  Using infrared data from the Herschel Space Observatory and Karl G. Jansky
Very Large Array (VLA) 3 GHz observations in the COSMOS field, we investigate
the redshift evolution of the infrared-radio correlation (IRRC) for
star-forming galaxies (SFGs) we classify as either spheroid- or disc-dominated
based on their morphology. The sample predominantly consists of disc galaxies
with stellar mass ${\gtrsim}10^{10}\,M_{\odot}$, and residing on the
star-forming main sequence (MS). After the removal of AGN using standard
approaches, we observe a significant difference between the redshift-evolution
of the median IR/radio ratio $\overline{q}_{\mathrm{TIR}}$ of (i) a sample of
ellipticals, plus discs with a substantial bulge component
(`spheroid-dominated' SFGs) and, (ii) virtually pure discs and irregular
systems (`disc-dominated' SFGs). The spheroid-dominated population follows a
declining $\overline{q}_{\mathrm{TIR}}$ vs. $z$ trend similar to that measured
in recent evolutionary studies of the IRRC. However, for disc-dominated
galaxies, where radio and IR emission should be linked to star formation in the
most straightforward way, we measure very little change in
$\overline{q}_{\mathrm{TIR}}$. This suggests that low-redshift calibrations of
radio emission as an SFR-tracer may remain valid out to at least
$z\,{\simeq}\,1\,{-}\,1.5$ for pure star-forming systems. We find that the
different redshift-evolution of $q_{\rm TIR}$ for the spheroid- and
disc-dominated sample is mainly due to an increasing radio excess for
spheroid-dominated galaxies at $z\,{\gtrsim}\,$0.8, hinting at some residual
AGN activity in these systems. This finding demonstrates that in the absence of
AGN the IRRC is independent of redshift, and that radio observations can
therefore be used to estimate SFRs at all redshifts for genuinely star-forming
galaxies.
