Testing the BH$^*$ Model: a UV-to-Optical Spectral Fitting of The Cliff

Rosa M. Mérida, Marcin Sawicki, Gaia Gaspar, Chris J. Willott, Kartheik G. Iyer

First listed 2026-05-08 | Last updated 2026-05-08

Abstract

In the black hole star (BH*) model, the characteristic "V"-shaped SED of LRDs is produced by an accreting BH embedded in a dense neutral-gas envelope with a near-unity covering factor. This envelope reprocesses radiation and emits as a ~5,000K blackbody, producing the optical continuum. Meanwhile, the UV is powered by a low-mass, dust-free, metal-poor host. The BH* scenario is promising, but it has yet to undergo detailed testing; conducting a self-consistent UV-to-optical spectral-fitting analysis of LRDs would provide a robust assessment of the model. In this work, we test the BH* scenario by fitting the full JWST/NIRSpec PRISM spectrum of The Cliff ($z_{spec}=3.55$), an LRD that played a pivotal role in the development of this model. A Bagpipes fit that allows stellar, nebular, AGN, and blackbody components naturally yields a BH*-like solution for The Cliff, even with broad priors. Our method allows us to characterize its host, despite remaining unresolved in JWST imaging. From the continuum, we infer the host to be low-mass (log $M_\star/M_\odot$~7.7), star-forming, metal-poor, affected by non-negligible dust attenuation ($A_V$~0.5 mag) acting on both stellar and nebular components. Larger $M_\star$ (up to log $M_\star/M_\odot$~8.1) and attenuations (up to $A_V$~1 mag) are obtained depending on the assumed dust attenuation law. Modest AGN UV leakage is consistently allowed by the code, but remains weak and not robustly constrained, with both AGN+host and host-dominated UV scenarios yielding equivalent fits. The star formation history of the host is relatively smooth, with the galaxy already assembling log $M_\star/M_\odot$~7 about 200 Myr before $z_{spec}=3.55$. The BH-to-$M_\star$ ratio exceeds the values expected from BH-host scaling relations, especially at recent times. This tension may indicate either inaccurate estimates of the BH properties or non-coeval BH-host evolution.

Short digest

Using the full JWST/NIRSpec PRISM spectrum of the z=3.55 little red dot The Cliff, this paper performs a self-consistent UV-to-optical Bagpipes fit with stellar, nebular, AGN, and blackbody components to test the BH* scenario. The fit naturally lands on a BH*-like solution: a ~5000 K reprocessing envelope accounts for the optical continuum, while the UV is consistent with a compact low-mass, star-forming, metal-poor host with non-negligible attenuation, with a fiducial log Mstar/Msun ~7.7 and A_V ~0.5. That matters because The Cliff was a key motivating object for the BH* picture, and here the model works even with broad priors while also extracting plausible host properties despite the source being unresolved in JWST imaging. The main remaining ambiguity is the UV mix: modest AGN leakage is allowed but not robustly required, and the inferred BH-to-stellar-mass ratio stays above standard BH-host scaling expectations.

Key figures to inspect

• Inspect the full PRISM spectrum with the component decomposition; this is the figure that shows whether one combined stellar+nebular+AGN+blackbody fit can really reproduce The Cliff's V-shaped UV-to-optical continuum in a self-consistent way.
• Look for the continuum zoom around the Balmer-limit region and red optical slope; this should make clear why a ~5000 K blackbody-like envelope is favored for the optical light and why a purely stellar interpretation is disfavored for this source.
• Check the posterior or model-comparison figure for the UV component; this is where to see why host-dominated UV and AGN+host UV solutions remain statistically equivalent, and how weakly the AGN leakage is actually constrained.
• Inspect the dust-law sensitivity figure or table-equivalent plot; this is the place to learn how the inferred host properties shift from the fiducial solution to larger values, reaching roughly log Mstar/Msun ~8.1 and A_V ~1 depending on the attenuation law.
• Look for the recovered star-formation history and BH-to-Mstar comparison panels; together they show the host's relatively smooth buildup, including assembly of ~10^7 Msun about 200 Myr before z=3.55, and the resulting tension with standard BH-host scaling relations.