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

TL;DR

This study investigates the spatially resolved correlation between radio and FIR emissions in normal galaxies at ~1 kpc scales, revealing differences between arm and interarm regions and implications for magnetic field and cosmic ray electron propagation.

Contribution

It provides the first detailed analysis of the radio-FIR correlation at ~1 kpc scales in normal galaxies, highlighting regional variations and magnetic field coupling.

Findings

Radio-FIR correlation slope differs between arm and interarm regions.

Coupling between magnetic field and gas density is close to MHD simulation predictions.

Propagation of cosmic ray electrons affects the correlation at 333 MHz.

Abstract

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 (~20 K) dust at \lambda 70\mu m 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 \lambda 20 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 (\rho_{gas}) as B_{eq} \propto \rho_{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.