  We investigate the astrophysics of radio-emitting star-forming galaxies and
ac- tive galactic nuclei (AGNs), and elucidate their statistical properties in
the radio band including luminosity functions, redshift distributions, and
number counts at sub-mJy flux levels, that will be crucially probed by
next-generation radio continuum surveys. Specifically, we exploit the
model-independent approach by Mancuso et al. (2016a,b) to compute the star
formation rate functions, the AGN duty cycles and the conditional probability
of a star-forming galaxy to host an AGN with given bolometric luminosity.
Coupling these ingredients with the radio emission properties associated to
star formation and nuclear activity, we compute relevant statistics at
different radio frequencies, and disentangle the relative con- tribution of
star-forming galaxies and AGNs in different radio luminosity, radio flux, and
redshift ranges. Finally, we highlight that radio-emitting star-forming
galaxies and AGNs are expected to host supermassive black holes accreting with
different Eddington ratio distributions, and to occupy different loci in the
galaxy main sequence diagrams. These specific predictions are consistent with
current datasets, but need to be tested with larger statistics via future radio
data with multi-band coverage on wide areas, as it will become routinely
achievable with the advent of the SKA and its precursors.
