Simulating radio synchrotron emission in star-forming galaxies:
  small-scale magnetic dynamo and the origin of the far infrared-radio
  correlation
  In star-forming galaxies, the far-infrared (FIR) and radio-continuum
luminosities obey a tight empirical relation over a large range of
star-formation rates (SFR). To understand the physics, we examine
magneto-hydrodynamic galaxy simulations, which follow the genesis of cosmic ray
(CR) protons at supernovae and their advective and anisotropic diffusive
transport. We show that gravitational collapse of the proto-galaxy drives a
small-scale dynamo, which exponentially amplifies weak seed magnetic fields.
After saturation at small scales, they grow in scale to reach equipartition
with thermal and CR energy densities in Milky Way-mass galaxies. In small
galaxies, the magnetic energy saturates at the turbulent energy while it fails
to reach equipartition with thermal and CR energies. We solve for steady-state
spectra of CR protons, secondary electrons/positrons from hadronic CR-proton
interactions with the interstellar medium, and primary shock-accelerated
electrons at supernovae. The modeled radio-synchrotron emission is dominated by
primary electrons, irradiates the magnetised disc and bulge of our simulated
Milky Way-mass galaxy and weakly traces bubble-shaped magnetically-loaded
outflows. Our star-forming and star-bursting galaxies with saturated magnetic
fields match the global FIR-radio correlation (FRC) across four orders of
magnitude. Its intrinsic scatter arises due to (i) different magnetic
saturation levels that result from different seed magnetic fields, (ii)
different radio synchrotron luminosities for different specific SFRs at fixed
SFR and (iii) a varying radio intensity with galactic inclination. In agreement
with observations, several 100-pc-sized regions within star-forming galaxies
also obey the FRC, while the centres of starbursts exceed the FRC by a
substantial amount.
