Supermassive black hole accretion disk seen 'on the edge' for 1st time

An illustration of a black hole with an accretion disk, accompanied by diagrams showcasing the team's research.
An illustration of a black hole with an accretion disk, accompanied by diagrams showcasing the team's research. (Image credit: NOIRLab/NSF/AURA/P. Marenfeld)

Astronomers have observed the outer edge of a disk of matter surrounding a feeding supermassive black hole for the first time. 

These observations could help scientists better measure the structures that surround these cosmic monsters, understand how black holes feed on those structures and put together how this feeding influences the evolution of galaxies that house such phenomena.

Feeding supermassive black holes sit at the hearts of regions of incredible brightness called active galactic nuclei (AGN). Immediately around these black holes, which can have masses millions or even billions of times greater than that of the sun, lies a swirling disk of gas and dust that is being gradually fed to the central supermassive object.

The incredible gravitational influence of such supermassive black holes causes the matter in accretion disks to reach temperatures as great as 18 million degrees Fahrenheit (10 million degrees Celsius). This causes the structure to emit radiation all across the electromagnetic spectrum, from high-energy gamma-rays and X-rays to visible light, infrared light and radio waves. These emissions from AGNs, which are also called "quasars," can be so bright that they outshine the combined light from every star in the galaxies that surround them.

Related: James Webb Space Telescope reveals active supermassive black holes were surprisingly rare in early universe

Yet, even with this powerful output, because accretion disks are relatively small and many are located in incredibly distant galaxies, they are difficult to directly image. But as an alternative, astronomers can use the full spectrum of light from an accretion disk to understand its physics and even determine its size.

This is the technique adopted by a team led by researchers from the Instituto Nacional de Pesquisas Espaciais, Brazil. Denimara Dias dos Santos and Alberto Rodriguez-Ardila studied the accretion disk of the distant quasar, III Zw 002, located at the heart of the galaxy Messier 106 (M 106). M 106 lives some 24 million light-years away from Earth in the constellation Canes Venatic.

The team saw, for the first time, near-infrared emission lines in the spectrum of light coming from this quasar's accretion disk. Those lines helped the researchers size this plate-like structure from which the supermassive black hole, determined to have a mass between 400 and 500 times the mass of the sun, feeds.

"This discovery gives us valuable insights into the structure and behavior of the broad line region in this particular galaxy, shedding light on the fascinating phenomena happening around supermassive black holes in active galaxies," Rodriguez-Ardila said in a statement.

Excitement around accrection disks

Emission lines like those studied by the team occur when an atom absorbs energy and adopts what physicists call an "excited state." Eventually, these atoms have to return to their lowest energy state, or "ground state." That drop back to ground state releases light that, because every element has a unique set of energy levels, is at wavelength and energy characteristic to an atom of a specific element.

That means these emissions in light spectra can help identify what elements are present in a star, the atmosphere of a planet and, in this case, in the accretion disk around a black hole.

Emission lines from stars and other sources take the appearance of thin spikes in spectra, but the violent conditions around a supermassive black hole cause accretion disk emission lines to adopt a different appearance. 

As matter close to the supermassive black hole is accelerated to speeds approaching that of light, associated emission lines are broadened and take on shallower peaks. The region these emissions come from is referred to as the broad line region of the accretion disk. 

As one side of an accretion disk moves towards Earth, the other side moves away. This results in short wavelengths of light on the side rotating towards us and longer wavelengths of light on the side of the accretion disk moving away.

This is similar to what happens to sound here on Earth as an ambulance races toward you on a city street. The siren's sound waves bunch up, resulting in short-wavelength and high-frequency sound. As the ambulance moves away, the soundwaves stretch out, and the frequency of the siren drops.

This phenomenon is called the Doppler shift, and for light emerging from an accretion disk, it gives rise to two peaks — one related to the side moving away from Earth and the other to the side moving rapidly toward Earth.

When these double-peaked, broadened emissions are seen coming from the inner region of an accretion disk, they don't give astronomers any hints about the size of the accretion disks. However, if those lines could be seen from the outer edge, they would.

This team of astronomers made the unambiguous detection of two near-infrared, double-peaked profiles in the broad line region of III Zw 002, a hydrogen-originating line from an inner area of the broad line region disk and an oxygen-generated line at the outer limit of this region.

The emission lines were found within data collected by the Gemini Near-Infrared Spectrograph (GNIRS), which is capable of observing the entire near-infrared spectrum at once. This allowed the team to catch a single, clean and consistently calibrated spectrum for the quasar. 

"We didn't know previously that III Zw 002 had this double-peaked profile, but when we reduced the data, we saw the double peak very clearly," Rodriguez-Ardila said. "In fact, we reduced the data many times thinking it could be a mistake, but every time we saw the same exciting result."

This helped to constrain the size of the accretion disk as the team could see the hydrogen line comes from a distance of 16.77 light-days from the central supermassive black hole, while the oxygen line originates at a radius of 18.86 light-days.

The astronomers were also able to determine the size of the broad line region, estimating that to be at an outer radius of 52.43 light-days. Plus, the team was able to calculate that the broad line region of the accretion disk is tilted at an angle of 18 degrees in relation to Earth. 

The team will continue to monitor the quasar III Zw 002, watching as its profile changes over time as well as considering the use of near-infrared to study other AGNs. 

The research was published in August in the Astrophysical Journal Letters. 

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Robert Lea
Senior Writer

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.