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 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∗=(3.01.4+2.5) × 109 M⊙, with M∗=(2.70.5+0.5) × 109 M⊙ for the companion galaxy. Using the Hβ Balmer line, we estimated a virial black hole mass of MBH = (1.91.1+2.9) × 109 M⊙. Thus, J1120+0641 has an extreme black hole-stellar mass ratio of MBH/M∗ = 0.630.31+0.54, 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.