Astronomers used the very large Karl G. Jansky Ensemble of the National Science Foundation (VLA) to create the first direct image of a dusty donut-like feature that surrounds the supermbadive black hole in the center of one of the most powerful radio galaxies in the Universe. – a feature that theorists published for the first time almost four decades ago as an essential part of such objects.
Scientists studied Cygnus A, a galaxy about 760 million light years from Earth. The galaxy harbors a black hole 2,500 million times more mbadive than the Sun in its nucleus. As the powerful gravitational pull of the black hole attracts the surrounding material, it also drives super-fast jets of material that move outward at almost the speed of light, producing spectacular "lobes" of bright radio emission.
The "central engines" fed by the black hole that produce bright emission at various wavelengths, and the jets that extend beyond the galaxy are common to many galaxies, but show different properties when observed. Those differences led to a variety of names, such as quasars, blazars or Seyfert galaxies. To explain the differences, the theorists built a "unified model" with a common set of characteristics that would show different properties depending on the angle from which they are seen.
The unified model includes the central black hole, a rotating disk of infaling material that surrounds the black hole and the jets that rush outward from the poles of the disk. In addition, to explain why the same type of object looks different when viewed from different angles, a thick, dusty, donut-shaped "bull" is included that surrounds the internal parts. The bull hides some characteristics when viewed from the side, which leads to apparent differences for the observer, even for intrinsically similar objects. Astronomers generally call this common set of features an active galactic core (AGN).
"The bull is an essential part of the AGN phenomenon, and there is evidence of such structures in nearby AGNs of low luminosity, but we have never seen one directly in such a bright radio-galaxy," said Chris Carilli of the National Radio Astronomy Observatory. (NRAO) "The bull helps explain why the objects known by different names are actually the same thing, only observed from a different perspective," he added.
In the 1950s, astronomers discovered objects that emitted strong radio waves, but appeared in the form of dots, similar to distant stars, when they were later observed with telescopes of visible light. In 1963, Maarten Schmidt, of Caltech, discovered that one of these objects was extremely distant, and more discoveries of this type followed quickly. To explain how these objects, called quasars, could be so bright, theorists suggested that they should be taking advantage of the tremendous gravitational energy of supermbadive black holes. The combination of the black hole, the spinning disk, called the accretion disk, and the jets was called the "central engine" responsible for the proliferation of energy of the objects.
The same type of central engine also seemed to explain the output of other types of objects, including the radio galaxies, blazars and galaxies of Seyfert. However, each one showed a different set of properties. The theorists worked to develop a "unification scheme" to explain how the same thing might look different. In 1977, darkening by dust was suggested as an element of that scheme. In a 1982 article, Robert Antonucci of the University of California, Santa Barbara, presented a drawing of an opaque bull, a donut-shaped object, that surrounds the central engine. From that moment, a hidden bull has been a common feature of the unified vision of astronomers of all types of active galactic nuclei.
"Cygnus A is the closest example of a powerful radio-emitting galaxy: 10 times closer than any other similarly bright radio emission, that proximity allowed us to find the bull in a high-resolution VLA image of the galaxy's nucleus. "said Rick Perley, also from NRAO. "Doing more work of this kind on weaker and more distant objects will almost certainly require improvement in the magnitude and resolution of the magnitude and resolution that the proposed Next Generation Very Large Generation (ngVLA) would bring," he added.
The VLA observations directly revealed the gas in the Cygnus A bull, which has a radius of almost 900 light-years. The long-standing models for the bull suggest that the dust is in the clouds embedded in the somewhat agglomerated gas.
"It's really great to finally see direct evidence of something that for a long time we have presumed should be there," Carilli said. "To determine more precisely the shape and composition of this bull, we must continue to observe, for example, the Atacama millimeter / submillimeter matrix (ALMA) can observe the wavelengths that will directly reveal the dust," he added.
Carilli and Perley, along with colleagues Vivek Dhawan, also of NRAO, and Daniel Perley, of John Moores University in Liverpool, in the United Kingdom, discovered the bull by following its surprising discovery in 2016 of a new bright object near the center of Cygnus A. They said that this new object is probably a second supermbadive black hole that recently found new material that could devour, causing it to produce a bright emission in the same way as the central black hole does. The existence of the second black hole, they said, suggests that Cygnus A merged with another galaxy in the astronomically recent past.
Cygnus A, so called because it is the most powerful radio emission object of the Cygnus constellation, was discovered in 1946 by the English physicist and radio astronomer J.S. Hears. It was paired with a giant galaxy of visible light by Walter Baade and Rudolf Minkowski in 1951. It became an early target for the VLA shortly after its completion in the early 1980s. The detailed images of Cygnus A VLA published in 1984 produced important advances in the understanding of astronomers of such galaxies.
Scientists are reporting their findings in the Astrophysics of letters magazine.
The National Radio Astronomy Observatory is an installation of the National Science Foundation, operated under a cooperative agreement by Associated Universities, Inc.