  We argue that the difference in infrared-to-radio luminosity ratio between
local and high-redshift star-forming galaxies reflects {the alternative
physical conditions} -- including magnetic field configurations -- of the
dominant population of star-forming galaxies in different redshift ranges. We
define three galactic types, based on our reference model, with reference to
ages of stellar populations. ``Normal'' late-type galaxies dominate the star
formation in the nearby Universe; ``starburst'' galaxies take over at higher
redshifts, up to z ~1.5; while ``protospheroidal'' galaxies dominate at high
redshift. A reanalysis of data from the COSMOS field combined with literature
results shows that, for each population, the data are consistent with an almost
redshift-independent mean value of the parameter q_IR, which quantifies the
infrared-radio correlation. However, we find a hint of an upturn of the mean
q_IR at z>~3.5 consistent with the predicted dimming of synchrotron emission
due to cooling of relativistic electrons by inverse Compton scattering off the
cosmic microwave background. The typical stellar masses increase from normal,
to starburst, and to protospheroidal galaxies, accounting for the reported
dependence of the mean q_IR on stellar mass. Higher values of q_IR found for
high-z strongly lensed dusty galaxies selected at 500 micron might be explained
by differential magnification.
