A Bright New Object found near Cygnus A Galaxy’s Central Black Hole

Artist’s conception of the newly-discovered object orbiting the main, central supermassive black hole of the galaxy Cygnus A. Image credit: Bill Saxton / NRAO / AUI / NSF.

When astronomers pointed for the first time the in more than 20 years the Karl G. Jansky Very Large Array (VLA) at the Cygnus A galaxy, they got a big surprise. This object is well-known and often-studied, now a new object has appeared near the galaxy’s supermassive black hole.

The object, the scientists concluded, is either a very rare type of supernova explosion or, more likely, an outburst from a second supermassive black hole closely orbiting the galaxy’s primary, central supermassive black hole. 

This radio galaxy, Cygnus A, also known as LEDA 63932, NRAO 620 and IRAS 19577+4035, was discovered by radio-astronomy pioneer Grote Reber in 1939, it is located at 800 million light-years away in the constellation Cygnus, this galaxy is one of the strongest radio sources in the sky.

Back in the early 1980s when the VLA was built, Cygnus A was one of the objects studied, detailed images were published in 1984, which led to major advances in scientist’s understanding of the superfast ‘jets’ of subatomic particles propelled into intergalactic space by the gravitational energy of supermassive black holes at the cores of galaxies.

“The VLA images of Cygnus A from the 1980s marked the state of the observational capability at that time,” said National Radio Astronomy Observatory astronomer Dr. Rick Perley, one of the researchers who made the original Cygnus A observations with the VLA in the 1980s.

The RM distribution in Cygnus A based on multi-frequency, multi-configuration
VLA observations. The resolution is 0.35'' (Dreher et al. 1987).

“Because of that, we didn’t look at Cygnus A again until 1996, when new VLA electronics had provided a new range of radio frequencies for our observations. The new object does not appear in the images made then.”

“However, the VLA’s upgrade that was completed in 2012 made it a much more powerful telescope, so we wanted to have a look at Cygnus A using the VLA’s new capabilities.”

“The new object may have much to tell us about the history of this galaxy,” said Dr. Daniel Perley, an astronomer with the Astrophysics Research Institute at Liverpool John Moores University and Dr. Rick Perley’s son. The researchers began the new observations in 2015, and continued them in 2016.

“To our surprise, we found a prominent new feature near the galaxy’s nucleus that did not appear in any previous published images,” Dr. Rick Perley said.

“This new feature is bright enough that we definitely would have seen it in the earlier images if nothing had changed. That means it must have turned on sometime between 1996 and now.”

Then, the scientists observed Cygnus A with the Very Long Baseline Array (VLBA) in November of 2016, clearly detecting the new object. A faint infrared object also is seen at the same location in Hubble Space Telescope and Keck observations, originally made between 1994 and 2002. The infrared astronomers, from Lawrence Livermore National Laboratory, had attributed the object to a dense group of stars, but the dramatic radio brightening is forcing a new analysis.

An image taken at radio wavelengths of the dramatic jets of charged particles being ejected from the nucleus of the galaxy Cygnus A. Image credit: NRAO/AU. 2016

What is the new object? Based on its characteristics, the astronomers concluded it must be either a supernova explosion or an outburst from a second supermassive black hole near the galaxy’s center. While they want to watch the object’s future behavior to make sure, they pointed out that the object has remained too bright for too long to be consistent with any known type of supernova.

“Because of this extraordinary brightness, we consider the supernova explanation unlikely,” said Dr. Vivek Dhawan, an astronomer at the National Radio Astronomy Observatory. While the new object definitely is separate from Cygnus A’s central supermassive black hole, by about 1500 light-years, it has many of the characteristics of a supermassive black hole that is rapidly feeding on surrounding material.

“We think we’ve found a second supermassive black hole in this galaxy, indicating that it has merged with another galaxy in the astronomically-recent past,” said Dr. Christopher Carilli, an astronomer at the National Radio Astronomy Observatory. “These two would be one of the closest pairs of supermassive black holes ever discovered, likely themselves to merge in the future.”

VLA radio images (orange) of central region of Cygnus A, overlaid on Hubble Space Telescope image, from 1989 on the left and 2015 on the right. Credit: Perley, et al., NRAO/AUI/NSF, NASA

The astronomers suggested that the second black hole has become visible to the VLA in recent years because it has encountered a new source of material to devour. That material could either be gas disrupted by the galaxies’ merger or a star that passed close enough to the secondary black hole to be shredded by its powerful gravity.

“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,” Daniel Perley said.

Rick Perley was one of the astronomers who made the original Cygnus A observations with the VLA in the 1980s. Daniel Perley is his son, now also a research astronomer.

“Daniel was only two years old when I first observed Cygnus A with the VLA,” Rick said. As a high school student in Socorro, New Mexico, Daniel used VLA data for an award-winning science fair project that took him to the international level of competition, then went on to earn a doctoral degree in astronomy.

The research paper reporting this discovery has been accepted for publication in the Astrophysical Journal (arXiv.org preprint).

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Daniel A. Perley et al. 2017. Discovery of a Luminous Radio Transient 460 pc from the Central Supermassive Black Hole in Cygnus A. ApJ, accepted for publication; arXiv: 1705.07901

Sources: Sci News, NRAO, Wikipedia,