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

Abstract: 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 $\sim10^{10}M_\odot$, suggesting that this is a major galaxy interaction. Through detailed quasar subtraction and SED fitting using the NIRCam data, we obtain an estimate of the host stellar mass of $M_{\ast}=(3.0^{+2.5}{-1.4})\times10^9M\odot$, with $M_{*}=(2.7^{+0.5}{-0.5})\times10^9M\odot$ for the companion galaxy. Using the H$\beta$ Balmer line we estimate a virial black hole mass of $M_{\rm{BH}}=(1.9^{+2.9}_{-1.1})\times10^9 M_\odot$. Thus, J1120+0641 has an extreme black hole-stellar mass ratio of $M_{\rm{BH}}/M_\ast=0.63^{+0.54}_{-0.31}$, 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.