A small and vigorous black hole in the early Universe

TL;DR

This work presents JWST-NIRSpec evidence that GN-z11 hosts an accreting black hole at z=10.6, as indicated by AGN-tracing lines [NeIV]2423 and CII*1335 and ultra-dense BLR gas, plus a high-velocity CIV outflow. The black hole mass is inferred to be ≈1.6×10^6 M⊙ with super-Eddington accretion (~5 L_Edd), compatible with both heavy seed and light-seed growth scenarios that include episodic super-Eddington phases. Photoionization modelling and multi-line diagnostics argue for BLR-like densities (n_H > 10^9 cm^-3) and a spectrum consistent with AGN broad-line features, while WR-star contributions appear insufficient to explain the data. These findings demonstrate that massive black holes can form and grow very early in the Universe, offering a natural explanation for GN-z11's luminosity and informing seed formation models and early AGN demographics in JWST surveys.

Abstract

Multiple theories have been proposed to describe the formation of black hole seeds in the early Universe and to explain the emergence of very massive black holes observed in the first billion years after Big Bang. Models consider different seeding and accretion scenarios, which require the detection and characterisation of black holes in the first few hundred million years after Big Bang to be validated. Here we present an extensive analysis of the JWST-NIRSpec spectrum of GN-z11, an exceptionally luminous galaxy at z=10.6, revealing the detection of the [NeIV]2423 and CII*1335 transitions (typical of Active Galactic Nuclei, AGN), as well as semi-forbidden nebular lines tracing gas densities higher than 10^9 cm-3, typical of the Broad Line Region of AGN. These spectral features indicate that GN-z11 hosts an accreting black hole. The spectrum also reveals a deep and blueshifted CIV1549 absorption trough, tracing an outflow with velocity 800-1000 km/s, likely driven by the AGN. Assuming local virial relations, we derive a black hole mass of log(M_BH/Msun) = 6.2 +- 0.3, accreting at about 5 times the Eddington rate. These properties are consistent with both heavy seeds scenarios, or scenarios envisaging intermediate/light seeds experiencing episodic super-Eddington phases. Our finding naturally explains the high luminosity of GN-z11 and can also provide an explanation for its exceptionally high nitrogen abundance.

Figures (9)

• Figure 1: Zoom in of the spectra of GN-z11 around specific spectral features of interest, along with their single/multiple Gaussian models (see Methods). Dashed lines indicate the rest-frame wavelengths of the lines at z=10.603. a) [NeIV]$\lambda\lambda$2422,2424 doublet; b) NIII] multiplet, illustrating the detection of the resolved NIII]$\lambda$1754 emission; c) NIV] doublet, showing the absence of [NIV]$\lambda$1483 despite the strong NIV]$\lambda$1486; d) CIV blueshifted absorption trough and redshifted resonant emission, compared with the CIV P-Cygni profile observed in low-metallicity, young star-forming galaxies (stack: orange dashed line; most extreme case: orange dotted line), showing inconsistency with the latter. e̱) CII/CII$^*\lambda\lambda$1334,1335 doublet (seen in emission, without P-Cygni, only in type 1 AGN); f) expected flux of the NIV1718 line in the case that NIV]1486 was associated with WR stars. In panels a, b, c, e, and f the continuum is subtracted, while in panel d the continuum is normalised to one. The grey dotted lines indicate the noise level (1 $\sigma$).
• Figure 2: Flux ratios of density-sensitive nitrogen lines as a function of hydrogen gas density, $n_\text{H}$. A large range of Cloudy models (see Methods) are compared with the values observed in GN-z11. Models with metal-poor ($Z_\text{neb} = 0.1 \, \mathrm{Z_\odot}$) and metal-rich ($Z_\text{neb} = 1 \, \mathrm{Z_\odot}$) gas are shown with solid lines and dashed lines, respectively, (color-coded according to the ionization parameter $U$) in the scenario where either an AGN (filled symbols demarcating different black body temperatures for the accretion disc, $T_\text{AGN}$) or stellar populations (open markers for various ages, $t_*$) is responsible for the incident radiation field. Top: [NIV]$\lambda$1483/NIV]$\lambda$1486 flux ratio. Bottom: ratio of NIII]$\lambda$1754 to total flux of the multiplet. The black dashed lines and blue shaded regions (in decreasing darkness for $1\sigma$, $2\sigma$, and $3\sigma$ confidence level as indicated) show the observed fractional contribution of NIII]$\lambda$1754 and upper limit on [NIV]$\lambda$1483/NIV]$\lambda$1486 obtained for GN-z11, indicating that the gas emitting these lines has high density ($n_\text{H} \gtrsim 10^{9} \, \mathrm{cm^{-3}}$ at $3\sigma$). The light green shaded areas highlight the range of densities typical of the Broad Line Regions (BLRs), while the gray shaded regions highlight the range of densities typical of the ionized ISM.
• Figure 3: Black hole mass as a function of redshift (on a logarithmic scale) and age of the Universe. The black hole mass inferred for GN-z11 is shown with the large golden symbol. The red shaded region indicates the evolution expected in the case of super-Eddington accretion at the level inferred for GN-z11. The darker blue shaded region shows the black hole mass evolution assuming Eddington-limited accretion, while the lighter blue shaded region shows the case of evolution in the case of sub-Eddington accretion (between 0.1 and 1 the Eddington rate). The horizontal gray shaded regions indicate the range of black hole seeds expected by different scenarios. Solid and dashed lines indicate the evolutionary tracks of various simulations and models Bennett23Zhang_Trinity_23_IVSchneider23 that can reproduce the GN-z11 BH mass, with different seeding and accretion rate assumptions, as detailed in the Methods. The small gray symbols indicate the black holes measured in quasars at z$\sim$6--7.5 inayoshi+2020fan_quasars_2022 (whose representative 1$\sigma$ errorbar is shown in the top-left), most of which can originate from a progenitor like the black hole in GN-z11.
• Figure 4: Black hole versus stellar mass diagram, showing the location of GN-z11 (large golden symbol), compared to local galaxies as indicated by the small red symbols and their best-fit relation (black solid line and uncertainty traced by the gray shaded region) Reines15. The grey symbols show the values estimated for quasars at z$\sim 6-7$izumi_subaru_2019, although in these cases the galaxy mass is inferred from dynamical tracers. The blue symbols are AGN at z$>$4 for which the black hole and galaxy stellar mass has been measured with JWST data (see Methods) using the same calibration as Reines15 for consistency.
• Figure 5: Zoom in on the additional emission lines fitted. a) CIII]$\lambda$1906,1908 doublet. As the doublet is unresolved, the fit turns out degenerate between line width and fluxes of the two components; moreover it is also contributed to by star formation in the host galaxy (see text for details); b) MgII$\lambda$2796,2804 doublet; c) HeII$\lambda$1640; d) Ly$\alpha$, NV$\lambda$1238,1242 doublet (undetected) and SiII$\lambda$1260,1264 (undetected), corrected for the Ly$\alpha$ damping wing; e) [NeIII]$\lambda$3869 profile compared with the Balmer lines H$\delta$ and H$\gamma$. In all panels, the continuum is subtracted. The black dotted lines indicate the noise level.