The microjansky and nanojansky radio sky: source population and multiwavelength properties

I present simple but robust estimates of the types of sources making up the faint, sub-microjansky radio sky. These include, not surprisingly, star-forming galaxies and radio-quiet active galactic nuclei but also two 'new' populations, i.e. low radio power ellipticals and dwarf galaxies, the latter likely constituting the most numerous component of the radio sky. I then estimate for the first time the X-ray, optical and mid-infrared fluxes these objects are likely to have, which are very important for source identification and the synergy between the upcoming Square Kilometre Array (SKA) and its various pathfinders with future missions in other bands. On large areas of the sky the SKA, and any other radio telescope producing surveys down to at least the microjansky level, will go deeper than all currently planned (and past) sky surveys, with the possible exception of the optical ones from the Panoramic Survey Telescope and Rapid Response System and the Large Synoptic Survey Telescope. The Space Infrared Telescope for Cosmology and Astrophysics, the James Webb Space Telescope (JWST) and in particular the Extremely Large Telescopes (ELTs) will be a match to the next generation radio telescopes but only on small areas and above similar to 0.1-1 mu Jy (at 1.4 GHz), while even the International X-ray Observatory will only be able to detect a small (tiny) fraction of the microjansky (nanojansky) population. On the other hand, most sources from currently planned all-sky surveys, with the likely exception of the optical ones, will have a radio counterpart within the reach of the SKA. JWST and the ELTs might turn out to be the main, or perhaps even the only, facilities capable of securing optical counterparts and especially redshifts of microjansky radio sources. Because of their sensitivity, the SKA and its pathfinders will have a huge impact on a number of topics in extragalactic astronomy including star formation in galaxies and its co-evolution with supermassive black holes, radio loudness and radio quietness in active galactic nuclei, dwarf galaxies and the main contributors to the radio background.