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

TL;DR

This study uses JWST observations to analyze a metal-poor galaxy at redshift 3.55 hosting an overmassive black hole, providing insights into black hole growth and galaxy evolution in the early universe.

Contribution

It presents high-resolution JWST data of a unique low-metallicity galaxy with an overmassive black hole, linking observations to black hole formation models.

Findings

Low metallicity ($Z=0.017 ext{ }Z_\odot$) confirmed by emission line ratios.

The galaxy's properties align with some black hole growth simulations requiring high seed masses.

The galaxy and black hole are rare, matching simulation volumes and highlighting their unusual nature.

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$\beta$ ratio, supported by the non-detection of low-ionisation emission lines such as [OII]$\lambda\lambda3727,3729$ and [NII]$\lambda\lambda6548,6583$. 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.