The Cliff: a metal-poor little red dot hosting an overmassive black hole at <i>z</i> = 3.55

ABSTRACT The James Webb Space Telescope (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 ($Z=0.016\pm 0.003 \, \mathrm{ Z}_\odot$) based on the low narrow-line [O iii]$\lambda$5007/ $\mathrm{H} \beta$ ratio, supported by the non-detection of low-ionization emission lines such as [O ii]$\lambda \lambda$3727,3729 and [N ii]$\lambda \lambda$6548,6583. We find that the observed properties of The Cliff, including its overmassive BH, can be reproduced by some simulations of BH growth and evolution down to $z\sim 3.5$. However, such outcomes require high seed masses ($10^4 {\text{-}} 10^5\, \mathrm{ M}_\odot$) and appear rarely within a simulation volume comparable to the $3\lt z\lt 4$ 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 BH and persist to such low redshift.