Eight New Ultramassive Black Hole Masses Confirm Best Correlation with Galaxy Core Sizes

We analyze black hole (BH) scaling relations at the high-mass end, focusing in particular on the regime of ultramassive black holes, M BH > 10 10 M ⊙ (UMBHs). Building on the new dynamical BH mass measurements presented in our companion paper for a sample of 16 brightest cluster galaxies (BCGs), including eight newly identified UMBHs based on triaxial Schwarzschild models, we investigate the implications of these measurements for BH scaling relations. This first sample of triaxial BH mass determinations increases the number of known UMBHs by a factor of 2 and dramatically increases the constraints for BH mass scaling relations at the high-mass end. We find that BCGs are outliers in the canonical M BH ∝ σ 4−5 relation, while the size of their depleted cores—the central light-deficient region—offers a tighter correlation with M BH with a scatter of 0.22 dex in this mass regime, confirming and extending earlier findings that core size provides a robust proxy at the high-mass end. Our measurements smoothly join the trend already established in previous studies. This also holds for tight correlations between core size and sphere-of-influence radius and core size and core density, which, along with the correlation between M BH and core size, strongly support the BH binary model for the formation of the centers of the most massive galaxies, as the observed core sizes are inconsistent with gravitational-wave recoil predictions and the galaxies display central tangential orbital anisotropy expected from scouring.