2012ApJ...756..141B

We study the radio-FIR correlation between the nonthermal (synchrotron) radio continuum emission at λ90 cm (333 MHz) and the far-infrared emission due to cool (∼20 K) dust at λ70 µm in spatially resolved normal galaxies at scales of ∼1 kpc. The slope of the radio-FIR correlation significantly differs between the arm and interarm regions. However, this change is not evident at a lower wavelength of λ20 cm (1.4 GHz). We find the slope of the correlation in the arm to be 0.8±0.12 and we use this to determine the coupling between equipartition magnetic field (B_eq) and gas density (ρ_gas) as B_eq∝ρ^0.51±0.12_gas. This is close to what is predicted by magnetohydrodynamic simulations of turbulent interstellar medium, provided the same region produces both the radio and far-infrared emission. We argue that at 1 kpc scales this condition is satisfied for radio emission at 1.4 GHz and may not be satisfied at 333 MHz. The change of slope observed in the interarm region could be caused by propagation of low energy (∼1.5 GeV) and long-lived (∼10⁸ yr) cosmic-ray electrons at 333 MHz.