Mind the Gap

29 August 2023

How astronomers can measure the width of an accretion disk. NOIRLab/NSF/AURA/P. Marenfeld
How astronomers can measure the width of an accretion disk.

When you think of a black hole, you might think its defining feature is its event horizon. That point of no return not even light can escape. While it’s true that all black holes have an event horizon, a more critical feature is the disk of hot gas and dust circling it, known as the accretion disk. And a team of astronomers have made the first direct measure of one.1

According to Newton, if you drop an object from rest near a planet or star, the object will fall straight down, tracing a linear path until it strikes the planet or star. Einstein says something slightly different. That straight path is only possible if the planet or star isn’t rotating. If it is rotating, then space near the planet or star is twisted. It’s an effect known as frame dragging, and it means our object will be pulled around an object as it falls. We have measured frame dragging on satellites near Earth, so we know it is a real effect.

Near fast-rotating black holes the frame-dragging effect can be immense. This means as gas and dust start to fall toward the black hole it’s swept out into a disk around the equatorial plane of the black hole. All the gas and dust are superheated, which builds up tremendous pressure. An accretion disk can generate strong magnetic fields, emit powerful X-rays, and even power jets of gas that stream away from the black hole at nearly the speed of light. Most of the black holes we’ve identified in the Universe have been through the high-energy effects of their accretion disks. But the physics of black hole accretion disks are complex, and we don’t yet fully understand their dynamics or even have a precise gauge of their size.

We do have a basic gauge of the size of accretion disks. One of the things we’ve noticed with quasars is that they can fluctuate in brightness. Quasars are supermassive black holes with a radio-bright accretion disk. Given the finite speed of light, the rate of fluctuations gives us an upper bound on the size of the accretion disk. So for example, if a quasar fluctuates on the scale of a year, we know the accretion disk can’t be larger than about a light-year across. The most accurately measured fluctuating quasar is 3C 273, and we know its accretion disk is about 1.5 light-years across, or about 100,000 AU.

But this is only an upper bound, and the accretion disk could be smaller. Without a direct measure of an accretion disk, we rely on computer simulations to estimate its size. But this recent work has measured the accretion disk of a supermassive black hole directly, which gives us a step up in understanding black holes.

The double peak spectra of the oxygen emission line. dos Santos, et al
The double peak spectra of the oxygen emission line.

To achieve this, the team used a different approach. Rather than using brightness fluctuations, they measured the emission lines of a supermassive black hole at the center of a galaxy known as III Zw 002. Using the Gemini North telescope, they were able to study a particularly bright emission line of hydrogen and one of oxygen. Both of these spectra presented a double peak feature. This double peak is caused by the rotation of the accretion disk. As the disk rotates, light from the portion of the disk rotating toward us is shifted toward the blue spectrum, while light on the portion of the disk rotating away from us is redshifted. The effect is most significant on the outer edges of the disk, hence the appearance of a double peak.

From this spectral data, the team determined that the black hole is about 400 - 900 million solar masses, and its axis of rotation is tilted about 18 degrees relative to our line of sight. The peaks of the hydrogen line are about 16.8 light-days from the black hole, and the peaks of the oxygen line are about 18.9 light-days from the black hole. That means the accretion disk is around 40 light-days across.

This result is just the first step. The team continues to observe III Zw 002 and hopes to be able to study how the accretion disk precesses around the black hole over time, which will tell us about the dynamics between the two.

  1. dos Santos, Denimara Dias, et al. “First Observation of a Double-peaked O i Emission in the Near-infrared Spectrum of an Active Galaxy.” The Astrophysical Journal Letters 953.1 (2023): L3. ↩︎