Big and young supermassive black holes in the early Universe

Blazars are Active Galactic Nuclei characterized by relativistic jets launched in the vicinity of the central engine (i.e. a supermassive black hole), that are oriented close to our line of sight. Their peculiar orientation makes them very efficient tracers of the overall jetted population, and due to their brightness they can be visible up to very high redshifts. A deep knowledge of these objects can provide fundamental clues to the models of formation and growth of the first supermassive black holes, but their search in the early Universe must be careful and follow a systematic approach. The discovery in the last $\sim15$ years of extremely massive blazars at very high redshifts ($M_{\rm BH}>10^9M_\odot, \, z>4$) revolutionized our perception of their earliest evolution: there seem to be different formation epochs for extremely massive black holes hosted in jetted ($z\sim4$) and non-jetted systems ($z\sim2.5$). This is not easy to explain, since one would expect that jetted sources accrete less efficiently. Small differences in the population are also derived from the search of such high-$z$ sources: we will go through the open questions in order to understand where the common knowledge stands and which steps must be undertaken to better understand the formation and common evolution of supermassive black holes and jets in the early Universe.