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Scientists have discovered that some supermassive black holes are spinning much faster than expected. The discovery is the result of a new form of “black hole archaeology,” which links the rotation of black holes to the gas and dust they consumed to grow over 7 billion years of cosmic history. The findings, courtesy of the Sloan Digital Sky Survey (SDSS), suggest a few things. First, the early universe may have been more ordered than previously thought. And second, the growth of supermassive black holes through a chain of mergers of larger and larger black holes (caused by the collision and merger of galaxies) may be supplemented by objects that greedily feast on the surrounding gas and dust.
"We studied giant black holes found at the centers of galaxies from today to seven billion years ago," team member Logan Freese of the University of Connecticut said in a statement. "We unexpectedly found that they rotate too quickly to have formed solely from galaxy mergers. "They probably formed largely from infalling material, the smooth growth of the black hole, and the acceleration of its rotation."
Measuring the rotation of a black hole is not easy
Despite being cosmic monsters that shape entire galaxies around them, supermassive black holes, with masses millions or billions of times that of the Sun (and their smaller, stellar-mass counterparts), are generally quite simple. They can be uniquely identified by just three characteristics: mass, spin, and, less importantly, electric charge. As physicist John Wheeler wittily explained this lack of distinguishing features, “black holes don’t have hair.”
"Black holes seem so exotic, but they can be completely described by just two numbers: mass and rotation speed," Fries explained. "The problem is that mass is hard to measure, and rotation is even harder."
The speed at which a black hole rotates is difficult to distinguish from the speed at which the surrounding flattened cloud of gas and dust, the accretion disk, rotates.
“The challenge is to separate the rotation of the black hole from the rotation of the accretion disk that surrounds it,” said Jonathan Trump, a team member and researcher at the University of Connecticut. “The key is to look at the innermost region where gas falls into the black hole’s event horizon. “The rotating black hole drags this internal material along with it, which makes a noticeable difference when we look at the details in our measurement.”
Cosmic fossil record
The team tackled the difficult task of determining the rotation of black holes using the SDSS Reverberation Mapping project. This project has made extremely precise measurements of the masses of hundreds of black holes, as well as detailed observations of the structure of the void’s accretion disks. This data comes in the form of spectra, or light emitted across the electromagnetic spectrum. With this in hand, scientists can begin to measure the rotation rate of the central black hole.
The subtle shift in the wavelength of light reveals a lot about the black hole's rotation. As material falls into the black hole, it also brings with it angular momentum—this rotation reveals details about the black hole's past feeding.
"I call this approach 'black hole archaeology' because we're trying to understand how the mass of a black hole grew over time," Fries said. "When you look at the rotation of a black hole, you're essentially looking at its fossils."
This “fossil record” can be deciphered by comparing observed rotation rates with predicted ones. The currently favored model suggests that supermassive black holes grow through mergers that occur when their parent galaxies collide and merge. Because these individual galaxies have their own rotation rates and random orientations, when they merge, those rotations could cancel out. Or, at least, they could merge. Both results have be equally likely.
Given this, scientists expect black holes to rotate very slowly. However, this team found that this is not the case. This study not only found that many black holes rotate faster than expected, but also showed that black holes in more distant galaxies rotate even faster than in the local Universe.
This suggests that the rotation of black holes can evolve gradually over time. One way this could happen is if the black hole accumulates angular momentum by gradually accreting dust and gas. The researchers could further test this idea and confirm these results using observations from the James Webb Space Telescope (JWST), which has been searching for supermassive black holes from earlier eras of the universe for three years.
“Black holes are truly at the frontier of human understanding,” Juna Kollmeier, director of SDSS-V, the current phase of SDSS, said in a statement. “We conduct large-scale surveys like SDSS to build an empirical astrophysical picture of their fundamental properties against which to test our theoretical models.”
Fries presented the team's findings on January 14 at the 245th meeting of the American Astronomical Society (AAS) in National Harbor, Maryland.
