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In powerful radio-quiet active galactic nuclei (AGN), black holes heavierthan one billion solar masses form at a redshift ~1.5-2. Supermassive blackholes in jetted radio-loud AGN seems to form earlier, at a redshift close to 4.The ratio of active radio-loud to radio-quiet AGN hosting heavy black holes istherefore a rather a strong function of redshift. We report on some recentevidence supporting this conclusion, gathered from the Burst Alert Telescope(BAT, onboard Swift) and by the Large Area Telescope (LAT, onboard Fermi). Wesuggest that the more frequent occurrence of relativistic jets in the mostmassive black holes at high redshifts, compared to later times, could be due tothe average black hole spin being greater in the distant past, or else to thejet helping a fast accretion rate (or some combination of the two scenarios).We emphasize that the large total accretion efficiency of rapidly spinningblack holes inhibits a fast growth, unless a large fraction of the availablegravitational energy of the accreted mass is not converted into radiation, butused to form and maintain a powerful jet.

The role of relativistic jets in the heaviest and most active supermassive black holes at high redshift