A mature quasar at cosmic dawn revealed by JWST rest-frame infrared spectroscopy

The rapid assembly of the first supermassive black holes is an enduring mystery. Until now, it was not known whether quasar `feeding' structures (the `hot torus') could assemble as fast as the smaller-scale quasar structures. We present JWST/MRS (rest-frame infrared) spectroscopic observations of the quasar J1120+0641 at z = 7.0848 (well within the epoch of reionization). The hot torus dust was clearly detected at λ_rest ≃ 1.3 μm, with a black-body temperature of T_dust=1,413.5_−7.4^+5.7 K, slightly elevated compared to similarly luminous quasars at lower redshifts. Importantly, the supermassive black hole mass of J1120+0641 based on the Hα line (accessible only with JWST), M_BH = 1.52 ± 0.17 × 10⁹ M_⊙, is in good agreement with previous ground-based rest-frame ultraviolet Mg II measurements. Comparing the ratios of the Hα, Paα and Paβ emission lines to predictions from a simple one-phase Cloudy model, we find that they are consistent with originating from a common broad-line region with physical parameters that are consistent with lower-redshift quasars. Together, this implies that J1120+0641's accretion structures must have assembled very quickly, as they appear fully `mature' less than 760 Myr after the Big Bang.