Low frequency radio-FIR correlation in normal galaxies at ~1 kpc scales

Abstract

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 λ70μm 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 \pm 0.12 and we use this to determine the coupling between equipartition magnetic field (B_{eq}) and gas density (ρ_{gas}) as B_{eq} \propto ρ_{gas}^{0.51 \pm 0.12}. This is close to what is predicted by MHD simulations of turbulent ISM, 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. Change of slope observed in the interarm region could be caused by propagation of low energy (~1.5 GeV) and long lived (~ 10^8 yr) cosmic ray electrons at 333 MHz.

8 Pages, 5 Figures, Accepted to be published in ApJ