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

TL;DR

This study uses JWST observations to measure the stellar mass of a z~6.25 quasar's host galaxy, revealing it is significantly less massive than the black hole, which appears overmassive compared to local relations.

Contribution

First direct measurement of a z~6.25 quasar host galaxy's stellar mass using JWST, showing black holes are overmassive relative to their hosts at high redshift.

Findings

Host galaxy stellar mass ~10^10 M_sun

Black hole is ~15 times more massive than predicted by local M_BH-M* relation

High-redshift quasars show larger scatter in the M_BH-M* relation

Abstract

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.