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

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 We analyse the full 2.2 Ms Chandra imaging dataset -- including 0.95 Ms of unpublished observations -- and present new JWST/MIRI photometry at $λ_\mathrm{obs}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.

Figures (3)

• Figure 1: Distribution of hard-band detection significance obtained by randomly shifting the astrometric registration of each observation. Both histograms are normalized to unit sum. Blue and orange histograms correspond to the full 2.2 Ms dataset and the B24 subset Bogdan24, respectively. The cyan shaded band marks the range of detection significances measured in this work (Table \ref{['tbl: chandra_detection']}); the orange shaded band marks the significance reported by Bogdan24. The $4.2$--$4.4\sigma$ value of Bogdan24 is recovered in fewer than $0.1\%$ of Monte Carlo realizations, demonstrating its sensitivity to the assumed astrometric alignment.
• Figure 2: Hard-band net count rates in the $1"$ source aperture. Each orange point stands for one observation, where the filled (open) ones represent those in (not in) the B24 subset. The open squares are binned to reach $\approx100-300$ ks. Their abscissa error bars represent the timespan of the corresponding bin, and their ordinate error bars reflect $1\sigma$ source-region count uncertainties calculated based on Gehrels86. The net count rate is consistent with zero in the more recent 0.95 Ms observations.
• Figure 3: Observed SED of UHZ1. Filled circles are photometric detections and downward triangles are $3\sigma$ upper limits. The vertical black line marks $\lambda_{\rm obs}=8~\mu\mathrm{m}$ (rest-frame $\sim\!0.7~\mu\mathrm{m}$), separating the NIRCam and MIRI LRS from the redder MIRI imaging bands. At $\lambda_{\rm obs}<8~\mu\mathrm{m}$, the SED is fully consistent with an unobscured, low-mass, metal-poor, star-forming galaxy with no AGN contribution. At $\lambda_{\rm obs}>8~\mu\mathrm{m}$, the MIRI non-detections place a direct upper bound on the bolometric luminosity of any buried AGN. The dashed orange and red curves show the maximally allowed normalization of the Torus Polletta2006 and Hot DOG Fan16 AGN SED templates, respectively, each scaled to the F1500W and F1800W upper limits in Table \ref{['tbl: miri']}. The bolometric luminosity implied by these upper envelopes falls one to two orders of magnitude below the intrinsic X-ray luminosity inferred by Bogdan24 under a Compton-thick model, ruling out a luminous buried AGN of that magnitude in UHZ1.