One persistent question in cosmology is about supermassive black holes (or SMBHs): how did they get so supermassive?

We see black holes with millions and billions of times the Sun’s mass in every big galaxy we look at, but how they formed is still a problem that’s not solved. We know massive stars can form black holes when the outer layers explode in a supernova, and the core collapses. But the biggest black hole you can get that way now might be a hundred times the Sun’s mass. 

In the very distant past when the Universe was young, it’s possible some could form that were bigger, but nowhere near what we see. So how did they get so big?

The Milky Way, our home galaxy, is a gigantic flat disk and has an SMBH called Sgr A*, which itself has about 4 million solar masses. That’s actually small for a galaxy as big as ours, but still pretty danged big. Recently, the Event Horizon Telescope consortium took images of Sgr A*, revealing a ring of material around it called the accretion disk. This is matter swirling around the black hole just above The Point of No Return. Using that data, scientists have an idea on at least one way our local SMBH got so SM: it ate another SMBH [link to journal paper].

The conclusion rests on a weird property of black holes: they can spin. That’s a bit mind twisty, since they are surrounded by an event horizon, a region where no light can escape and nothing inside it can be seen. However, there are properties of the black hole we can still measure. Mass is one. 

Spin is another. Almost every object in the Universe spins, including planets, stars, and even whole galaxies. If a spinning object shrinks, that rotation rate increases (like an ice dancer bringing their arms in during a spin and speeding it way up). This is called conservation of angular momentum, and it’s a basic property of physics. Because of this, many (probably all) black holes are born spinning rapidly.

Moreover, if material falls in to the black hole that can increase its spin. The accretion disk feeds the black hole, and if the material spins around it in the same direction the black hole spins, that spin can increase. Usually, that matter is falling in from the greater galaxy around the SMBH, so it aligns with the galaxy.

But not always. There’s a misalignment in the spin of Sgr A* to the plane of the Milky Way. Not only that, Sgr A* is spinning faster than expected. The best fit the scientists get to the data is that Sgr A* was once lower mass, but another galaxy collided with the Milky Way and merged with it. That galaxy’s SMBH fell down to the center, then eventually collided with Sgr A*. They merged, and what was left was the bigger black hole we see today. Sgr A* probably had a little less 3 million solar masses at the time, and the other SMBH about a quarter of that.

When galaxies merge they can do so at any angle, so that explains the misalignment, and as the black holes merged their spins added together, increasing that of the merged SMBH. So that hangs together.

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