Nearby galaxies in the LOFAR Two-metre Sky Survey: III. Influence of cosmic-ray transport on the radio-SFR relation

Context. To understand galaxy evolution, it is essential to measure star formation rates (SFRs) across cosmic time. Aims. The use of radio continuum emission as an extinction-free tracer of star formation necessitates a good understanding of the influence of cosmic-ray electron (CRE) transport. Our aim in this work is to improve this understanding. Methods. We analysed the spatially resolved radio continuum-star formation rate (radio-SFR) relation in 15 nearby galaxies using data from the LOw Frequency ARray (LOFAR) and the Westerbork Synthesis Radio Telescope (WSRT) at 144 and 1365 MHz, respectively. The hybrid SFR maps are based on observations with Spitzer at 24 μm and with GALEX at 156 nm. Our pixel-by-pixel analysis at 1.2 kpc resolution reveals the usual sublinear radio-SFR relation for local measurements. This can be linearised with a smoothing experiment, convolving the hybrid SFR map with a Gaussian kernel that provides us with the CRE transport length. Results. CRE transport can be described as energy-independent isotropic diffusion. If we consider only young CREs as identified with the radio spectral index, we find a linear relation showing the influence of cosmic-ray transport. We then define the CRE calorimetric efficiency as the ratio of radio-to-hybrid SFR surface density and show that it is a function of the radio spectral index. If we correct the radio-SFR relation for the CRE calorimetric efficiency parametrised by the radio spectral index, it becomes nearly linear with a slope of 1.01 ± 0.02, independent of frequency. Conclusions. The corrected radio-SFR relation is universal and it holds for both global and local measurements.