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

The study examines black-hole scaling relations at the high-mass end, specifically the regime of ultramassive black holes with masses above 10^10 solar masses. The goal is to improve constraints on how black-hole mass correlates with host-galaxy properties where the canonical relations are poorly tested.

Using a sample of 16 brightest cluster galaxies that lacked previous black-hole mass measurements, the authors performed direct dynamical detections with triaxial Schwarzschild models. This yielded eight new ultramassive black holes, making it the first sample of triaxial black-hole mass determinations and increasing the number of known ultramassive black holes by a factor of two. These measurements dramatically improve constraints on black-hole mass scaling relations at the high-mass end.

The results show that brightest cluster galaxies are outliers in the canonical black-hole–stellar velocity dispersion relation. In contrast, the size of their depleted cores — the central light-deficient region — is a much better unbiased predictor of black-hole mass and should be used as a proxy at the high-mass end. The brightest cluster galaxies smoothly join the trend already established for massive core galaxies in previous studies. This also holds for tight correlations between core size and sphere-of-influence radius, and between core size and core density.

Taken together, these relations strongly support the black-hole binary model for the formation of the centers of the most massive galaxies.