Apr 3 2019
The Karl G. Jansky Very Large Array (VLA) of the National Science Foundation was used by astronomers to capture the first direct image of a doughnut-shaped, dusty feature around the supermassive black hole located at the core of one of the most powerful radio galaxies in the Universe—a feature first proposed by theorists almost 40 years ago as a fundamental part of such objects.
Artist’s conception of active galactic nucleus, with labels. (Image credit: Bill Saxton, NRAO/AUI/NSF)
The researchers explored Cygnus A, a galaxy located nearly 760 million light-years from Earth. The galaxy includes a black hole at its core, with a mass that is 2.5 billion times that of the Sun. While pulling in the surrounding material, the powerful gravitational effect of the black hole also propels superfast jets of material moving outward at speeds close to that of light, generating breathtaking “lobes” of bright radio emission.
“Central engines” that are powered by black holes generate bright emission at different wavelengths, and in many galaxies, it is very common to find jets extending far beyond the galaxy, but exhibiting distinct properties when observed. Those differences resulted in different names, like blazars, quasars, or Seyfert galaxies. Theorists developed a “unified model” with a common set of features to account for the differences, which would exhibit different properties based on the angle from which they are observed.
In the unified model, the central black hole is surrounded by a rotating disk of infalling material, and the jets accelerate outward from the disk’s poles. Moreover, a dusty, thick, doughnut-shaped “torus” is included, around the inner parts, to account for the fact that the same type of object appears different when observed from different angles. The torus conceals certain features when observed from the side, resulting in evident differences to the observer, even in the case of intrinsically similar objects. Astronomers term this common set of features as an active galactic nucleus (AGN).
The torus is an essential part of the AGN phenomenon, and evidence exists for such structures in nearby AGN of lower luminosity, but we’ve never before directly seen one in such a brightly-emitting radio galaxy. The torus helps explain why objects known by different names actually are the same thing, just observed from a different perspective.
Chris Carilli, National Radio Astronomy Observatory
In the 1950s, astronomers found out objects that emitted strong radio waves, but seemed to be point-like, analogous to distant stars, when later viewed with visible-light telescopes. In 1963, Maarten Schmidt of Caltech found out that one such object was very far away, and more such discoveries followed soon. In order to give an explanation for how these objects, named quasars, could be so bright, theorists proposed that they must be absorbing the tremendous gravitational energy of supermassive black holes. The combination of black hole, the rotating disk (known as an accretion disk), and the jets was called as the “central engine,” which was responsible for the copious bursts of energy from the objects.
It was observed that the same type of central engine also explained the output of other types of objects, such as blazars, radio galaxies, and Seyfert Galaxies. However, each demonstrated a different set of properties. Theorists strived to create a “unification scheme” to elucidate how the same thing could show up differently. In 1977, concealment by dust was proposed as one element of that scheme. As part of a 1982 paper, Robert Antonucci, of the University of California, Santa Barbara, introduced a drawing of an opaque torus—a doughnut-shaped object—around the central engine. Since that time, an obscuring torus has turned out to be a common feature of astronomers’ unified view of all kinds of active galactic nuclei.
Cygnus A is the closest example of a powerful radio-emitting galaxy—10 times closer than any other with comparably powerful radio emission. That proximity allowed us to find the torus in a high-resolution VLA image of the galaxy’s core. Doing more work of this type on weaker and more distant objects will almost certainly need the order-of-magnitude improvement in sensitivity and resolution that the proposed Next Generation Very Large Array (ngVLA) would bring.
Rick Perley, National Radio Astronomy Observatory
The VLA observations directly unraveled the gas in the torus of Cygnus A’, with a radius of almost 900 light-years. Longstanding torus models propose that the dust is in the clouds embedded in the slightly clumpy gas.
It’s really great to finally see direct evidence of something that we’ve long presumed should be there. To more accurately determine the shape and composition of this torus, we need to do further observing. For example, the Atacama Large Millimeter/submillimeter Array (ALMA) can observe at the wavelengths that will directly reveal the dust.
Chris Carilli, National Radio Astronomy Observatory.
Carilli and Perley, with their team members Vivek Dhawan, also of NRAO, and Daniel Perley of Liverpool John Moores University in the United Kingdom, found out the torus while following up their astonishing discovery in 2016 of a new, bright object located close to the center of Cygnus A. According to the researchers, the new object is most probably a second supermassive black hole that came into contact new material it could gobble only recently, making it generate bright emission the same manner in which the central black hole does. According to them, the occurrence of the second black hole indicates that Cygnus A merged with another galaxy in the astronomically recent past.
English physicist and radio astronomer J.S. Hey was the first to discover Cygnus A, called so since it is the most powerful radio-emitting object in the constellation Cygnus, in 1946. In 1951, Walter Baade and Rudolf Minkowski matched it to a giant, visible-light galaxy. It turned out to be an early target for the VLA right after it was completed in the early 1980s. Major progress in astronomers’ insights into such galaxies was achieved through detailed VLA images of Cygnus A published in 1984.
The study findings have been reported in the Astrophysical Journal Letters.