A small and vigorous black hole in the early Universe

The formation of black hole seeds in the early Universe and the subsequent emergence of very massive black holes within the first billion years after the Big Bang remain significant questions in astrophysics. Several theoretical models propose different seeding and accretion scenarios to explain these observations. These models, however, require observational validation through the detection and detailed characterization of black holes in the early Universe, specifically within the first few hundred million years after the Big Bang. This study focuses on GN-z11, an exceptionally luminous galaxy at redshift z = 10.6, recently observed with the James Webb Space Telescope (JWST). Previous NIRCam imaging resolved an unresolved nuclear component and a larger disk-like component with a radius of a few hundred parsecs. A preliminary NIRSpec spectrum analysis suggested star formation as the primary energy source, though the presence of an AGN was not ruled out. This study delves into the AGN scenario using a deeper, more extensive NIRSpec spectrum of GN-z11 to investigate the possibility of a black hole powering this early universe galaxy.

Several papers have explored the formation of the first massive black holes. Inayoshi et al. (2020) reviewed the assembly of the first massive black holes, Fan et al. (2023) examined quasars and the intergalactic medium at cosmic dawn, and Volonteri et al. (2023) discussed the possibility of massive black holes in young, high redshift galaxies observed by JWST. Other works have focused on the low-end of the black hole mass function at cosmic dawn (Trinca et al., 2022), the formation of supermassive black holes from Population III seeds (Banik et al., 2019; Singh et al., 2023), and the growth of gargantuan black holes powering high-redshift quasars and their impact on galaxy formation (Bennett et al., 2024). The properties of GN-z11 itself have been studied previously, particularly its interstellar medium and stellar populations (Tacchella et al., 2023), and its Lyman-alpha emission and potential nitrogen abundance (Bunker et al., 2023). This paper builds upon this prior research to provide further insights into the nature of GN-z11's energy source.

The study utilized JWST-NIRSpec spectroscopic data of GN-z11. The analysis focused on specific spectral features to determine the presence and properties of an active galactic nucleus (AGN). The detection of the [Ne V]λ2422,2424 doublet, requiring photons with energy greater than 63.5 eV, served as a key indicator of AGN activity, as this line is absent in star-forming galaxies even those with Wolf-Rayet stars. Further evidence was provided by the detection of C II*λ1335 emission, a common AGN tracer. The analysis included detailed modeling of semi-forbidden nebular lines, such as [N IV]λ1483, [N IV]λ1486, and the N III] multiplet, to determine gas densities. The observed line ratios were compared to a wide range of photoionization models generated using Cloudy, considering variations in metallicity, ionization parameter, and the nature of the ionizing source (either AGN or star formation). The presence of a deep and blueshifted CIVλ1549 absorption trough was also examined, indicative of a high-velocity outflow. Comparison with stacked spectra of local galaxies with similar metallicity helped rule out a stellar-wind origin for this feature, further supporting an AGN-driven outflow. Black hole mass estimation employed local virial relations, considering line widths and continuum luminosity, with potential uncertainties and caveats discussed. Bolometric luminosity was calculated, and the accretion rate was determined relative to the Eddington limit. Finally, the analysis considered the high nitrogen abundance observed in GN-z11, evaluating its compatibility with the AGN scenario and comparing to the literature on 'nitrogen-loud' AGN.

The key findings of this paper strongly support the presence of an accreting black hole, specifically an AGN, within GN-z11. The detection of [Ne V]λ2423 and C II*λ1335 transitions provided unambiguous evidence for AGN activity. Analysis of semi-forbidden nebular lines ([N IV]λ1483, [N IV]λ1486, N III] multiplet) revealed gas densities exceeding 10⁹ cm⁻³, consistent with the broad-line regions (BLRs) of AGN and far exceeding densities observed in star-forming galaxies. The observed deep, blueshifted CIVλ1549 absorption trough, tracing an outflow with velocities of 800–1,000 km s⁻¹, further corroborates the AGN scenario. Using local virial relations, the black hole mass was estimated at log(M_BH/M_☉) = 6.2 ± 0.3, accreting at about five times the Eddington rate. This super-Eddington accretion is consistent with the high luminosity observed in GN-z11. The high nitrogen abundance in GN-z11 is also explained within the AGN context, as several 'nitrogen-loud' AGN have been previously identified. The mass of the BLR, estimated based on the observed Hγ luminosity, is small enough that only a few supernovae would be sufficient to enrich it to near solar or super-solar metallicity, easily achievable considering the very high gas densities in the BLR. The evolutionary tracks of the black hole mass at earlier cosmic epochs, considering various accretion rates (Eddington, super-Eddington, and sub-Eddington), are consistent with several semi-analytical models and cosmological simulations of black hole seed formation and growth.

The findings of this study directly address the question of early black hole formation and growth. The detection of an AGN in GN-z11 at redshift z = 10.6 provides strong observational evidence supporting models proposing heavy seeds or episodic super-Eddington accretion in intermediate or light seeds. The high luminosity of GN-z11, previously challenging to explain solely through star formation, is readily accounted for by the presence of a rapidly accreting black hole. The results are significant as they confirm the existence of an accreting black hole in the very early Universe, offering insight into the processes that lead to the formation of supermassive black holes observed at higher redshifts. The observed properties of GN-z11, particularly its super-Eddington accretion rate, are consistent with numerous simulations predicting the rapid growth of supermassive black holes in the early Universe. If other hyperluminous galaxies at high redshift share similar characteristics, it would alleviate some of the tension between observations and theoretical models of galaxy evolution.

This study provides compelling evidence for a small but rapidly accreting black hole within the early universe galaxy GN-z11. The detection of characteristic AGN spectral features, high gas densities, and a high-velocity outflow, coupled with black hole mass and accretion rate estimates, strongly supports this conclusion. These findings offer crucial insights into the early formation and growth of black holes, favoring models incorporating either heavy seeds or episodic super-Eddington accretion of lighter seeds. Future research should focus on spectroscopic observations of similar hyperluminous galaxies at high redshift to confirm if the AGN scenario is prevalent among these objects and to refine our understanding of black hole seed formation and growth in the early universe. The analysis of more high-redshift galaxies with JWST may also allow for more robust studies of AGN evolutionary paths and black hole-galaxy co-evolution.

The study relies on local virial relations to estimate black hole mass, which may not be entirely accurate for AGN at such high redshifts. Some of the spectral features could potentially be influenced by contributions from both the AGN and the host galaxy’s star formation, and separating these contributions could be challenging. The uncertainty associated with the determination of the bolometric luminosity and the associated Eddington ratio should be kept in mind.