Astronomers using the Very Large Array (VLA) have spotted a curious object orbiting Cygnus A, a famous supermassive black hole and one of the strongest sources of radio waves in the universe.
The new discovery, accepted in the Astrophysical Journal, is a bright object close to the galaxy core and was spotted when a new emission of radio waves appeared. The culprit is either an extremely rare example of a supernova or, more likely, a second supermassive black hole. If the object is indeed another supermassive black hole, we might have missed the whole picture of Cygnus A.
“This new object may have much to tell us about the history of this galaxy. Further observations will help us resolve some of these questions. In addition, if this is a secondary black hole, we may be able to find others in similar galaxies.” lead author Daniel Perley, of the Astrophysics Research Institute of Liverpool John Moores University, said in a statement.
The presence of a second black hole indicates that Cygnus A experienced a collision with another galaxy in the recent cosmic past. The object is only 1,500 light-years away from the central black hole, and if its nature is confirmed, this would be the closest pair of supermassive black holes ever discovered.
GIF showing the 1989 and 2015 observations of Cygnus A.
The new object is under the right beam from the central black hole.
Perley, et al., NRAO/AUI/NSF, NASA
More observations are necessary to actually confirm what the object is. Until then, astronomers are looking for other explanations, for example an incredibly powerful supernova. The supernova hypothesis is unlikely because the source seems to be very bright and long-lasting, factors that go against expected supernova behavior.
The object was detected in the latest observations of Cygnus A, which is over 800 million light-years away. Being such an incredibly bright object, Cygnus A has been observed many times by many instruments since it was discovered in 1939. It was also an early target for the VLA when it opened in the 1980s and it has been observed again in 1996. Only one radio source, the central supermassive black hole, was ever seen.
The region was also observed both by Hubble and the Keck observatory, which considered it a dense agglomeration of stars. It’s only in the latest observation campaign from 2015/2016 that the new source appears. If it’s a black hole, it means that it’s feeding.
“To our surprise, we found a prominent new feature near the galaxy’s nucleus that did not appear in any previous published images. That means it must have turned on sometime between 1996 and now.” Rick Perley from the National Radio Astronomy Observatory added.
Soon enough, the mystery of the Cygnus A’s companion won’t be much of a mystery.