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

Abstract

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 ($A_V\simeq3$) broad emission-line Active Galactic Nucleus (AGN) at a redshift of $z_{\mathrm{spec}}=7.0451\pm0.0005$. From the width of H$β$ ($\mathrm{FWHM}=2800\pm250\,\frac{\mathrm{km}}{\mathrm{s}}$) we derive a black hole mass of $M_{\mathrm{BH}}=4_{-1}^{+2}\times10^7\,\mathrm{M}_{\odot}$. 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 ($L_{\mathrm{bol}}=(1.1\pm0.3)\times10^{45}\,\frac{\mathrm{erg}}{\mathrm{s}}$) 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).

Figures (3)

• Figure 1: Composite-color image cutouts of the three images of A2744-QSO1. The $2.4"\times2.4"$ cutouts of the UNCOVER JWST-NIRCam composite-color image furtak23c (Blue: F115W+F150W, Green: F200W+F277W, Red: F2356W+F410M+F444W) show the three images of A2744-QSO1 overlaid with the NIRSpec MSA slitlets of one of our MSA masks as an example (the three images were targeted with several masks and the exact position of the object in each slit may slightly shift).
• Figure 2: NIRSpec-prism spectrum of A2744-QSO1. Panel a) shows the de-magnified stacked spectrum (black), equivalent to $\sim1700$ h integration time on target once the lensing is factored-in, and its $1\sigma$-uncertainty range (grey). In addition to its red rest-frame optical continuum furtak23d, the source clearly displays strong Hydrogen emission lines in the Balmer-series and Lyman-$\alpha$. The emission lines allow us to precisely measure the spectroscopic redshift at $z_{\mathrm{spec}}=7.0451\pm0.0005$. We overlay the NIRCam photometry measured previously furtak23d as red circles. Securely identified and detected ($\geq3\sigma$) emission lines are marked in purple and selected non-detected lines are shown in orange. Panel b) shows the individual (magnified) spectra of the three images. All three images perfectly align in wavelength space, thus confirming the triply-imaged nature of our object. Panel c) shows the continuum-subtracted H$\beta$ line (black) and our Gaussian fit to the line (purple). The red shaded areas delimit the region in which the fit is performed and the dark red curve illustrates the NIRSpec-prism LSF at the wavelength of H$\beta$. All errors shown represent $1\sigma$ uncertainties.
• Figure 3: A2744-QSO1 compared to other broad-line AGN. The top row shows A2744-QSO1 (red star) in the black hole mass-luminosity space of high-redshift broad-line AGN. Purple circles show UV-bright AGN observed from the ground yang21izumi21, the violet circles show AGN detected with JWST at $z\sim6-7$fujimoto22harikane23maiolino23b, and the orange and yellow symbols show recently confirmed AGN at higher redshifts larson23goulding23maiolino23a. While A2744-QSO1 is notably fainter in the UV than other AGN recently detected with JWST (a) because of its strong dust attenuation, it is typical in the $M_{\mathrm{BH}}-L_{\mathrm{bol}}$ plane (b). The bottom row shows A2744-QSO1 in $M_{\mathrm{BH}}-M_{\star}$-space (c) and in number-densities (d) compared to the DELPHI simulations dayal19piana21 (blue) and AGN observed at low redshift reines15suh20 (green). The dark blue points show the default DELPHI runs, which predict much higher host stellar masses and much lower number density for a black hole of this mass. The light blue points show the models with feedback turned off, in which case the models can approach the observations of A2744-QSO1. Error bars represent $1\sigma$ uncertainties.