A dormant, overmassive black hole in the early Universe

TL;DR

The paper reports the discovery of a broad Hα line in a z=6.68 galaxy GN-1001830 with a BH mass of about $4\times10^{8}$ $M_\odot$ accreting at ~2% of Eddington, yielding an extreme BH-to-stellar-mass ratio (~0.4).Using JWST/JADES NIRSpec/NIRCam data, the authors perform robust spectral and spatial decompositions to derive BH mass, bolometric luminosity, host stellar mass, star formation rate, dynamical mass, and gas content, and they compare the system to BH scaling relations and simulations.The results support a scenario in which short bursts of super-Eddington accretion drive rapid BH growth, followed by long dormant phases that leave the host galaxy with low SFR and a large BH-to-stellar-mass offset, challenging purely Eddington-limited, heavy-seed models.Comparisons with FABLE and other simulations indicate that bursty growth with super-Eddington episodes can reproduce objects like GN-1001830, suggesting that dormant, overmassive BHs may be common in the early Universe.

Abstract

Recent observations have found a large number of supermassive black holes already in place in the first few hundred million years after Big Bang. The channels of formation and growth of these early, massive black holes are not clear, with scenarios ranging from heavy seeds to light seeds experiencing bursts of high accretion rate. Here we present the detection, from the JADES survey, of broad Halpha emission in a galaxy at z=6.68, which traces a black hole with mass of ~ 4 * 10^8 Msun and accreting at a rate of only 0.02 times the Eddington limit. The host galaxy has low star formation rate (~ 1 Msun/yr, a factor of 3 below the star forming main sequence). The black hole to stellar mass ratio is ~ 0.4, i.e. about 1,000 times above the local relation, while the system is closer to the local relations in terms of dynamical mass and velocity dispersion of the host galaxy. This object is most likely the tip of the iceberg of a much larger population of dormant black holes around the epoch of reionisation. Its properties are consistent with scenarios in which short bursts of super-Eddington accretion have resulted in black hole overgrowth and massive gas expulsion from the accretion disk; in between bursts, black holes spend most of their life in a dormant state.

Figures (19)

• Figure 1: Prism spectrum and H$\alpha$ line of GN-1001830.a shows the 2D prism spectrum, b - the 1D prism (black line in the bottom panel) with marked emission lines. The spectrum around H$\alpha$ is shown in c illustrating the presence of a broad component. Lines shown are the observed spectrum (black solid line, with grey shading indicating $1\sigma$ uncertainties) along with the best fit line to the narrow (red dashed) and broad (green dashed) components. The [NII] doublet is shown in blue and it is only marginally detected at 2$\sigma$. The magenta solid line shows the total fit. The greyed line portion at $\sim 4.95\ \mu$m of the spectrum shows the region, which was masked due to a possible artefact or H$\alpha$ emission from a lower redshift interloper. d shows the fit residuals for a simple narrow H$\alpha$ and [N ii] fit (black line) and the best-fit, containing a broad component (purple line). It can be seen that the narrow line only fit does not account for the broad wings of the line, leaving significant systematic residuals.
• Figure 2: Comparison of GN-1001830 with other high-z AGN and models in terms of accretion rate, black hole mass and stellar mass of the host galaxy.Left panels (a, c): accretion rate relative to the Eddington limit, $\lambda_{\rm Edd}$, versus black hole mass, $\log{M_{\rm BH}}$. Right panels (b, d): Black hole mass versus stellar mass of the host galaxy $\log{M_{\rm star}}$. Green dashed lines indicate constant $M_{\rm BH}/M_{\rm star}$ ratios, while the solid green line represents the local relation from VolonteriBHmass, the shaded region shows the scatter. In all panels GN-1001830 is indicated with a magenta circle with errorbars. Top panels (a, b): Comparison with other JWST-discovered AGN at high redshift (blue symbols Harikane_AGN, Maiolino_AGN, Matthee2023,Ubler2023, CArnall2023_AGN, Goulding2023_AGN, Kokorev2023_AGN,Ubler23a, Furtak2023_AGN) and with the QSO population at similar redshifts (orange/yellow symbols XQR30, Hyperion,Stone2023_QSO, Ding2023_QSO, Yue2023_QSO). The observed negative correlation between $\lambda_{\rm Edd}$ and $M_{\rm BH}$ is likely reflective of Eddington luminosity's dependence on black hole mass and observational incompleteness and not a separate physical phenomenon. Bottom panels (c, d): Comparison with the predictions (at z$\sim$7) from the semi-analytical models from Schneider2023, Trinca2022 in the scenario of Eddington-limited accretion (grey points) and the scenario of light or heavy seeds that can experience super-Eddington accretion (red shaded contours).
• Figure 3: Completeness simulation results on the Eddington ratio versus black hole mass plane. Points colored in blue hues show the previously discovered JWST sources at 6$<$z$<$8, as in Fig.\ref{['fig:comparisons']}. The dark green points show the simulated AGN (at z$\sim$7) in the scenario of super-Eddington bursts. GN-1001830 is indicated with magenta circle with errorbars. The color shading indicates the completeness of the JADES spectroscopic survey in detecting black holes with a given mass and accreting at a given rate relative to Eddington. It can be readily seen that most of the low accretion rate AGN predicted by super-Eddington bursts lie in the sub-50% completeness region and that GN-1001830 overlaps them at the edge of the high completeness region.
• Figure 4: Prism spectrum of GN-1001830. The top panel shows the 2D spectrum with the y axis representing the shutter pitch and yellower portions showing more positive flux. The bottom panel shows the extracted 1D prism spectrum with emission line locations indicated by colored vertical lines. The (noisier) R1000 spectrum is shown in blue, the wavelength range is narrowed with respect to Fig.\ref{['fig:halpha_fit']} to leave out the noisiest parts of R1000. The panel to the right shows a zoomed-in view on the blended H$\gamma$ and [O iii]$\lambda$4363 feature along with its decomposition into two Gaussian profiles.
• Figure 5: Image of GN-1001830. A red-green-blue (RGB) image of the AGN and galaxy in the F444W, F277W and F115W bands. A 1 kpc physical scale bar is overplotted alongside the FWHM of the PSF in the F444W band. The position of the NIRSpec slit is also overplotted.