&Bullet; physics 14, s52

The elusive masses of two merging black holes could be explained if they were the central black holes in two distant, tiny galaxies that are merging.

A. Palmese / Fermilab

On May 21, 2019, the LIGO and Virgo collaborations recorded an unexpected detection of gravitational waves. The signal, known as GW190521, was based on the merger of two black holes, the masses of which fall within a range that is forbidden by conventional stellar evolution theories (see position: a fusion in heavy weights). Now, Antonella Palmese of the Fermi National Accelerator Laboratory (Fermilab) and the University of Chicago and Christopher Conselice of the University of Manchester, UK, propose that the observation can be explained by the fusion of black holes in the centers of two low-mass dwarf galaxies [1] . Such “forbidden mass” black holes could form in dwarf galaxies from the successive merging of several smaller black holes – without having to rewrite models of the formation of star black holes.

To test their proposal, the researchers first extrapolated the masses of black holes in the centers of dwarf galaxies based on known relationships between the central mass of black holes and the total stellar mass for larger galaxies. They then used observation data sets that describe in detail how the population of dwarf galaxies has changed over time to calculate the rate of fusion of these galaxies as a function of their distance from Earth. Ultimately, they determined how long it should take for the central black holes of two galaxies to merge after the galaxies themselves began to merge.

Palmese and Conselice estimated how many dwarf galaxies are likely to host black holes with masses similar to those suspected for GW190521, and concluded that a merger of dwarf galaxies is a plausible explanation. Observations of other fusions with black holes in this mass range and further numerical analyzes of black holes in nearby galaxies and clusters could confirm the scenario and open a new way to understand the formation of galaxies and the still mysterious origin of supermassive black holes.

–Rachel Berkowitz

Rachel Berkowitz is Corresponding Editor for physics based in Vancouver, Canada.


  1. A. Palmese and CJ Conselice, “GW190521 from the Fusion of Ultradwarf Galaxies”, Phys. Rev. Lett.126181103 (2021).

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