2025A&A...702A..50M

The James Webb Space Telescope is revolutionising our ability to understand the host galaxies and local environments of high-z quasars. Here we obtain a comprehensive understanding of the host galaxy of the z = 7.08 quasar J1120+0641 by combining NIRSpec integral field spectroscopy with NIRCam photometry of the host continuum emission. Our emission-line maps reveal that this quasar host is undergoing a merger with a bright companion galaxy. The quasar host and the companion have similar dynamical masses of ∼10¹⁰ M_⊙, suggesting that this is a major galaxy interaction. Through detailed quasar subtraction and SED fitting using the NIRCam data, we obtained an estimate of the host stellar mass of $ M_{\ast}=(3.0^{+2.5}_{-1.4})\times10^9\,M_\odot $, with $ M_{*}=(2.7^{+0.5}_{-0.5})\times10^9\,M_\odot $ for the companion galaxy. Using the Hβ Balmer line, we estimated a virial black hole mass of $ M_\mathrm{{BH}}=(1.9^{+2.9}_{-1.1})\times10^9\,M_\odot $. Thus, J1120+0641 has an extreme black hole-stellar mass ratio of $ M_\mathrm{{BH}}/M_\ast=0.63^{+0.54}_{-0.31} $, which is ∼3 dex larger than expected by the local scaling relations between black hole and stellar mass. J1120+0641 is powered by an overmassive black hole with the highest reported black hole-stellar mass ratio in a quasar host that is currently undergoing a major merger. These new insights highlight the power of JWST for measuring and understanding these extreme first quasars.