Detection of the Low-Stellar Mass Host Galaxy of a $z\sim6.25$ quasar with JWST

Résumé : 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 PSF 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 ~10^10 M_sun, 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 IFU 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.