Study finds eight ultramassive (>10 billion M⊙) black holes in brightest cluster galaxies

An accepted Astrophysical Journal paper reports eight new direct detections of ultramassive black holes, each weighing more than 10 billion times the mass of the sun, in 16 giant galaxies at the centers of galaxy clusters. The result roughly doubles the number of directly measured black holes known in that extreme mass range.

The paper, posted Sept. 30 on arXiv as 2512.04178v2 and accepted for publication in The Astrophysical Journal, argues that the finding matters for more than the tally. The authors say the standard relationship astronomers often use to estimate black-hole masses from a galaxy’s stellar velocity dispersion breaks down for the biggest systems. At the high-mass end, they report, the size of a galaxy’s central core tracks black-hole mass better.

“In a sample of 16 Brightest Cluster Galaxies (BCGs) without previous black-hole mass measurements we discover 8 UMBHs based on direct dynamical detections with triaxial Schwarzschild models,” the paper says.

Brightest cluster galaxies are the huge galaxies that sit near the centers of galaxy clusters, making them natural places to search for the universe’s largest black holes. For the new study, Stefano de Nicola and colleagues at institutions including Universitäts-Sternwarte München and the Max Planck Institute for Extraterrestrial Physics analyzed 16 such galaxies that previously had no black-hole mass measurements.

Rather than inferring masses indirectly, the team used deep imaging and long-slit spectroscopy from the Large Binocular Telescope to model how stars move inside each galaxy. Those stellar motions were then fit with triaxial Schwarzschild orbit-superposition models, a dynamical technique the paper describes as the first sample of triaxial black-hole mass determinations.

The study found eight ultramassive black holes, defined as having masses above 10 billion solar masses. They are in brightest cluster galaxies in Abell 160, 292, 1185, 1749, 1775, 2107, 2147 and 2256. The reported masses range from about 1.05 × 10^10 to 2.47 × 10^10 solar masses.

Among the largest in the new sample, the paper reports a black hole in Abell 2256 with a mass of 2.47 ± 0.69 × 10^10 solar masses, one in Abell 2107 at 2.24 ± 0.33 × 10^10 solar masses, and one in Abell 2147 at 1.621 ± 0.090 × 10^10 solar masses. The authors say those eight detections “more than double” the previously known sample, which they say stood at seven examples in the literature.

The second main result is about how to predict black-hole mass in giant galaxies. Astronomers have long relied on the MBH-σ relation, which links black-hole mass to the random speeds of stars in a galaxy. But the authors say the galaxies in this sample are outliers to that canonical relation.

“We confirmed that the canonical MBH-σ relation is inadequate for predicting black hole masses at the high-mass end, and the effect is particularly severe for BCGs... In contrast, the core size predicts MBH much more accurately...,” the paper says.

A galaxy’s depleted core is the dimmer central region thought to have been excavated as pairs of supermassive black holes, brought together by galaxy mergers, fling stars away. The paper reports that core size shows about half the scatter of velocity dispersion when used to track black-hole mass in these extreme systems. The authors also say links between black-hole mass, core size, core density and the black hole’s sphere of influence support the “core scouring” picture.

That could matter because core size can be measured from imaging more easily than black-hole mass can be measured with detailed dynamical modeling, though the paper presents that as an implication rather than a new standard practice.

The findings come with important caveats. This was a targeted sample, selected from a larger survey for especially large cores or high mass, so it is not an unbiased census of cluster-central galaxies. And the masses come from model-dependent dynamical analyses. Still, with the paper now accepted by ApJ, the study adds a substantially larger direct sample at the highest black-hole masses and strengthens the case that the usual velocity-dispersion rule does not hold up at the top end.