Revisiting the Claim for a Direct-Collapse Black Hole in UHZ1 at $z=10.05$

Fan Zou, Elena Gallo, Zihao Zuo, Edmund Hodges-Kluck, Dieu D. Nguyen, Guido Roberts-Borsani, Piero Madau, Fabio Pacucci, Anil C. Seth, Tommaso Treu

First listed 2026-03-26 | Last updated 2026-03-26

Abstract

We reassess the direct collapse black hole (DCBH) interpretation of UHZ1 (UNCOVER-26185), a gravitationally lensed galaxy at $z_\mathrm{spec}=10.054$. That interpretation rests on a hard ($2-7$ keV) X-ray excess detected with Chandra, attributed to a Compton-thick AGN with an inferred $2-10$ keV luminosity of $L_\mathrm{X,int}\sim10^{46}~\mathrm{erg~s^{-1}}$ (Bogdan et al. 2024). The resulting extreme X-ray to rest-frame optical-IR ratio was taken as the hallmark signature of an "outsize black hole galaxy" at cosmic dawn. We analyse the full 2.2 Ms Chandra imaging dataset -- including 0.95 Ms of unpublished observations -- and present new JWST/MIRI photometry at $λ_\mathrm{obs}>5~μ\mathrm{m}$. Across the full range of plausible Chandra data reductions, the $2-7$ keV excess at the position of UHZ1 reaches a significance of only $2.3-2.9σ$; the originally reported $4.2-4.4σ$ detection is sensitive to the specific astrometric alignment adopted and is not robustly reproducible. Moreover, the hard X-ray signal does not grow with the additional exposure, contrary to expectations for a steady source, indicating that any excess is not persistent. UHZ1 is also undetected in all nine MIRI imaging bands. Fitting red/obscured AGN SED templates to the tightest MIRI upper limit, we constrain the bolometric luminosity of any buried AGN to $L_\mathrm{bol}<1.3\times10^{45}~\mathrm{erg~s^{-1}}$. These conclusions are further supported by independent JWST spectroscopy (Alvarez-Marquez et al. 2026), which reveals no AGN signatures in the rest-frame UV or optical. Taken together, the multiwavelength data paint a consistent picture of UHZ1 as a low-mass, metal-poor, star-forming galaxy in the early Universe, with no compelling evidence for a luminous obscured AGN, regardless of its proposed formation channel.

Short digest

Reanalyzing the full 2.2 Ms Chandra/ACIS data set for the lensed z=10.054 galaxy UHZ1 and adding new JWST/MIRI imaging, the authors reassess the DCBH claim. The hard 2–7 keV signal reaches only 2.3–2.9σ depending on astrometric registration and does not strengthen with the extra 0.95 Ms, arguing against a persistent source. UHZ1 is undetected in all nine MIRI bands, with SED fits limiting any buried AGN to L_bol < 1.3×10^45 erg s^-1; independent JWST spectroscopy also finds no AGN features. The multiwavelength picture favors a low-mass, metal-poor, star-forming galaxy rather than a luminous obscured DCBH, underscoring the sensitivity of such claims to Chandra astrometry and cluster-background treatment.

Key figures to inspect

• Figure 1: Inspect the Monte Carlo histograms of detection significance versus astrometric registration to see how rarely the Bogdán et al. (2024) 4.2–4.4σ value is recovered and how the preferred 2.3–2.9σ range emerges under plausible alignments.
• Figure 2: Check the per-epoch and binned 2–7 keV net count rates; the flat trend and consistency with zero in the recent 0.95 Ms show the excess does not build with exposure, disfavoring a steady hard X-ray source.
• Figure 3: Examine the SED with MIRI upper limits and the scaled Torus/Hot DOG templates; the mid-IR calorimetric constraint (set by F1500W/F1800W limits) caps L_bol well below the Compton-thick interpretation, aligning with a star-forming, metal-poor host.