies, respectively. Methods . We measured the radio continuum luminosities at 144 MHz using observations with the LOw Frequency ARray (LOFAR) and measured the radio spectral indices using ancillary 1.4 GHz data. We selected 70 nearby (distance d < 30 Mpc) galaxies, 15 of which were used for local measurements. Taking advantage of their rich ancillary data, we carried out a study of the radio-SFR relation using the total infrared (TIR), mid-infrared (MIR), H α , and far-ultraviolet (FUV) as complementary SFR tracers. About a third of our sample is made up of at least moderately star-forming edge-on galaxies with the remaining ones chosen to form a representative sample of a wide range of morphological types and SFR values. Results . For the first time, we show that the radio luminosity ( L 144 ), the SFR, and the radio spectral index ( α ) define a ‘fundamental plane’ in the [log( L 144 ), log(SFR), α ] space. This enables us to define a unified radio-SFR relation that is applicable to both for global and local data when using the radio spectral index as a secondary parameter. Conclusions . A unified radio-SFR relation for both global and local data could serve as a litmus test for galaxy simulations that include the effect of cosmic rays and magnetic fields. It also strengthens the case for using the radio-SFR relation as an extinction-free tracer of star formation.