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Resolved maps of the thermal and nonthermal radio continuum (RC) emission ofdistant galaxies are a powerful tool for understanding the role of theinterstellar medium (ISM) in the evolution of galaxies. We simulate the RCsurface brightness of present-day star forming galaxies in the past at$0.15<z<3$ considering two cases of radio size evolution: (1)~no evolution, and(2)~same evolution as in the optical. We aim to investigate the a)~structure ofthe thermal and nonthermal emission on kpc scales, b)~evolution of the thermalfraction and synchrotron spectrum at mid-radio frequencies ($\simeq$1-10\,GHz),and c)~capability of the proposed SKA1-MID reference surveys in detecting theRC emitting structures. The synchrotron spectrum flattens with $z$ causingcurvature in the observed mid-radio SEDs of galaxies at higher $z$. Thespectral index reported in recent observational studies agrees better with theno size evolution scenario. In this case, the mean thermal fraction observed at1.4\,GHz increases with redshift by more than 30\% from $z=0.15$ to $z=2$because of the drop of the synchrotron emission at higherrest-frame~frequencies. More massive galaxies have lower thermal fractions andexperience a faster flattening of the nonthermal spectrum. The proposedSKA1-MID band~2 reference survey, unveils the ISM in M51- and NGC6946-likegalaxies (with ${\rm M_{\star}}\simeq10^{10}\,{\rm M}_{\odot}$) up to $z=3$.This survey detects lower-mass galaxies like M33 (${\rmM_{\star}}\simeq10^{9}\,{\rm M}_{\odot}$) only at low redshifts $z\lesssim0.5$. For a proper separation of the RC emitting processes at the peak of starformation, it is vital to include band~1 into the SKA1-MID reference surveys.

Evolution of thermal and nonthermal radio continuum emission on kpc scales--Predictions for SKA