2026A&A...706A..99C

We present the first near-infrared interferometric data of a QSO at z = 4. The K-band observations were performed with GRAVITY+ on the VLTI using all four UTs, detecting a differential phase signal that traces the spatially resolved kinematics for both the Hβ and Hγ lines in the broad-line region (BLR). We fit the two lines simultaneously with an updated model that includes distinct rotating and conical outflowing components. For the best-fit model, more than 80% of the H I line emission from the BLR originates in an outflow with a velocity up to 10⁴ km s^–1. This is oriented so that our line of sight is along an edge of the conical structure, which produces the prominent blue wing on the line profile. A combination of anisotropic line emission and mid-plane opacity leads to the single-sided phase signal. The model is able to qualitatively match both the outflowing C IV line profile and the systemic O I fluorescent emission. The black hole mass of 8 × 10⁸ M_⊙ that we derive is the highest redshift black hole mass measurement to date obtained directly from BLR dynamics. It is an order of magnitude lower than that inferred from various single epoch scaling relations, and it implies that the accretion is highly super-Eddington. With reference to recent simulations, the data suggest that this QSO is emitting close to its radiative limit in a regime where strong outflows are expected around a polar conical region.