Stochastic black hole growth tracks stellar mass in ultramassive galaxies

Abstract

Over the past three decades, it has been established that galaxy spheroids contain supermassive black holes (BHs) whose masses correlate tightly with the properties of their host galaxies. The most predictive of these correlations is between BH mass and stellar velocity dispersion (the $M_{\rm BH}-σ$ relation), alongside a relation with bulge luminosity or stellar mass (the $M_{\rm BH}-M_\star$ relation). Theoretical models of galaxy evolution predict that for the most massive galaxies, whose late-time growth proceeds mainly via gas-poor `dry' mergers, $M_\star$ should become a better predictor of BH mass than $σ$. However, genuine deviations from the $M_{\rm BH}-σ$ relation have remained difficult to observe due to the extreme rarity and low central surface brightness of such galaxies. Here we report James Webb Space Telescope (JWST) measurements of central BHs in a sample of eight ultramassive galaxies. We find the largest deviations from the $M_{\rm BH}-σ$ relation ever reported, with most BH masses exceeding $10^{10}$ solar masses. In this extreme mass regime, the galaxies tightly follow the $M_{\rm BH}-M_\star$ relation with minimal scatter, successfully confirming theoretical predictions. Furthermore, we observe significant stochasticity: For one ultramassive galaxy, the central dispersion drops rather than peaks, revealing no evidence for a central mass; while we can formally only place an upper limit, the data are entirely consistent with no BH at all. Crucially, this physical dichotomy is already evident directly in the raw kinematic maps prior to any fitting, and is robustly confirmed by our dynamical modelling. This unexpected diversity suggests that extreme multi-body dynamical processes, such as gravitational recoil or three-body ejections, significantly disrupt BH growth in the Universe's most massive galaxies.