  The central nuclei of galaxies, where super-massive black holes (SMBHs) are
thought to reside, can experience phases of activity when they become Active
Galactic Nuclei (AGN). An AGN can eject winds, jets, and produce radiation
across the entire electromagnetic spectrum. The fraction of the bolometric
emission in the radio spans a factor of ~10^5 across the different AGN classes.
The weakest radio sources, radio-quiet (RQ) AGN, are typically 1,000 times
fainter than the radio-loud (RL) AGN, and represent the majority of the AGN
population. In RL AGN, radio emission is essentially all produced by
synchrotron emission from a relativistic jet. In contrast, in RQ AGN the
absence of luminous jets allows us to probe radio emission from a wide range of
possible mechanisms, from the host galaxy kpc scale down to the innermost
region near the SMBHs: star formation, AGN driven wind, free-free emission from
photo-ionized gas, low power jet, and the innermost accretion disc coronal
activity. All these mechanisms can now be probed with unprecedented precision
and spatial resolution, thanks to the current and forthcoming generation of
highly sensitive radio arrays.
