Black Holes are Spinning Faster Than Expected

There’s a Universe full of black holes out there, spinning merrily away—some fast, others more slowly. A recent survey of supermassive black holes reveals that their spin rates reveal something about their formation history.

If you want to describe a supermassive black hole’s characteristics, there are two important numbers to use. One is its mass and the other is its spin rate. Some black hole spin rates are thought to be very close to the speed of light. According to Logan Fries, a PhD student at the University of Connecticut, those numbers are tough to get. “The problem is that mass is hard to measure, and spin is even harder,” he said. Yet, having accurate numbers is important if we want to understand black hole evolution.

Fries and his colleagues in the Sloan Digital Sky Survey’s Reverberation Mapping Project took on a tough job. They measured the spin rates of black holes over cosmic history. “We have studied the giant black holes found at the centers of galaxies, from today to as far back as seven billion years ago,” said Fries, a primary author of a paper about this work. The mapping project also made detailed observations of the associated accretion disks. Those are the areas nearest the black hole where matter accumulates and heats up as it spirals in. Measuring that region is important since knowing the black hole’s mass and its accretion disk’s structure provides data that allows them to measure the spin rate. Astronomers typically estimate the spin rate by observing how matter behaves as it falls into the black hole.

Black Holes and their Archaeology

The results of the SDSS Survey of mass measurements of hundreds of black holes were a surprise, according to Fries. That’s because the spin rates reveal something about the black holes’ formation history. “Unexpectedly, we found that they were spinning too fast to have been formed by galaxy mergers alone,” he said. “They must have formed in large part from material falling in, growing the black hole smoothly and speeding up its rotation.”

Fries described his work at a recent meeting of the American Astronomical Society. “I have read research papers that examine black hole spin, theoretically, from the lens of like black hole mergers, and I was curious if spin could be observationally measured,” said Fries. He pointed out that the history of black hole growth requires more precise measurements than have been available. And, they’re not easy, according to Fries’s thesis advisor, Physics professor Jonathan Trump. “The challenge lies in separating the spin of the black hole from the spin of the accretion disk surrounding it,” said Trump. “The key is to look at the innermost region, where gas is falling into the black hole’s event horizon. A spinning black hole drags that innermost material along for the ride, which leads to an observable difference when we look at the details in our measurements.”

Digging into the mass and spin of a black hole requires spectral measurements. Those made by the SDSS contain subtle shifts in the spectra toward shorter wavelengths of light. That shift is a major clue to the black hole’s rotation rate. “I call this approach ‘black hole archaeology,’” said Fries “because we’re trying to understand how the mass of a black hole has grown over time. By looking at the spin of the black hole, you’re essentially looking at its fossil record.”

What The Black Holes Tell Us

So, what does that fossil record tell us? First of all, it challenges the prevailing wisdom that black holes are always created in galaxy collisions. In other words, when galaxies merged, so did their central black holes. Each galaxy brings a rotation rate and orientation to the merger. The rotations could just as easily cancel each other out as they are to add together. If that is true, then the astronomers should have seen a wide range of spins. Some black holes should have a lot of spin, others… not so much.

The big surprise is that many black holes appear to spin very quickly. Even more amazing, the most distant ones seem to be spinning faster than the ones nearest to us (i.e. the “nearby” Universe). It’s as if they spin faster in the early Universe, and more slowly in more recent epochs. “We find that about 10 billion years ago, black holes acquired their mass primarily through eating things,” Fries explained.

The early fast spin rate implies that most supermassive black holes (like the one in our own Milky Way Galaxy) built up over time by taking in gas and dust in a very smooth and controlled manner. In other words, the more they eat (in the way of stars and gas), the faster their spin rate. It also turns out that merger growth actually slows the spin of supermassive black holes. That could explain why those we measure today have a mix of spin rates, rather than the more uniform rates of earlier epochs.

Future Directions

The idea of black holes forming smoothly over time provides a new direction for black hole research. Observations by JWST will help give more targets to study. Surveys such as the SDSS Reverberation Mapping project will follow up with more precise measurements of the huge supermassive black holes JWST continually finds as it studies the Universe.

For More Information

Spinning Black Holes Reveal How They Grew
‘Black Hole Archaeology’: Understanding How Black Holes Gained Their Mass

Black Hole Archaeology: Mapping the Growth History of Black Holes Across Cosmic Time (PDF of AAS presentation)

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