The Cliff: A Metal-Poor Little Red Dot Hosting an Overmassive Black Hole at $z = 3.55$

Lucy R. Ivey, Francesco D'Eugenio, Roberto Maiolino, Yuki Isobe, Ignas Juodžbalis, Sophie Koudmani, Michele Perna, Saiyang Zhang, Volker Bromm, Andrew J. Bunker, Stefano Carniani, Andrew C. Fabian, Kohei Inayoshi, Xihan Ji, Gareth C. Jones, Boyuan Liu, Robert Pascalau, Pierluigi Rinaldi, Brant Robertson, Jan Scholtz, Sandro Tacchella

First listed 2026-04-10 | Last updated 2026-04-10

Abstract

JWST has revealed a large population of massive black holes (BHs) in the early Universe with unusual properties which mark them as distinct from low-redshift active galactic nuclei. Such findings have prompted the development of new models of BH formation and growth, and of their co-evolution with host galaxies. Linking the gas-phase metallicity of BH environments to seed masses is key to understanding which evolutionary pathways could explain the population of JWST-discovered BHs. We present new high-resolution JWST NIRSpec/IFU observations covering the rest-frame optical emission lines of a Little Red Dot (LRD) at $z=3.55$, known as The Cliff, from the `Red Unknowns: Bright Infrared Extragalactic Survey' (RUBIES). We find evidence for low metallicity ($Z=0.017\pm0.004 \ Z_\odot$) based on the low narrow-line [OIII]$\lambda5007$/H$β$ ratio, supported by the non-detection of low-ionisation emission lines such as [OII]$λ\lambda3727,3729$ and [NII]$λ\lambda6548,6583$. We find that the observed properties of The Cliff, including its overmassive BH, can be reproduced by some simulations of black hole growth and evolution down to $z\sim3.5$. However, these simulation runs require high seed masses ($10^4 - 10^5\ M_\odot$) and appear as rarely in the simulation volume as in the RUBIES survey volume over redshifts $3<z<4$, highlighting the unusual nature of The Cliff. Future simulations and numerical models will help to uncover how such a metal poor system managed to develop a massive black hole and persist to such low redshift.

Short digest

This paper studies a metal-poor little red dot at z = 3.55 that appears to host an overmassive black hole. The main result is that the system combines low metallicity with black-hole-to-host properties that are hard to reconcile with simple local scaling expectations. The paper matters because it sharpens the argument that at least some LRD-like sources may trace unusually rapid or early black-hole growth.

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