Astronomers Detect Innermost Dusty Ring of Active Galactic Nucleus

Astronomers using the Center for High Angular Resolution Astronomy (CHARA) Array interferometer at the Mount Wilson Observatory have observed the long-sought, innermost dusty ring around a supermassive black hole in NGC 4151, an active spiral galaxy located 62 million light-years away in the constellation of Canes Venatici.

An active galactic nucleus is a relatively small region at the center of some galaxies that has abnormally high luminosity.

The ‘engine’ of an active galactic nucleus is a disk of gas orbiting a supermassive black hole.

“As material in the surrounding region gets pulled toward the center, the gas forms a hot and bright disk-like structure,” said Kyoto Sangyo University astronomer Makoto Kishimoto and colleagues.

“In some cases, a jet emerges from the vicinity of the black hole in a direction at a right angle to the disk.”

“However, this flat structure has never been directly seen because it’s too small to be captured by conventional telescopes.”

“One way to approach this key structure is to directly see an outer ‘dusty ring’ — interstellar gas contains dust grains, tiny solid particles made of heavy elements, which can only survive in the outer region where temperature is low enough (less than 1,500 K),” they added.

“The heated dust emits thermal infrared radiation, and thus would look like an outer ring around the black hole, if the central system indeed has a flat structure.”

“The detection of its structure would be a key step to delineate how the central engine works.”

With the CHARA Array at the Mount Wilson Observatory, the astronomers detected the dusty ring, at a right angle to the emerging jet in the center of NGC 4151.

“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 authors said.

“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, the detection probably is not just an indication of a flat structure.”

“Additional studies have been showing that the structure seen at slightly longer infrared wavelengths, corresponding to an even larger outer region, seems elongated along the jet, and not at a right angle to it.”

“This has been interpreted as an indication for a dusty wind being blown out toward the jet direction.”

“The present finding that the inner structure looks 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 measured the size and orientation of the dusty disk.”

The findings were published in the Astrophysical Journal.

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Makoto Kishimoto et al. 2022. The Dust Sublimation Region of the Type 1 AGN NGC 4151 at a Hundred Microarcsecond Scale as Resolved by the CHARA Array Interferometer. ApJ 940, 28; doi: 10.3847/1538-4357/ac91c4

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