Detection of the Low-stellar-mass Host Galaxy of a z ∼ 6.25 Quasar with JWST

We characterize the stellar mass of J2239+0207, a z ∼ 6.25 sub-Eddington quasar (M ₁₄₅₀ = −24.6), using dedicated JWST/NIRCam medium-band observations of a nearby point-spread function star to remove the central point source and reveal the underlying galaxy emission. We detect the host galaxy in two bands longward of the Balmer break, obtaining a stellar mass of ∼10¹⁰ M _⊙, more than an order of magnitude less than this quasar’s existing measured [C ii] dynamical mass. We additionally calculate the mass of J2239+0207's central supermassive black hole using JWST/NIRSpec integral field unit observations, and determine that the black hole is ∼15 times more massive than predicted by the local M _BH-M _* relation, similar to many high-redshift quasars with dynamical masses determined via millimeter-wave line widths. We carefully consider potential selection effects at play, and find that even when z ∼ 6 quasars are compared to a local sample with similarly determined dynamical masses, many of the high-redshift quasars appear to possess overmassive black holes. We conclude z ∼ 6 quasars are likely to have a larger spread about the M _BH-M _* relation than observed in the local Universe.