Revealed: The doughnut around a black hole
Critical part of ‘unified model’ confirmed. Andrew Masterson reports.
The blue-to-green approximate oval in the centre of this image is a phenomenon long theorised but never before directly seen.
It is a thick, dusty “doughnut” surrounding a rotating disk of material that is itself falling into the maw of a supermassive black hole that lies at the centre of Cygnus A, a galaxy some 760 million light-years from Earth.
In the so-called “unified model” for black holes, the existence of the torus, which is positioned approximately at right angles to the titanic jets extending fore and aft, is deployed to explain why the structures can look to different to observers, despite being pretty much identical.
All black holes, astronomers conclude, are surrounded by a ring of infalling material, and spew outflowing jets. Together, this set of features is called the active galactic nucleus (AGN).
However, black holes in different parts of the universe, and set at different angles of view to terrestrial or orbiting telescopes, don’t all look the same – a condition that led to them being divided into categories, such as quasars, blazars, or Seyfert galaxies.
The AGN model works on the assumption that such divisions represent a false taxonomy. To explain this, the notion of the torus was introduced. All black hole-powered “central engines” are basically similar (except in the matter of mass), the theory runs, but the torus is massive and thick, and obscures whatever is behind or inside it, relative to an observer, altering their appearance.
And now, astronomers led by Chris Carilli, of the National Radio Astronomy Observatory (NRAO) in Virginia, US, have succeeded in directly observing one for the first time.
“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,” Carilli says.
“The torus helps explain why objects known by different names actually are the same thing, just observed from a different perspective.”
To obtain the image, the researchers used the Karl G Jansky Very Large Array (VLA) radio-astronomy observatory in New Mexico, US.
The result clearly shows the supermassive black hole – 2.5 billion times more massive than the sun – at the centre of the structure, in the form of two red circles. The orange and yellow rings represent a swirling, orbiting mass of infalling material, inside the darker torus.
Jets flying from either side of the structure can also clearly be shown. Also of interest is a smaller bright object in the lower right-hand part of the image. It was first observed in 2016 and, at present, its exact nature remains unknown.
“Cygnus A is the closest example of a powerful radio-emitting galaxy — 10 times closer than any other with comparably powerful radio emission,” says researcher Rick Perley.
“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,” he added.
Carilli and colleagues plan to publish their findings in The Astrophysical Journal Letters.