2023ApJ...950...68E

We present the first rest-frame optical spectrum of a high-redshift quasar observed with JWST/NIRCam in Wide Field Slitless mode. The observed quasar, J0100+2802, is the most luminous quasar known at z > 6. We measure the mass of the central supermassive black hole (SMBH) by means of the rest-frame optical H β emission line, and find consistent mass measurements of the quasar's SMBH of M_• ≈ 10¹⁰M_☉ when compared to the estimates based on the properties of rest-frame UV emission lines C IV and Mg II, which are accessible from ground-based observatories. To this end, we also present a newly reduced rest-frame UV spectrum of the quasar observed with X-Shooter/Very Large Telescope (VLT) and FIRE/Magellan for a total of 16.8 hr. We readdress the question whether this ultraluminous quasar could be effected by strong gravitational lensing making use of the diffraction limited NIRCam images in three different wide band filters (F115W, F200W, F356W), which improves the achieved spatial resolution compared to previous images taken with the Hubble Space Telescope by a factor of 2. We do not find any evidence for a foreground deflecting galaxy, nor for multiple images of the quasar, and determine the probability for magnification due to strong gravitational lensing with image separations below the diffraction limit of Δθ ≲ 0.″05 to be ≲2.2 × 10^–3. Our observations therefore confirm that this quasar hosts a 10 billion solar mass black hole less than 1 Gyr after the Big Bang, which is challenging to explain with current black hole formation models.