We have been unable to observe such a structure directly in galaxies because intervening material obscures our line of vision.

We continue to learn just how fascinating black holes are. Astronomers have studied these massive cosmic monsters for years. Nonetheless, little is known about them. Imaging them directly is nearly impossible, so much about them remains a mystery. Nonetheless, science is making progress in better understanding them. Astronomers have recently revealed to have directly imaged the long-sought innermost dusty ring around a supermassive black hole at right angles to its jet. We have been unable to observe such a structure directly in galaxies because intervening material obscures our line of vision. Using the highest spatial resolution ever applied to an extragalactic object, the inner disk has now been detected. “Observing the inner regions of distant galaxies in such fine detail is an exciting step forward,” said Gail Schaefer, Associate Director of the Center for High Angular Resolution Astronomy (CHARA) Array.

Black Holes and cosmic dust

It is believed that every large galaxy has a supermassive black hole at its center. The gas in the surrounding region forms a hot disk-like structure as it is pulled toward the center. Black holes occasionally produce jets that emerge from their surroundings at right angles to their disks. However, because this flat structure is too small to be captured by conventional telescopes, we’ve never actually seen this ‘engine’ of the supermassive black hole system. One option is to view the outer ‘dusty ring.’ It has been found that interstellar gas contains dust grains. Dust grains are solid particles made of heavy elements that can only survive at temperatures below 1500 K (otherwise, metals evaporate).

A black hole engine

A view of the dusty inner region structure. Image Credit: HLA/Mundell.

Upon further investigation, if a flat structure were to be present, the heated dust would emit thermal infrared radiation, forming an outer ring around the black hole. To determine how the central engine works, it is important to determine its structure. Due to the same dust acting as an absorber of light, it is difficult to see this structure from edge-on directions. For the new study, the team examined a system with a face-on view, the brightest such object in the vicinity. It was necessary, however, to have a very high spatial resolution in the infrared region of the spectrum to detect these signals. Additionally, a large array of telescopes arranged so that different objects can be observed from different angles.

CHARA

The only facility that meets both requirements is the Georgia State University CHARA Array interferometer at Mount Wilson Observatory in California. It is composed of 6 telescopes, each with a 1-meter diameter mirror, which are combined to achieve a spatial resolution that is comparable to a much larger telescope. The layout of the array enables telescopes to observe objects from various angles and with a great deal of distance between them, despite each telescope being relatively small. This results in a very high level of spatial resolution. CHARA has the world’s sharpest eyes for infrared wavelengths. Finally, the CHARA Array was able to detect the dusty ring in NGC 4151 at a right angle to the jet. A research paper detailing the discovery was published in The Astrophysical Journal.

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