Supermassive black holes are powerful engines, pumping matter and energy into their environments. Despite their size and influence, they are also relatively small, making detailed observations a major challenge. For that reason, most of the data we have on them involves tracking the matter swirling around the black hole rather than the properties of the black hole itself.

Astronomers have now used gravitational magnification to measure the rotation rate of a supermassive black hole in a very distant galaxy. From four separate images of the same black hole, R.C. Reis, M.T. Reynolds, J.M. Miller, and D.J. Walton found it was spinning nearly as fast as possible. That likely means it was spun up by a small number of mergers with other black holes rather than a gradual increase from eating smaller amounts of mass.

This marks the first measurement of black hole rotation outside the local Universe, and it was only possible because the light from this black hole was magnified by a galaxy lying serendipitously between it and the Milky Way. However, the fortuitous alignment means it will be difficult to replicate the method for most other black holes.

Bright black holes

Black holes don't emit light of their own, but matter swirling around them heats up through collisions and acceleration by magnetic fields. The matter does emit light, making supermassive black holes some of the brightest objects in the cosmos.



Supermassive black holes reside in the center of nearly every galaxy. They weigh in at millions or billions of times the mass of the Sun. Since astronomers observe them even in distant galaxies, they must have grown that large very early in the history of the Universe.

Regardless of how they formed, black holes are characterized by just three physical quantities: mass, spin, and (in rare cases) electric charge. That simplicity means it is impossible to recreate a black hole's history from present observation. In particular, if it is spinning rapidly, that might be because it formed via the merger of two smaller black holes—an event that creates very fast rotation over a short time—or more gradually by devouring smaller amounts of mass. Either of those processes can impart the momentum needed to spin the black hole.

As a result, the best way to understand the evolution of the largest black holes is to study a variety of them over the history of the Universe. By comparing quasars—black holes emitting powerful jets—in early galaxies to their more quiet cousins in nearby galaxies, astronomers can create a time-lapse of black hole evolution. (Supermassive black holes in the local Universe are not as bright as in previous times because of a relative lack of nearby material.)

The present study involved measuring the amount of light reflected off the matter orbiting very quickly near a quasar 200 million times more massive than the Sun. This quasar, known as RX J1131-1231, is about 6.1 billion light-years away, much too far to be studied at high resolution under ordinary circumstances.

However, a galaxy happens to be directly in line between us and the quasar, creating a gravitational lens. According to general relativity, gravity bends the paths of photons, meaning that a sufficiently massive object can focus light. The strongest gravitational lenses create multiple magnified images of more distant objects. In this case, the galaxy's gravity split the quasar's light into four separate images, each of which was comparable to the size of the galaxy—much larger than it would appear unmagnified.

The researchers combined the four lensed images, allowing them to effectively quadruple the data. They looked for light "reflected" off iron atoms—meaning absorbed and re-emitted photons—and imaged the iron orbiting at high speeds in the immediate vicinity of RX J1131-1231. The distinct iron reflection spectrum was modified by the atoms' fast motion, providing the means to measure how fast they were being spun around by the black hole.

That near the quasar, things don't orbit in quite the way that they do in the Solar System. Rather than the objects themselves moving quickly, the spin and gravity of the black hole combine to twirl the fabric of space itself in a phenomenon called frame dragging. Close enough to the black hole, this phenomenon pulls the iron atoms along at nearly the speed of light. Determine the speed of the atoms, and you have a measure of the black hole's spin.

This reflection produces a relatively small amount of light, so it's not possible to measure it for many quasars. However, the researchers collated data from nearly 140 hours of observation, using both the Chandra and XMM-Newton X-ray observatories. The amount of data let them separate the reflection signature from the other light from the quasar.

They found that the black hole was spinning at least 66 percent of the maximum possible value allowed by general relativity and probably as much as 87 percent of the maximum. That means that even 6.1 billion years ago, this black hole was rotating more rapidly than would be possible if it was slowly devouring gas—it must have merged with at least one other black hole earlier in time.

Since this observation required magnification of the quasar by a gravitational lens, it won't be possible to repeat it for every distant black hole. However, other lensed quasars exist, so with sufficient patience and observation time, astronomers should be able to determine if RX J1131-1231 is typical or if other black holes might be spinning less rapidly. Still, even knowing that one supermassive black hole has grown through mergers suggests that others out there have done so as well.

Nature, 2014. DOI: 10.1038/nature13031 (About DOIs).