The Cliff: A Metal-Poor Little Red Dot Hosting an Overmassive Black Hole at <i>z</i> = 3.55

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 = 0.016 ± 0.003 Z⊙) based on the low narrow-line [O iii]λ5007/Hβ ratio, supported by the non-detection of low-ionisation emission lines such as [O ii]λλ3727,3729 and [N ii]λλ6548,6583. 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 ∼ 3.5. However, such outcomes require high seed masses (104 − 105 M⊙) and appear rarely within a simulation volume comparable to the 3 &amp;lt; z &amp;lt; 4 RUBIES volume, 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.