UNCOVER: The growth of the first massive black holes from JWST/NIRSpec –
spectroscopic redshift confirmation of an X-ray luminous AGN at z=10.1

Until the launch of the James Webb Space Telescope (JWST), the earliest black holes known were a handful of extremely UV-luminous $z\approx 7$ quasars (e.g., Mortlock et al., 2011; Bañados et al., 2018; Matsuoka et al., 2018, 2023). While these sources are quite rare, given their high estimated masses, their existence leads to a timing challenge for the formation of supermassive black holes (SMBHs). If their initial seeds originate from the death of the first massive stars, with a typical remnant mass of $\sim 100$ $M_{\odot}$  these black holes would need to accrete at or above the Eddington limit continuously for $\sim 700-800$ million years in order to reach the observed masses of $>10^{9}$ $M_{\odot}$ (see Natarajan, 2011; Fan et al., 2019, for a review). Theorists have therefore explored alternate seed formation models, with heavier seed BH mass functions ($\sim 10^{4}$ $M_{\odot}$) that could form from the direct collapse of gas in the high redshift Universe (Lodato & Natarajan, 2006, 2007; Volonteri et al., 2008; Inayoshi et al., 2022, see for instance). These heavy seeds are expected to be rare in general (e.g., Agarwal et al., 2013; Dayal et al., 2019; Habouzit et al., 2022), and their individual future growth trajectories are unclear. Therefore, it is unclear if UHZ-1 is a likely progenitor for the luminous optically detected SDSS quasars. Guidance from cosmological simulations that track BH growth, the MASSIVEBLACK suite in particular, have shown that the most massive BH at $z\sim 10$ does not necessarily grow to remain the most massive BH by $z=6$ (Di Matteo et al., 2017, 2023). The details of the environment play an important role in shaping the accretion and, therefore, growth history of BHs.

The situation has gotten decidedly more interesting with the launch of JWST. A number of intriguing active galactic nuclei (AGN) candidates have been spectroscopically confirmed at more moderate luminosities (Kocevski et al., 2023; Matthee et al., 2023), with some discovered at $z>7$ (Harikane et al., 2022; Furtak et al., 2023a; Larson et al., 2023; Maiolino et al., 2023a, b). In the absence of other direct indicators of AGN activity (e.g., broadened Balmer emission lines), the most unambiguous identification of AGN activity is through the detection of high-energy X-ray emission. Due to the negative k-correction at X-ray energies, high redshift AGN are preferentially detected at increasingly harder X-ray energies, making their identification more distinct from lower energy X-rays produced by contaminating star-formation emission.

Harnessing the exquisite spatial resolution afforded by the Chandra X-ray Observatory, the current record-holder to-date for the most distant ($z_{\rm phot}\sim 10.3$; Castellano et al. 2023) X-ray luminous AGN is UHZ-1, discovered behind the lensing cluster Abell 2744 (Bogdan et al., 2023). Previously identified as an extremely high-redshift candidate (Castellano et al., 2023), UHZ-1 is reported with a robust $4.2-4.4\sigma$ detection in the observed-frame 2–7 keV band with 20.6 net counts. At $z>10$, these are extremely hard X-ray photons with rest-frame energies of $E\sim 22-80$ keV, which can only arise from an accreting BH. UHZ-1 is undetected in the softer 0.5–2 keV band, which the authors explain is likely due to UHZ-1 being heavily obscured with a Compton-thick column density of $N_{\rm H}\sim 10^{24}-10^{25}$ cm^-2. Assuming their adopted lensing magnification of $\mu=3.81$, and given the degeneracies related to the X-ray spectral fitting, the resultant intrinsic 2–10 keV X-ray luminosity is $L_{\rm X}\gtrsim 2\times 10^{44}$ erg/s.

In this paper, we present the JWST/NIRSpec Prism spectroscopy confirming that UHZ1 is at a redshift of $z=10.07$ (§ 2). This deep spectrum was recently collected as part of the UNCOVER JWST treasury program (JWST-GO-2561, PIs: Labbe, Bezanson), which includes deep ($\sim 2.7-17$ hr) low-resolution spectroscopy for $\sim 700$ JWST-selected targets. We further examine the rest-frame UV/optical spectral properties of UHZ1 (§ 3), and estimate the stellar mass and star-formation rate of the host galaxy (§ 4). Throughout, we assume a $\Lambda$CDM cosmology with $\Omega_{M}=0.29$, $\Omega_{\Lambda}=0.71$ and $H_{0}=69.6$ km s^-1 Mpc^-1, with a Chabrier (2003) initial mass function.

Based upon the non-detection of broadened emission lines as well as lack of prominent high-ionization lines and a blue UV continuum shape that is consistent with star formation, there is no clear and direct evidence for a significant contribution from the AGN to the UV/optical spectrum. Therefore, we can fit the spectrophotometry robustly, where the light contribution is dominated by the host galaxy, despite knowledge of the presence of an X-ray AGN.

We use the Bagpipes SED fitting code (Carnall et al., 2018, 2019) to perform this SED fit. We use Bruzual & Charlot (2003) stellar population models, the MILES spectral library (Sánchez-Blázquez et al., 2006; Falcón-Barroso et al., 2011), a Chabrier (2003) IMF, and Cloudy nebular emission models (Ferland et al., 2017). We utilize PyMultinest (Buchner et al., 2014; Feroz et al., 2019) to perform our sampling using the default Bagpipes convergence criteria. We parameterize the star formation history with a delayed-$\tau$ model (SFR $\propto\ te^{-t/\tau}$) with the age and $\tau$ as free parameters ($0.1\ \mathrm{Gyr}<\mathrm{age}<t_{\mathrm{universe}}$ and $0.01\ \mathrm{Gyr}<\tau<5\ \mathrm{Gyr}$). We additionally allow the metallicity (with the stellar and gas phase metallicity fixed to the same value) and ionization parameter to vary, with $-2<\mathrm{log(Z)}<0.3$ and $-3.5<\mathrm{log(U)}<-1.0$. We assume a Charlot & Fall (2000) dust model, with $0<A_{v}<5$ and $0.3<n<2.5$ as free parameters. We allow the redshift to vary around the best fitting spectroscopic redshift of 10.07$\pm$0.05. Additionally, we apply the wavelength-dependent instrumental resolution curve to all models before fitting, assuming that the resolution is a nominal 1.3 times better than the pre-flight curve provided by STSci²²2https://jwst-docs.stsci.edu/jwst-near-infrared-spectrograph/nirspec-instrumentation/nirspec-dispersers-and-filters (Curtis-Lake et al., 2023), and we leave the wavelength-independent velocity dispersion free between 1 km/s and 2000 km/s as a nuisance parameter. We fit for a polynomial calibration vector of order 2 and the Bagpipes white noise model to allow for underestimated errors up to a factor of 10. Finally, we place a signal-to-noise ceiling of 20 on both our photometry and spectroscopy to account for potential systematic issues with the flux calibration. We fit all available JWST/NIRCam (F115W/F150W/F200W/F277W/F356W/F410M/F444W) and HST/ACS (F435W/F606W/F814W) and WFC3 (F105W/F125W/F160W) photometry in addition to the full NIRSPEC/Prism spectrum.

We present the best-fit model spectrum in Figure 4a along with the pairwise posterior distributions of relevant parameters in Figure 4b, which have been corrected for the magnification of UHZ1 with the median value of $\mu=3.866$ ($\pm^{0.1}_{0.4}$, see Furtak et al., 2023b). The median, 16th and 84th percentiles of the host galaxy parameter posteriors are presented in Table 2. The magnification-corrected stellar mass of log$(M_{\star}/M_{\odot})=8.1\pm 0.1$ is fairly typical for $z\approx 10$ galaxies being spectroscopically confirmed with JWST (e.g., Curtis-Lake et al., 2023; Arrabal Haro et al., 2023), but is now more robustly constrained to the higher mass end of the stellar mass previously inferred from photometry alone (Castellano et al., 2022; Bogdan et al., 2023) owing to the precise nature of the spectroscopic redshift in the Bagpipes fit. We note that fitting only the photometric data at the spectroscopic redshift of UHZ-1 produces a consistent measure of the stellar mass with log$(M_{\star}/M_{\odot})=8.1\pm 0.1$. The Bagpipes fit prefers a star-formation rate of SFR$\sim 1.3\pm 0.2$ $M_{\odot}$/yr, i.e., a sSFR$\sim 10^{-8}$ yr^-1, which is similar to within a factor $\sim 2-3$ of sSFR values inferred for galaxies of similar mass at this early epoch (e.g., Castellano et al., 2023). Finally, the fit prefers a moderately low stellar metallicity of $Z=0.2\pm 0.05$ compared to solar metallicity, similar to other early galaxies.

We perform a parametric Sersic-profile fit to the NIRCam F444W image using pysersic (Pasha & Miller, 2023), utilizing the NUTS sampler to explore the posterior (Hoffman et al., 2014; Phan et al., 2019). We find the Sersic index is consistent with $n\sim 1$, typical of disk-like structures, and with an effective radius of $r_{\rm eff}\sim 0.14\pm 0.02$ arcseconds, which at $z=10.07$ translates to a physical scale of $r_{\rm eff,physical}\sim 0.592$ kpc. We therefore find that UHZ-1 is also fairly typical in physical scales and extent measured for high-z galaxies in the GLASS field (Castellano et al., 2022).

We have performed JWST NIRSpec/Prism spectroscopic follow-up of the $z\approx 10$ X-ray luminous AGN, UHZ-1, and confirm the highest-redshift (currently known) galaxy hosting an X-ray AGN, with a firm spectroscopic redshift of $z=10.073\pm 0.002$. The spectrum is likely dominated by host galaxy light in the rest-frame UV/optical and stellar population synthesis analysis yields a stellar mass for the system of log $M_{*}=8.1\pm 0.1$ $M_{\odot}$. The size, mass, emission-line properties and star-formation rate of the host of UHZ-1 seem relatively typical of other known $z\approx 10$ galaxies. Here, we discuss the implications of UHZ1 for the growth of supermassive BHs, and for their signatures in the very early Universe.

A large open question for the formation of supermassive black holes has been whether they originate from “light” or stellar-mass black holes, remnants from the death of massive stars ($\sim 100$ $M_{\odot}$ seeds; Loeb & Rasio, 1994; Bromm & Loeb, 2003) or whether there are mechanisms that operate to form heavier initial seeds ($M_{\rm BH}$$\sim 10^{3}-10^{5}$ $M_{\odot}$; Miller & Hamilton, 2002; Portegies Zwart & McMillan, 2002; Freitag et al., 2006; Devecchi & Volonteri, 2009; Koushiappas et al., 2004; Lodato & Natarajan, 2006, 2007; Alexander & Natarajan, 2014; Begelman, 2010; Natarajan et al., 2017; Natarajan, 2021; Greene et al., 2020). As shown by Bogdan et al. (2023), to form the BH in UHZ-1 requires either continuous growth exceeding the Eddington limit for $>200$ Myr, or a massive seed. Similar timing arguments have been made for UV-luminous quasars at $z>6$; while they have more time to grow, they pose challenges for our current understanding of accretion models (e.g., Haiman & Loeb, 2001; Natarajan, 2011).

The ratio of BH mass to galaxy mass provides a complementary clue to the seeding mechanism. A key prediction of the heavy seed formation scenarios by the direct collapse of gas is that this ratio at early times, close to the seeding epoch, is significantly higher than found locally (Natarajan, 2011; Natarajan et al., 2017). Heavy seeding models predict an early phase of overly massive BHs relative to the local $M_{\rm BH}/M_{*}$ relation (e.g., Natarajan et al., 2017). Under the assumption that the AGN is contributing little to the observed UV/optical continuum and emission lines, the measured host mass for UHZ-1 is roughly 0.5 dex higher with the benefit of spectroscopy compared to the median photometric value in Castellano et al. (2023). However, we still estimate a very high ratio of $M_{\rm BH}/M_{*}$ based on the X-ray emission. The X-ray source in UHZ-1 has an $L_{\rm X,int}\sim 2\times 10^{44}$ erg/s (assuming $N_{\rm H}\sim 2\times 10^{24}$ cm^-2), and an implied BH mass of $M_{\rm BH}\sim 10^{7-8}$ $M_{\odot}$ conservatively assuming Eddington limited accretion and relevant uncertainties regarding the high Compton-thick column density observed in the X-rays (Bogdan et al., 2023). The implied BH to stellar mass ratio in this system (combining relevant modeling uncertainties) based on our best-fit Bagpipes model is thus $M_{\rm BH}/M_{\rm gal}\approx 0.05-1.0$.

This BH is a much larger fraction of the galaxy mass than the typical ratio of $\sim 0.001-0.002$ seen locally (Kormendy & Ho, 2013; Reines & Volonteri, 2015). At face value, this observation would strongly disfavor light seed scenarios (in systems similar to UHZ-1) which typically result in BHs that are under massive with respect to the host galaxy until the galaxy mass exceeds $\sim{}10^{10}\,M_{\odot}$ Anglés-Alcázar et al. (2017); Çatmabacak et al. (2022). Thus far, all luminous AGN discovered at $z>5$ with estimates for both the BH mass and the galaxy mass have had high ratios, but this source is extreme even in that context (e.g., Izumi et al., 2019; Neeleman et al., 2021; Fan et al., 2022). This of course does not preclude a light-seed scenario concurrently taking place in other systems, particularly as such lower-mass BHs would be substantially more difficult to detect and may not be X-ray luminous (Natarajan, 2021). However, the combination of the high BH mass, low galaxy mass at $z\sim 10$ and early accretion history modeling suggest that the particular BH in UHZ-1 was likely formed from a heavy seed (Natarajan et al., 2023). Indeed, given the extremely small volume probed by the UNCOVER region ($\sim$40 arcmin²), the number density of BHs formed from a heavy seed or a scenario involving light seeds that require substantial super-Eddington growth, cannot be vanishingly small with the detection of UHZ-1 and other systems thus far.

Despite the $>4\sigma$ detection in the 2–7 keV band (see Fig. 1 and Bogdan et al., 2023), the lack of clear AGN signatures in the UV/optical given the luminous hard X-ray source present in this system is somewhat surprising. While highly unlikely given the precise locale and centroiding of the X-ray emission, it cannot be excluded that the X-ray association is coincidental. On the other hand, the lack of UV AGN signatures is not unusual. Even in other bonafide broad-line AGN detected by JWST, which interestingly lack X-ray emission despite their Type-1 AGN nature (e.g., Furtak et al. 2023c), these systems are also bereft of the typical broad and narrow AGN emission lines blueward of $H\beta$ (e.g., Harikane et al. 2023; Kocevski et al. 2023; Matthee et al. 2023; Labbe et al. 2023). Moreover, such objects are not entirely uncommon in the nearby Universe (e.g., NGC 1448; NGC 4945; see Alexander & Hickox 2012; Annuar et al. 2017) as even luminous AGN signatures can be swamped by strong star formation and/or extreme obscuration. Furthermore, the relatively high $L_{\rm X,int}/L_{\rm 1450,obs}\sim 0.7$ ratio also points towards a heavily buried AGN. Indeed, the relatively low $A_{\rm V}\sim 0.08$ mag suggests a low line-of-sight extinction to the galaxy, placing any obscuration on small nuclear scales, as suggested by the Compton-thick column measured in the X-rays. Longer wavelength data may provide useful clues about the AGN’s nature in UHZ-1. For example, due to the spectral coverage of the NIRSpec/Prism data, the current spectrum just misses the vital [OIII] and H$\beta$ complex (a typical diagnostic for AGN activity). Moreover, at significantly longer wavelengths, the rest-frame near and mid-IR wavebands can provide much cleaner information on the presence of AGN emission even in the most heavily obscured AGN due to the re-emission by dust of the AGN signatures. Such wavelengths are fully accessible by JWST/MIRI out to restframe $\lambda<2.5\mu$m, giving access to the hot AGN dust continuum as well as high-ionization emission lines and the Paschen series. Indeed, the detection of lower-order Balmer lines and the Paschen series, given the lack of Balmer emission lines in the current NIRSpec/Prism data may point towards heavy obscuration in UHZ-1.

Objects such as UHZ-1 are only now beginning to be uncovered in the JWST data. The UNCOVER program hints at the power of deep NIRSpec spectroscopy to characterize the first growing black holes, while the high magnification ($\mu\sim 3.8\pm 0.2$) certainly aids in the detection of objects such as that presented here. The detection of the unimpeded UV-photons produced due to star formation from UHZ-1 suggests that the obscuration of the growing BH is confined to the nuclear regions given the low inferred extinction along the line of sight. This suggests that at extremely high-redshift, even modest star-forming sources like UHZ-1, likely jump-started re-ionization. The number of AGN detected so far, given the relatively limited areas covered by JWST to date, suggests that an actively growing BH population may already be in place at this early epoch. However, because of the difficulty to detect the AGN signatures in UHZ-1, the further identification and characterization of the demographics of such systems may require a suite of diagnostics from JWST NIRCam, NIRSpec, MIRI and in the X-rays.

The authors thank the anonymous referee for a constructive and insightful report that improved several aspects of the manuscript. A.D.G would like to thank M.A. Strauss for useful and enlightening conversations. A.D.G and J.E.G. acknowledges support from NSF/AAG grant# 1007094. PD acknowledges support from the NWO grant 016.VIDI.189.162 (“ODIN”) and the European Commission’s and University of Groningen’s CO-FUND Rosalind Franklin program. RB acknowledges support from the Research Corporation for Scientific Advancement (RCSA) Cottrell Scholar Award ID No: 27587. This work is based in part on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. The specific observations analyzed can be accessed via http://dx.doi.org/10.17909/8k5c-xr27 (catalog ). These observations are associated with JWST Cycle 1 programs JWST-GO-2561 and JWST-ERS-1324. Support for program JWST-GO-2561 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Associations of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. The Cosmic Dawn Center is funded by the Danish National Research Foundation (DNRF) under grant #140. YF acknowledge support from NAOJ ALMA Scientific Research Grant number 2020-16B. YF further acknowledges support from support from JSPS KAKENHI Grant Number JP23K13149. HA and IC acknowledge support from CNES, focused on the JWST mission, and the Programme National Cosmology and Galaxies (PNCG) of CNRS/INSU with INP and IN2P3, co-funded by CEA and CNES. ÁB acknowledges support from the Smithsonian Institution through NASA contract NAS8-03060. PN acknowledges support from the Gordon and Betty Moore Foundation and the John Templeton Foundation that fund the Black Hole Initiative (BHI) at Harvard University where she serves as one of the PIs. AZ acknowledges support by Grant No. 2020750 from the United States-Israel Binational Science Foundation (BSF) and Grant No. 2109066 from the United States National Science Foundation (NSF), and by the Ministry of Science & Technology, Israel.