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Elusive, dusty inner region of a distant galaxy: The long-sought innermost dusty ring has been discovered with the highest infrared spatial resolution ever used

An international team of scientists has achieved the milestone of directly observing the long-sought innermost ring of dust around a supermassive black hole perpendicular to its nascent jet. Such a structure was thought to exist at the core of galaxies, but it was difficult to observe directly because intervening material obscured our line of sight.

Now the inner disk is detected with the highest spatial resolution in the infrared wavelength range ever performed for an extragalactic object. The new discovery was just published in The Astrophysical Journal.

“This is a very exciting step forward in looking at the inner region of a distant galaxy in such fine detail,” said Gail Schaefer, associate director of the Center for High Angular Resolution Astronomy (CHARA) Array.

A supermassive black hole is believed to exist at the center of every large galaxy. As surrounding material is pulled toward the center, the gas forms a hot and bright disk-like structure. In some cases, a jet emanates from near the black hole in a direction perpendicular to the disk. This flat structure, which is essentially the “engine” of this active supermassive black hole system, has never been seen directly, however, as it is too small to be seen by conventional telescopes.

One way to approach this key structure is to look directly at an outer “dust ring” – interstellar gas contains dust grains, tiny solid particles of heavy elements that can only survive in the outer region where the temperature is low enough ( < ~1500K -- otherwise metals evaporate). The heated dust emits thermal infrared radiation, so it would appear like an outer ring around the black hole if the central system were actually a flat structure. Discovering its structure would be a crucial step in delineating how the central engine works.

Attempts to view this structure from the side are difficult because the system is obscured by the same dust that acts as a light absorber. Instead, in the new study, the team focused on a front-view system, the brightest such object in the nearby Universe. However, detection required a very high spatial resolution in the infrared wavelength range and, at the same time, a large number of telescopes suitably designed to observe objects in different orientations.

Georgia State University’s CHARA Array interferometer at Mount Wilson Observatory in California is the only facility that meets both of these requirements. The array consists of 6 telescopes, each with a 1 meter diameter mirror, which combine to achieve the spatial resolution of a much larger telescope. While each individual telescope is relatively small, the array layout is optimized for observing objects at a variety of angles and with large distances between the telescopes. This achieves a very high spatial resolution. The CHARA array actually has the world’s sharpest eyesight in the infrared range.

Finally, using the CHARA array, the team spotted the dusty ring at right angles to the emerging jet called NGC 4151 at the center of the galaxy.

“We have long hoped to see this structure in a bare core object,” says Makoto Kishimoto, principal investigator on the project at Kyoto Sangyo University.

A big boost has been that each telescope has recently added a new system called “adaptive optics”.

Matt Anderson, a postdoctoral researcher at CHARA Array who played a crucial role in conducting the observations, says: “This significantly increased the injection rate of the light and compensated for the relatively small collecting mirror used to observe the extragalactic target, which is much fainter than the stellar targets , which are typically observed in our galaxy.”

For the past nearly 40 years, researchers in the field have believed that this dusty ring holds a key to understanding various properties of accreting supermassive black hole systems. The features we observe depend on whether we have an obscured side view or a clear front view of the core of the active galaxy. The detection of this ring-like structure confirms this model.

In addition, the detection is probably not just an indication of a flat structure. Additional studies have shown that the structure, seen at slightly longer infrared wavelengths, corresponding to an even larger outer region, appears elongated along the beam rather than at right angles to it. This has been interpreted as an indication that a dusty wind is being blown out in the direction of the jet. The recent finding that the inner structure appears flat and perpendicular to the jet is an important link to the windy structure and its interaction with the rest of the galaxy surrounding the active black hole system.

These groundbreaking observations have measured the size and orientation of the dust disk. The team is working to get an even more detailed picture of the central region by building a new instrument on the CHARA array that can see deeper into space and resolve finer structure of the source.

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