High-resolution radio continuum survey of M33 -: II.: Thermal and nonthermal emission

Context. Constraints on the origin and propagation of cosmic rays can be achieved by studying the variation in the spectral index of the synchrotron emission across external galaxies. Aims. We determine the variation in the nonthermal radio spectral index in the nearby spiral galaxy M33 at a linear resolution of 360 pc. Methods. We separated the thermal and nonthermal components of the radio continuum emission without the assumption of a constant nonthermal spectral index. Using the Spitzer FIR data at 70 and 160 m and a standard dust model, we dereddened the Ha emission. The extinction corrected Ha emission serves as a template for the thermal free-free radio emission. Subtracting this free-free emission from the observed 3.6 cm and 20 cm emission (Effelsberg and the VLA), we obtained the nonthermal maps. A constant electron temperature used to obtain the thermal radio intensity seems appropriate for M33, which, unlike the Milky Way, has a shallow metallicity gradient. Results. For the first time, we derive the distribution of the nonthermal spectral index across a galaxy, M33. We detect strong nonthermal emission from the spiral arms and starforming regions. Wavelet analysis shows that at 3.6 cm the nonthermal emission is dominated by contributions from starforming regions, while it is smoothly distributed at 20 cm. For the whole galaxy, we obtain thermal fractions of 51% and 18% at 3.6 cm and 20 cm, respectively. The thermal emission is slightly stronger in the southern than in the northern half of the galaxy. We find a clear radial gradient of mean extinction in the galactic plane. Conclusions. The nonthermal spectral index map indicates that the relativistic electrons suffer energy loss when diffusing from their origin in starforming regions towards interarm regions and the outer parts of the galaxy. We also conclude that the radio emission is mostly nonthermal at R > 5 kpc in M33.