Astronomers observe a donut-shaped torus surrounding a black hole

The artist's design depicts a donut-shaped dusty object surrounding the supermbadive black hole, a disk of material gravitating around the black hole, and jets of material ejected from the disk at the center of a galaxy. Credit: Bill Saxton, NRAO / AUI / NSF

Astronomers have used the Karl G. Jansky's very large network (VLA) of the National Science Foundation to create the first direct image of a donut-shaped dusty element surrounding the supermbadive. black hole at the heart of one of the most powerful radio galaxies in the universe – a feature that theorists first postulated almost forty years ago as an essential part of such objects.

Scientists studied Cygnus A, a galaxy located 760 million light-years away from Earth. The galaxy has a black hole 2.5 times bigger than the Sun. While the gravitational pulling force of the black hole attracts the surrounding materials, it also propels super fast jets of material moving outward at a speed close to that of light, producing "lobes" spectacular brilliant radio show.

"Central motors" powered by black holes, producing light emissions at different wavelengths, and jets extending well beyond the galaxy are common to many galaxies, but have properties different when they are observed. These differences have led to a variety of names, such as quasars, blazars or Seyfert galaxies. To explain the differences, the theorists constructed a "unified model" with a common set of characteristics showing different properties depending on the angle at which they are viewed.

Artistic design of the active galactic core, with labels. Credit: Bill Saxton, NRAO / AUI / NSF

The unified model includes the central black hole, a rotating disc consisting of an infiltrating material surrounding the black hole and the jets moving outward from the poles of the disc. In addition, to explain why the same type of object looks different from a different angle, a thick, dusty, donut-shaped "torus" surrounds the internal parts. The torus masks certain features when viewed from the side, resulting in apparent differences for the observer, even for intrinsically similar objects. Astronomers generally call this common set of features an active galactic core (AGN).

"The torus is an essential element of the AGN phenomenon, and there is evidence of the low luminosity of such structures in the neighboring AGN, but we have never seen it directly in such a bright radio galaxy," said Chris Carilli of the National Radio Astronomy Observatory (NRAO). "The torus helps to explain why objects known by different names are in fact the same, but viewed from a different angle," he added.

In the 1950s, astronomers discovered objects emitting strong radio waves, but appearing as dots, similar to distant stars, when they were later observed with visible light telescopes. In 1963, Maarten Schmidt of Caltech discovered that one of these objects was extremely remote and other discoveries quickly followed. To explain how these objects, called quasars, could be so brilliant, theorists have suggested drawing on the enormous gravitational energy of supermbadive black holes. The combination of the black hole, the rotating disk, called the accretion disk, and the jets has been described as the "central engine" responsible for lightning overflows of energy.

VLA image of the central region of Cygnus A, with labels. Credit: Carilli et al., NRAO / AUI / NSF

The same type of central engine also seems to explain the production of other types of objects, including Seyfert radio galaxies, blazars and galaxies. However, each showed a different set of properties. Theorists worked to develop a "unification scheme" to explain how the same thing might appear differently. In 1977, obscuration by dust was suggested as an element of this scheme. In 1982, Robert Antonucci, of the University of California at Santa Barbara, presented the drawing of an opaque torus – a donut-shaped object – surrounding the central engine. From that moment, a darkening torus has become a common feature of the unified vision of astronomers of all types of active galactic nuclei.

"Cygnus A is the closest example to a powerful transmitting galaxy – 10 times closer than any other with a comparable radio broadcast. This proximity has allowed us to find the torus in a high-resolution VLA image of the galaxy's core, "said Rick Perley, also of NRAO. "Doing more work of this type on weaker and more distant objects will almost certainly require an improvement in the order of magnitude of the sensitivity and resolution provided by the ngVLA, the very new generation proposed." , he added.

VLA observations directly revealed the presence of gas in the Cygnus A torus, which has a radius of nearly 900 light-years. Long-standing models for the torus suggest that the dust is in the clouds embedded in the somewhat coarse gas.

Astronomers have used the NSF's very large Karl Jansky network to observe the dusty donut-shaped torus surrounding the black hole and the accretion disk at the center of a powerful galaxy radio. The animation shows how the torus can hide different features when viewed from different angles. This explains how the same type of "central engine" may appear differently, which leads to giving different names to objects seen from different angles.

"It's really great to finally see some direct evidence of something that we presume has been there for a long time," said Carilli. "To determine more precisely the shape and composition of this torus, we must continue to observe. For example, the chart Atacama Large Millimeter / submillimeter Array (ALMA) can observe at wavelengths that will directly reveal dust, "he added.

Carilli and Perley, along with their colleagues Vivek Dhawan, also of NRAO, and Daniel Perley of the Liverpool John Moores University in the UK, discovered the torus following their surprising discovery in 2016 of a new luminous object near the center of Cygnus A. This new object, they said, is most likely a second supermbadive black hole that has only recently encountered a new material that it could devour, allowing it to produce a light emission in the same way as the central black hole. The existence of the second black hole, they say, suggests that Cygnus A fused with another galaxy in the recent astronomical past.

Cygnus A, so named because it is the most powerful radio transmitter object in the constellation Cygnus, was discovered in 1946 by English physicist and radio astronomer J.S. Hey. It was badociated with a giant visible-light galaxy by Walter Baade and Rudolf Minkowski in 1951. It became an early target for VLA soon after its completion in the early 1980s. Detailed VLA images of Cygnus A published in 1984 allowed for major advances in the understanding of astronomers of such galaxies.

Scientists report their findings in the journal Astrophysical Journal Letters.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under a cooperative agreement by Associated Universities, Inc.