GA-NIFS and EIGER: A merging quasar host at z=7 with an overmassive black hole

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 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 ∼10^ 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_*= (3. 0^ with M_ for the companion galaxy. Using the Balmer line, we estimated a virial black hole mass of M_ BH M_⊙. Thus has an extreme black hole--stellar mass ratio of M_ BH /M_*=0. 63^ which is ∼3 dex larger than expected by the local scaling relations between black hole and stellar mass. 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.