A high black hole to host mass ratio in a lensed AGN in the early Universe

Early JWST observations have uncovered a new population of red sources that might represent a previously overlooked phase of supermassive black hole growth (Kocevski et al. 2023; Matthee et al. 2023, Labbé et al. 2023). One of the most intriguing examples is an extremely red, point-like object that was found to be triply-imaged by the strong lensing (SL) cluster Abell 2744 (Furtak et al. 2023). Here we present deep JWST/NIRSpec observations of this object, Abell2744-QSO1. The spectroscopy confirms that the three images are of the same object, and that it is a highly reddened ($A_V\simeq 3$) broad emission-line Active Galactic Nucleus (AGN) at a redshift of $z_{\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}}=7.0451\pm 0.0005$. From the width of H$\beta$ ($\mathrm{F}\mathrm{W}\mathrm{H}\mathrm{M}=2800\pm 250\frac{\mathrm{k}\mathrm{m}}{\mathrm{s}}$) we derive a black hole mass of $M_{\mathrm{B}\mathrm{H}}=4_{-1}^{+2}\times {10}^7{\mathrm{M}}_{\odot }$. We infer a very high ratio of black hole to galaxy mass of at least 3%, an order of magnitude more than is seen in local galaxies (Bennert et al. 2011), and possibly as high as 100%. The lack of strong metal lines in the spectrum together with the high bolometric luminosity ($L_{\mathrm{b}\mathrm{o}\mathrm{l}}=(1.1\pm 0.3)\times {10}^{45}\frac{\mathrm{e}\mathrm{r}\mathrm{g}}{\mathrm{s}}$) indicate that we are seeing the black hole in a phase of rapid growth, accreting at 30% of the Eddington limit. The rapid growth and high black hole to galaxy mass ratio of A2744-QSO1 suggest that it may represent the missing link between black hole seeds (Volonteri et al. 2021) and the first luminous quasars (Fan et al. 2022).