The Radio-IR Correlation in the Context of Deep Radio Source Counts
Increasingly deep, confusion-limited radio surveys have pushed direct radio source-count measurements down to tens of $\mu$Jy at 1.4 GHz. Confusion-noise $P(D)$ analyses extend the statistical counts down below $1\,\mathrm{\mu Jy}$. Radio source counts have allowed for constraints on the radio-derived star formation rate density (SFRD) history through models of the backwards evolution of the local radio luminosity function, using the radio-FIR correlation, $q \propto \log(L_{\mathrm{FIR}}/L_{1.4})$, to convert radio luminosities to FIR luminosities and hence star-formation rates. Recent deep radio source counts from MeerKAT suggest a potential tension in the SFRD history between radio and UV/IR measurements at $1\lesssim z\lesssim 2$. This corresponds to a ${>}3\sigma$ discrepancy between the predicted and measured source counts near $10\,\mathrm{\mu Jy}$. We introduce a purely radio-luminosity based parameterization of the redshift evolution of the radio-FIR correlation based on changing cosmic ray losses. We find evidence (${\gtrsim}2\sigma$) that an evolution in the radio-FIR correlation consistent with a mild decrease in $q$ out to $z{\sim}2$ arising from strengthening magnetic fields can mitigate the source count tension. We additionally show that intrinsic scatter in the radio-FIR correlation is likely bounded $\sigma_q\lesssim 0.3\,\mathrm{dex}$ at these redshifts if $q$ decreases. Although we find no evidence that current radio source counts imply a breakdown in the radio-FIR correlation, future deep radio surveys from the Deep Synoptic Array (DSA) will be able to push radio source counts down to several nJy, providing stronger constraints on the allowed evolution.