Big and young SMBHs in the early Universe the case of B2 1023+25

The formation of supermassive black holes in the early Universe is still an obscure and intriguing mechanism. The presence of extremely massive black holes (M_BH>10(9M_&sun;) ) at redshift z>4, specifically in jetted systems, deeply challenges the SMBHs formation models. Justifying the formation of such massive black holes in less than one Gigayear is indeed not trivial, and the presence of a jet make the scenario even less easy to understand. In this perspective, the search of blazars at extremely high redshift is then crucial. Blazars are AGN with a jet directed toward our line of sight, and hence they are reliable tracers of the whole family of jetted AGN. Our research program is centered on a systematic search of these objects at z>4, starting from a complete quasar catalog. I will focus on the successful classification of our best blazar candidate, B2 1023+25 (z=5.3), achieved thanks to NuSTAR observations, and its consequences on the overall picture of SMBHs at extremely high redshift.