2018A&A...610L...7H

A prediction of the current paradigm of the hierarchical assembly of galaxies is the presence of supermassive dual black holes at separations of a few kpc or less. In this context, we report the detection of a narrow-line emitter within the extended Lyα nebula (∼120kpc diameter) of the luminous radio-quiet quasi-stellar object (QSO) LBQS 0302-0019 at z=3.286. We identify several high-ionization narrow emission lines (HeII, CIV, CIII) associated with this point-like source, which we have named "Jil", which is only ∼20kpc (2.9) away from the QSO in projection. Emission-line diagnostics confirm that the source is likely powered by photoionization of an obscured active galactic nucleus (AGN) three orders of magnitude fainter than the QSO. The system represents the tightest unobscured/obscured dual AGN currently known at z>3, highlighting the power of MUSE to detect these elusive systems.