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 (2308.02750v3)

Abstract: The James Webb Space Telescope is now detecting early black holes (BHs) as they transition from "seeds" to supermassive BHs. Recently Bogdan et al. (2023) reported the detection of an X-ray luminous supermassive BH, UHZ-1, with a photometric redshift at $z > 10$. Such an extreme source at this very high redshift provides new insights on seeding and growth models for BHs given the short time available for formation and growth. Harnessing the exquisite sensitivity of JWST/NIRSpec, here we report the spectroscopic confirmation of UHZ-1 at $z = 10.073 \pm 0.002$. We find that the NIRSpec/Prism spectrum is typical of recently discovered z~10 galaxies, characterized primarily by star-formation features. We see no clear evidence of the powerful X-ray source in the rest-frame UV/optical spectrum, which may suggest heavy obscuration of the central BH, in line with the Compton-thick column density measured in the X-rays. We perform a stellar population fit simultaneously to the new NIRSpec spectroscopy and previously available photometry. The fit yields a stellar mass estimate for the host galaxy that is significantly better constrained than prior photometric estimates ($M_\sim 1.4^{+0.3}_{-0.4} \times 10^8 M_\odot$). Given the predicted BH mass ($M_{\rm BH}\sim10^7-10^8 M_\odot$), the resulting ratio of $M_{\rm BH}/M_$ remains two to three orders of magnitude higher than local values, thus lending support to the heavy seeding channel for the formation of supermassive BHs within the first billion years of cosmic evolution.

• The paper demonstrates the spectroscopic confirmation of an X-ray luminous AGN at z=10.1 using JWST/NIRSpec.
• The study employs a multiwavelength approach, revealing a heavily obscured AGN and precise stellar mass estimates of its host galaxy.
• The findings indicate an elevated black hole to stellar mass ratio, supporting heavy seed models for early supermassive black holes.

Spectroscopic Confirmation of an X-ray Luminous AGN at Redshift 10.1 in the UNCOVER Study

The paper under discussion reports on significant findings regarding the early universe's supermassive black holes, specifically presenting spectroscopic insights into an X-ray luminous active galactic nucleus (AGN) known as UHZ-1 at a confirmed redshift of $z = 10.073 \pm 0.002$. This paper leverages the capabilities of the James Webb Space Telescope (JWST), particularly its Near-Infrared Spectrograph (NIRSpec), to provide a detailed spectroscopic account that contributes to understanding the seeding and growth mechanisms of supermassive black holes (SMBHs) shortly after the Big Bang.

Key Findings

1. Spectroscopic Confirmation: The researchers have utilized the JWST/NIRSpec to spectroscopically confirm the high-redshift AGN, UHZ-1, substantiating its position in the early universe at $z=10.1$. The spectrum revealed by NIRSpec is characterized primarily by features indicative of vigorous star formation, typical of galaxies at a similar redshift.
2. X-ray Emission and Obscuration: Despite the absence of clear AGN signatures such as broad emission lines in the UV/optical spectra, the detection of luminous, hard X-rays establishes UHZ-1 as an AGN. The lack of obvious UV/optical AGN markers combined with the detected X-ray emissions suggests that the AGN is heavily obscured, aligning with the detection of Compton-thick column densities.
3. Stellar Mass Estimation: The stellar population fitting—integrating both spectroscopic and photometric data—yields a stellar mass estimate for UHZ-1's host galaxy of $M_\star \sim 1.4^{+0.3}_{-0.4} \times 10^{8}$. This finding refines previous photometric estimates, highlighting the precision introduced by spectroscopic methods.
4. Black Hole to Stellar Mass Ratio: The predicted black hole mass for UHZ-1 ($M_{\rm BH} \sim 10^7-10^8$) suggests a $M_{\rm BH} / M_\star$ ratio that is two to three orders of magnitude higher than typical values observed in the local universe. This supports the hypothesis that early SMBHs may follow a "heavy seeding" growth channel, contrary to light-seed scenarios where SMBHs grow from stellar-mass precursors.

Theoretical and Practical Implications

The results from this paper have significant implications for both theoretical and observational astrophysics. The spectroscopic verification of such a distant and luminous AGN offers crucial evidence in support of SMBH formation models that propose 'heavy seeds' formed from direct gas collapse. These heavy seeds can potentially skip the long exploratory pathways anticipated in light-seed models, where continuous super-Eddington accretion is required over extended periods.

Furthermore, the findings underscore the importance of multi-wavelength observations in accurately characterizing early universe phenomena. The invisible (or obscured) nature of the AGN in rest-frame UV/optical spectra, coupled with robust X-ray detection, suggests that future AGN identifications should incorporate X-ray data to account for obscured black holes that rest only faintly visible in other wavebands.

Future Developments

The paper's results pave the way for more extensive observational campaigns using facilities like JWST and the Chandra X-ray Observatory to continue identifying and characterizing similar high-redshift AGNs. Continued spectroscopic efforts will refine our understanding of early black hole demographics, accretion processes, and their interactions with host galaxies. Additionally, cosmological simulations that incorporate these findings will enhance models predicting the distribution and characteristics of primordial SMBHs, potentially adjusting the assumed frequency and luminosity thresholds of such phenomena.

Overall, the paper by Goulding et al. exemplifies the capability of contemporary astronomical instrumentation to substantiate theoretical models of cosmic evolution, enhancing our grasp of the universe's formative years.