This is the story of the discovery of an intermediate black hole with a mass of roughly 142 solar masses that formed via the combination of a 66 and 85 solar mass black hole.

Now, I know some of you are thinking, “We know there are supermassive black holes that are millions and billions of solar masses. Why is a hundred-something solar mass black hole so exciting?

Well, in part, it’s exciting because we haven’t had definitive proof of this size black hole existing. 

It appears that black holes can form in at least two different ways. The giant black holes in the largest galaxies that formed at the beginning of the universe appear to have formed through a massive and potentially turbulent inflow of material at the center of the overdensities of material. In this way, the first galaxies and first supermassive black holes all grew in size together. 

Once stars began to form, the other confirmed way to form black holes started as well. The most massive stars rapidly fuse successively heavier elements in their cores until their cores are full of iron and energy can no longer be released in fusion processes. These stars, which may start out twenty or more times the mass of the sun, will undergo mass loss throughout their lives, and the final black hole may be as small as 3.5 solar masses, with most stellar-mass black holes being 5-10 solar masses.

The question on many cosmologists’ minds has been: how do we get new galaxies growing and containing growing supermassive black holes in the process? When we look at dwarf galaxies – the kind of galaxies we think are the building blocks of most large galaxies – we aren’t finding black holes in their centers. This implies that, somehow, black holes between stellar-size and supermassive-size have to have a way of forming and growing, allowing new galaxies to form and grow around them.

And these intermediate-mass black holes have refused to be found… until now. 

We can detect black holes in two different ways. First, we find them by looking for places in space where only a high-density massive object can explain observed phenomena like accretion disks and high speed orbiting companions. The second discovery method involves looking for the gravitational waves generated when black holes form and merge with other massive objects, including other black holes. 

Gravitational waves are literal waves in the space-time continuum that stretch and construct everything as they radiate through space, carrying with them energy lost during the merger or collapse. We detect these waves by looking for the literal change in the distance that occurs between two points in a massive laser system here on Earth. By building multiples of these systems scattered across our planet, scientists can confirm they are detecting real events and use wave arrival times to figure out the approximate direction the wave is coming from. The combined frequency and amplitude pattern of the incoming waves allows scientists to figure out the approximate sizes of the combining objects. 

The US detectors are the LIGO systems, and since their first detections in 2015, LIGO, working in collaboration with Europe’s VIRGO system, has made dozens of detections and candidate detections of gravitational waves from merging black holes and neutron stars. The majority of the confirmed detections have come from stellar-mass black hole mergers with systems just tens of solar masses in size. While resulting black hole masses had been as great as 80 solar masses, the initial black holes had all been below the theoretical limit of 65 solar masses, which is thought to be the maximum size black hole a single collapsing star can form.

This new merger – between a 65 and 82 mass black hole – has two inexplicably large black holes. 

While a 130 solar mass star can end its life as potentially a 65 solar mass black hole, large stars – stars initially between 120 and 200 solar masses – are unstable and should explode and leave nothing behind. It is unclear if mass loss allows more massive stars to stay massive long enough to form more massive black holes. If this is possible, those stars would create 120 solar mass black holes, leaving a gap of no black holes between 65 and 120 solar masses that form through the collapse of a single star.

For this new merger, these limitations mean that it is just on the edge of the possibility that its 66 solar mass smaller black hole formed from a single star, but that 85 solar mass monster must have resulted from a prior merger. I honestly feel this is most likely a system in which each black hole grew through the merger of two or more lower-mass black holes or neutron stars, giving us the 142 solar mass intermediate black hole we’ve been looking for.

You can read about these results in a pair of papers in The Astrophysical Journal Letters and Physical Review Letters, both authored simply by the LIGO and VIRGO collaborations. 

It’s unclear how black holes find one another across the merging generations, and we still have no clear idea of how often this happens. We still have a lot to learn and observe, but now we know these mergers are happening, growing bigger and bigger black holes over time.

More Information

LIGO press release 

Albert Einstein Institute press release 

Northwestern University press release 

The Australian National University press release 

CNRS press release 

“GW190521: A Binary Black Hole Merger with a Total Mass of 150 M_Sun,” LIGO Scientific Collaboration and Virgo Collaboration, 2020 Sep. 2, Physical Review Letters

“Properties and Astrophysical Implications of the 150 M_Sun Binary Black Hole Merger GW190521,” LIGO Scientific Collaboration and Virgo Collaboration, 2020 Sep. 2, Astrophysical Journal Letters