Long-anticipated dusty ring’s interior revealed at highest spatial resolution at infrared wavelengths ever used – ScienceDaily


An international team of scientists has achieved the milestone of directly observing the long-awaited and innermost dusty ring around a supermassive black hole, at right angles to its upward jet. Such a structure was thought to exist in the nuclei of galaxies but was difficult to observe directly because the intervening material blocked our field of view.

Now the inner disk is being detected using the highest spatial resolution at infrared wavelengths ever made for an extragalactic object. The new discovery has just been published in Astrophysical Journal.

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

The supermassive black hole is believed to be at the center of every major galaxy. When material in the surrounding region is pulled toward the center, the gas forms a hot, bright disk-like structure. In some cases, a jet emerges from the vicinity of the black hole toward the right corner of the disk. However, this flat structure, which is essentially the ‘engine’ of the active supermassive black hole system, has never been seen directly because it is too small to be picked up by conventional telescopes.

One way to approach this underlying structure is to see an outer “dusty ring” – interstellar gas contains dust grains and small solid particles made of heavy elements, which can only survive in the outer region where the temperature is low enough (<~1500 K ) - otherwise the minerals will evaporate). The hot dust emits thermal infrared radiation and would therefore look like an outer ring around the black hole, if the central system really had a flat structure. Discovering its structure will be a key step in determining how the central engine works.

Attempts to see this structure from edge directions is difficult, because the system is obscured by the same dust that acts as a light absorber. Instead, in the new investigation, the team focused on a face-to-face system, the brightest such object in the nearby universe. However, detection requires very high spatial resolution in infrared wavelengths and, at the same time, a large group of appropriately positioned telescopes to observe objects in different directions.

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, that are combined 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 wide variety of angles and with large distances between telescopes. This achieves a very high spatial resolution capability. CHARA Array actually has the sharpest eyes in the world at infrared wavelengths.

Using the CHARA array, the team finally detected the dusty ring, at right angles to the emerging jet at the galactic center called NGC 4151.

“We had been hoping to see this structure in an exposed core body for a very long time,” says Makoto Kishimoto, the project’s principal investigator at Kyoto Sangyo University.

The biggest driver was that each telescope recently added a new system called “adaptive optics.”

Matt Anderson, a CHARA Array postdoctoral researcher who played a critical role in making the observations, says, “This dramatically increased the rate of light injection, compensating for the relatively small collecting mirror for observing the extragalactic target, which is much lighter than stellar targets that are normally observed.” in our galaxy.”

For nearly the past 40 years, researchers in the field have believed that this dusty ring is key to understanding the various properties of accretion of supermassive black hole systems. The properties we observe depend on whether we have a dark, edge-on, or clear, head-on view of the active galactic nucleus. The discovery of this ring-like structure validates this model.

Moreover, the discovery is probably not just an indication of a flat structure. Additional studies have shown that the structure seen at slightly longer infrared wavelengths, which corresponds to a larger outer region, appears to be elongated along the flow, rather than at right angles to it. This has been interpreted as an indication of dusty winds blowing in the direction of the jet. The current finding that the inner structure appears flat and perpendicular to the plane, is an important link to the stormy structure and its interaction with the rest of the galaxy surrounding the active black hole system.

These pioneer observations measured the size and orientation of the dusty disk. The team is working to get a more detailed picture of the central region by building a new instrument in the CHARA Array that can see deeper into space and resolve the source’s finer scale structure.



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