Ultramassive black holes diverge from the M–sigma relation.

A study measured masses for eight ultramassive black holes in brightest cluster galaxies and found they lie above the extrapolated M–sigma relation, while the size of a galaxy's central light-deficit correlated more closely with black hole mass.

Nearly every galaxy hosts a supermassive black hole at its centre. Astronomers commonly use the M–sigma relation, which links the spread in stellar velocities (sigma) in a galaxy core to black hole mass, to estimate masses when direct signatures are absent. That relation is simple and useful for many systems but is harder to apply for very large black holes. A new study by de Nicola et al. examined brightest cluster galaxies to test how the relation performs for ultramassive black holes. Key findings: - The team examined 16 brightest cluster galaxies and applied triaxial Schwarzschild dynamical modelling where observations allowed, obtaining reliable mass estimates for 8 galaxies. - Those measured black holes are classified as ultramassive (masses above about 10 billion solar masses). - When placed on an M–sigma plot, these ultramassive black holes lie above the extrapolated M–sigma relation, indicating the relation would underestimate their masses. - Triaxial Schwarzschild modelling of stellar orbits provided strong mass constraints when the necessary data were available. - The authors report that the size of a galaxy's central light-deficit (a core dip in brightness) correlates better with black hole mass for these largest systems, offering an alternative estimator when dynamical modelling is not possible. Summary: The study indicates the commonly used M–sigma relation does not scale cleanly to the most massive black holes and can underestimate masses for ultramassive systems. The authors find that galaxy core light-deficit size shows a closer correlation with mass for these objects. Undetermined at this time.