Radio–FIR correlation: A probe into cosmic ray propagation in the nearby galaxy IC 342

Resolved studies of the correlation between the radio and far-infrared (FIR) emission from galaxies at different frequencies can unveil the interplay between star formation and the relativistic interstellar medium (ISM). Thanks to the LOFAR LoTSS observations combined with VLA, Herschel, and WISE data, we study the role of cosmic rays and magnetic fields in the radio–FIR correlation on scales of ≳200 pc in the nearby galaxy IC 342. The thermal emission traced by the 22 μm emission, constitutes about 6%, 13%, and 30% of the observed radio emission at 0.14, 1.4, and 4.8 GHz, respectively, in star-forming regions and less in other parts. The nonthermal spectral index becomes flatter at frequencies lower than 1.4 GHz (αn = −0.51 ± 0.09, Sν ∝ ναn) than between 1.4 and 4.8 GHz (αn = −1.06 ± 0.19) on average, and this flattening occurs not only in star-forming regions but also in the diffuse ISM. The radio–FIR correlation holds at all radio frequencies; however, it is tighter at higher radio frequencies. A multi-scale analysis shows that this correlation cannot be maintained on small scales due to diffusion of cosmic ray electrons (CREs). The correlation breaks at a larger scale (≃320 pc) at 0.14 GHz than at 1.4 GHz (≃200 pc), indicating that the CREs traced at lower frequencies have diffused a longer path in the ISM. We find that the energy index of CREs becomes flatter in star-forming regions, in agreement with previous studies. Cooling of CREs due to the magnetic field is evident globally only after compensating for the effect of star formation activity that both accelerates CREs and amplifies magnetic fields. Compared with other nearby galaxies, we show that the smallest scale of the radio–FIR correlation is proportional to the propagation length of the CREs on which the ordered magnetic field has an important effect.