A small and vigorous black hole in the early Universe.

Multiple theories have been proposed to describe the formation of black hole seeds in the early Universe and to explain the emergence of very massive black holes observed in the first billion years after Big Bang 1-3 . Models consider different seeding and accretion scenarios 4-7 , which require the detection and characterisation of black holes in the first few hundred million years after Big Bang to be validated. Here we present an extensive analysis of the JWST-NIRSpec spectrum of GN-z11, an exceptionally luminous galaxy at z = 10.6, revealing the detection of the [NeIV]λ2423 and CII*λ1335 transitions (typical of Active Galactic Nuclei, AGN), as well as semi-forbidden nebular lines tracing gas densities higher than 10 9 cm -3 , typical of the Broad Line Region of AGN. These spectral features indicate that GN-z11 hosts an accreting black hole. The spectrum also reveals a deep and blueshifted CIVλ1549 absorption trough, tracing an outflow with velocity 800 - 1000 km s -1 , likely driven by the AGN. Assuming local virial relations, we derive a black hole mass of log (M BH /M ⊙ ) = 6.2 ± 0.3, accreting at about 5 times the Eddington rate. These properties are consistent with both heavy seeds scenarios, or scenarios envisaging intermediate/ light seeds experiencing episodic super-Eddington phases. Our finding naturally explains the high luminosity of GN-z11 and can also provide an explanation for its exceptionally high nitrogen abundance.