PUB - Publikationen an der Universität Bielefeld

Context. Radio continuum (RC) emission in galaxies allows us to measure star formation rates (SFRs) unaffected by extinction due to dust, of which the low-frequency part is uncontaminated from thermal (free free) emission. Aims. We calibrate the conversion from the spatially resolved 140 MHz RC emission to the SFR surface density (Sigma(SFR)) at 1 kpc scale. Radio spectral indices give us, by means of spectral ageing, a handle on the transport of cosmic rays using the electrons as a proxy for GeV nuclei. Methods. We used recent observations of three galaxies (NGC 3184, 4736, and 5055) from the LOFAR Two-metre Sky Survey (LoTSS), and archival LOw-Frequency ARray (LOFAR) data of NGC 5194. Maps were created with the facet calibration technique and converted to radio Sigma(SFR) maps using the Condon relation. We compared these maps with hybrid Sigma(SFR) maps from a combination of GALEX far-ultraviolet and Spitzer 24 mu m data using plots tracing the relation at the highest angular resolution allowed by our data at 1.2 x 1.2 kpc(2) resolution. Results. The RC emission is smoothed with respect to the hybrid Sigma(SFR) owing to the transport of cosmic-ray electrons (CREs) away from star formation sites. This results in a sublinear relation (Sigma(SFR))(RC) alpha [(Sigma(SFR))(hyb)](a), where a = 0.59 +/- 0.13 (140 MHz) and a = 0.75 +/- 0.10 (1365 MHz). Both relations have a scatter of sigma = 0.3 dex. If we restrict ourselves to areas of young CREs (alpha > -0.65; I-nu alpha nu(alpha)), the relation becomes almost linear at both frequencies with a approximate to 0.9 and a reduced scatter of sigma = 0.2 dex. We then simulate the effect of CRE transport by convolving the hybrid Sigma(SFR) maps with a Gaussian kernel until the RC-SFR relation is linearised; CRE transport lengths are l = 1-5 kpc. Solving the CRE diffusion equation, assuming dominance of the synchrotron and inverse-Compton losses, we find diffusion coefficients of D = (0.13-1.5) x 10(28) cm(2) s(-1) at 1 GeV. Conclusions. A RC-SFR relation at 1.4 GHz can be exploited to measure SFRs at redshift z approximate to 10 using 140 MHz observations.