The Far-Infrared--Radio Correlation at High Redshifts: Physical Considerations and Prospects for the Square Kilometer Array

TL;DR

This paper predicts how the far-infrared--radio correlation evolves with redshift, emphasizing the potential of high-frequency radio surveys with the SKA to probe high-redshift star formation unaffected by dust.

Contribution

It introduces a model for FIR-radio correlation evolution considering CMB effects and proposes SKA sensitivity goals for detecting high-redshift star-forming galaxies.

Findings

FIR-radio ratio likely increases with redshift due to CMB effects.

Deep SKA surveys at >10 GHz can effectively trace high-z star formation.

Detection of galaxies with star formation rates >25 solar masses per year at all redshifts is feasible.

Abstract

(Abridged) I present a predictive analysis for the behavior of the FIR--radio correlation as a function of redshift in light of the deep radio continuum surveys which may become possible using the SKA. To keep a fixed ratio between the FIR and predominantly non-thermal radio continuum emission of a normal star-forming galaxy requires a nearly constant ratio between galaxy magnetic field and radiation field energy densities. While the additional term of IC losses off of the cosmic microwave background (CMB) is negligible in the local Universe, the rapid increase in the strength of the CMB energy density (i.e. $\sim(1+z)^{4})$ suggests that evolution in the FIR-radio correlation should occur with infrared (IR; $8-1000 \micron$)/radio ratios increasing with redshift. At present, observations do not show such a trend with redshift; $z\sim6$ radio-quiet QSOs appear to lie on the local FIR-radio correlation while a sample of $z\sim4.4$ and $z\sim2.2$ SMGs exhibit ratios that are a factor of $\sim$2.5 {\it below} the canonical value. I also derive a 5$\sigma$ point-source sensitivity goal of $\approx$20 nJy (i.e. $\sigma_{\rm RMS} \sim 4$ nJy) requiring that the SKA specified be $A_{\rm eff}/T_{\rm sys}\approx 15000$ m$^{2}$ K$^{-1}$; achieving this sensitivity should enable the detection of galaxies forming stars at a rate of $\ga25 M_{\sun} {\rm yr}^{-1}$, at all redshifts if present. By taking advantage of the fact that the non-thermal component of a galaxy's radio continuum emission will be quickly suppressed by IC losses off of the CMB, leaving only the thermal (free-free) component, I argue that deep radio continuum surveys at frequencies $\ga$10 GHz may prove to be the best probe for characterizing the high-$z$ star formation history of the Universe unbiased by dust.