LOW-FREQUENCY RADIO-FIR CORRELATION IN NORMAL GALAXIES AT ∼1 kpc SCALES

We study the radio-FIR correlation between the nonthermal (synchrotron) radio continuum emission at lambda 90 cm (333 MHz) and the far-infrared emission due to cool (similar to 20 K) dust at lambda 70 mu m in spatially resolved normal galaxies at scales of similar to 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 lambda 20 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 (rho(gas)) as B-eq proportional to rho(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 (similar to 1.5 GeV) and long-lived (similar to 10(8) yr) cosmic-ray electrons at 333 MHz.