Scientists reach milestone of directly observing the ring around black hole

The long-sought-after innermost dusty ring around a supermassive black hole has finally been directly observed by an international team of researchers, at a right angle to the jet it is producing.

Although it was believed that such a structure existed in the centre of galaxies, it had been challenging to directly detect due to intervening material blocking our view.

With the highest spatial resolution ever achieved for an extragalactic object, the inner disk is now discovered. The Astrophysical Journal has just published a new discovery.

"This is a fascinating step forward to view the inner region of a distant galaxy with such fine detail," said Gail Schaefer, associate director of the Center for High Angular Resolution Astronomy (CHARA) Array.

Every giant galaxy is expected to have a supermassive black hole. The surrounding material is drawn toward the centre, and the gas condenses into a hot, dazzling disk-like shape. Sometimes a jet travels away from the black hole in a direction that is perpendicular to the disk.

However, because it is too small to be seen with conventional telescopes, this flat structure, which is essentially the "engine" of this active supermassive black hole system, has never been directly observed.

A direct view of an outer "dusty ring" is one way to approach this important structure. Dust grains, which are tiny solid particles made of heavy elements found in interstellar gas, can only survive in the outer region where temperatures are low enough ( 1,500K -- otherwise metals evaporate) to prevent them from evaporating.

If the central system is flat, the heated dust would appear to be an outside ring surrounding the black hole since it emits thermal infrared radiation. Determining its structure would be a crucial first step in figuring out how the central engine operates.

The system is hidden by the same dust functioning as a light absorber, making it challenging to see this structure from edge-on directions. The team instead concentrated on a system with a face-on perspective in the latest investigation, which was the brightest such object in the local cosmos.

However, the detection required a large array of telescopes that is configured properly to observe objects at various orientations, as well as very high spatial resolution in the infrared wavelengths.

The only facility that satisfies both of these criteria is the Georgia State University CHARA Array interferometer at the Mount Wilson Observatory in California.

The Array is made up of 6 telescopes, each with a mirror diameter of 1 meter, that work together to produce spatial resolution comparable to that of much bigger telescopes.

Despite the fact that each telescope is rather modest on its own, the array arrangement is designed to see objects from a variety of angles and at great distances from one another.

By doing this, a very high spatial resolution capacity is achieved. Infrared wavelengths are where the CHARA Array's eyes are the sharpest in the world.

Finally, the team used the CHARA Array to identify a dusty ring that was perpendicular to the emerging jet in the galaxy known as NGC 4151's centre.

"We've been hoping to see this structure in a bare nucleus object for a long, long time," says Makoto Kishimoto, principal investigator of the project at Kyoto Sangyo University.

A big boost was that each telescope has recently added a new system called "adaptive optics."

Matt Anderson, a postdoctoral researcher at the CHARA Array who played a critical role in conducting the observations, said, "This greatly increased the injection rate of the light, compensating for the relatively small collecting mirror to observe the extragalactic target, which is much fainter than the stellar targets typically observed in our Galaxy."

Over the last nearly 40 years, researchers in the field believed that this dusty ring is a key to understanding different characteristics of accreting supermassive black hole systems. The properties we observe depend on whether we have an obscured, edge-on view or clear, face-on view of the nucleus of the active galaxy. The detection of this ring-like structure validates this model.

Furthermore, it's likely that the detection represents more than just a flat structure. The structure at somewhat longer infrared wavelengths, which corresponds to an even bigger outer zone, appears to be elongated along the jet rather than at a right angle to it, according to further research.

This has been taken as a sign that a dusty wind is being blasted out in the direction of the jet. An important connection between the windy structure and its interaction with the remainder of the galaxy surrounding the active black hole system is made by the current discovery that the inner structure appears flat and perpendicular to the jet.

These ground-breaking discoveries provided measurements of the dusty disk's size and orientation. By constructing a new instrument at the CHARA Array that can see deeper into space and resolve the finer scale structure of the source, the team hopes to obtain an even more precise image of the central region.