A new interpretation of the far-infrared - radio correlation and the expected breakdown at high redshift

TL;DR

This paper offers a new interpretation of the far-infrared-radio correlation in galaxies, linking it to turbulent magnetic field amplification, and predicts its breakdown at high redshift due to inverse Compton losses, with implications for upcoming observations.

Contribution

It introduces a model connecting star formation, turbulence, and magnetic fields to explain the correlation and predicts its breakdown at high redshift based on ISM density evolution.

Findings

Correlation explained by turbulent magnetic field amplification.

Predicted breakdown redshift depends on star formation surface density and ISM density evolution.

Upcoming SKA and ALMA observations will test these predictions.

Abstract

(Abrigded) Observations of galaxies up to z 2 show a tight correlation between far-infrared and radio continuum emission. We explain the far-infrared - radio continuum correlation by relating star formation and magnetic field strength in terms of turbulent magnetic field amplification, where turbulence is injected by supernova explosions from massive stars. We calculate the expected amount of turbulence in galaxies based on their star formation rates, and infer the expected magnetic field strength due to turbulent dynamo amplification. We estimate the timescales for cosmic ray energy losses via synchrotron emission, inverse Compton scattering, ionization and bremsstrahlung emission, probing up to which redshift strong synchrotron emission can be maintained. We find that the correlation between star formation rate and magnetic field strength in the local Universe can be understood as a result of turbulent magnetic field amplification. If the typical gas density in the interstellar medium increases at high z, we expect an increase of the magnetic field strength and the radio emission, as indicated by current observations. Such an increase would imply a modification of the far-infrared - radio correlation. We expect a breakdown when inverse Compton losses start dominating over synchrotron emission. For a given star formation surface density, we calculate the redshift where the breakdown occurs, yielding z (Sigma_SFR/0.0045 M_solar kpc^{-2} yr^{-1})^{1/(6-alpha/2)}. In this relation, the parameter \alpha describes the evolution of the characteristic ISM density in galaxies as (1+z)^\alpha. Both the possible raise of the radio emission at high redshift and the final breakdown of the far-infrared -- radio correlation at a critical redshift will be probed by the Square Kilometre Array (SKA) and its pathfinders, while the typical ISM density in galaxies will be probed with ALMA.