Artist's conception of newly-discovered secondary supermassive black hole orbiting the main,
central supermassive black hole of galaxy Cygnus A.
Credit: Bill Saxton, NRAO/AUI/NSF
central supermassive black hole of galaxy Cygnus A.
Credit: Bill Saxton, NRAO/AUI/NSF
VLA radio images (orange) of central region of Cygnus A, overlaid on Hubble Space Telescope image, from 1989 and 2015.
Animated GIF. Credit: Perley, et al., NRAO/AUI/NSF, NASA
Animated GIF. Credit: Perley, et al., NRAO/AUI/NSF, NASA
VLA radio image (orange) of central region of Cygnus A, overlaid on Hubble Space Telescope image, from 1989.
Credit: Perley, et al., NRAO/AUI/NSF, NASA
2015 VLA radio image (orange) of Cygnus A, overlaid on Hubble Space Telescope image.
Credit: Perley, et al., NRAO/AUI/NSF, NASA
1989 VLA radio image of the central region of Cygnus
A.
Credit: Perley, et al., NRAO/AUI/NSF
2015 VLA radio image of the central region of Cygnus A.
Credit: Perley, et al., NRAO/AUI/NSF
Pointing the National Science Foundation’s Very Large Array (VLA) at a
famous galaxy for the first time in two decades, a team of astronomers
got a big surprise, finding that a bright new object had appeared near
the galaxy’s core. 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.
The astronomers observed Cygnus A, a well-known and often-studied
galaxy discovered by radio-astronomy pioneer Grote Reber in 1939. The
radio discovery was matched to a visible-light image in 1951, and the
galaxy, some 800 million light-years from Earth, was an early target of
the VLA after its completion in the early 1980s. Detailed images from
the VLA published in 1984 produced major advances in scientists’
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.
“This new object may have much to tell us about the history of this
galaxy,” said Daniel Perley, of the Astrophysics Research Institute of
Liverpool John Moores University in the U.K., lead author of a paper in
the Astrophysical Journal announcing the discovery.
“The VLA images of Cygnus A from the 1980s marked the state of the
observational capability at that time,” said Rick Perley, of the
National Radio Astronomy Observatory (NRAO). “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,” Perley said.
Daniel and Rick Perley, along with Vivek Dhawan, and Chris Carilli,
both of NRAO, 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. This new
feature is bright enough that we definitely would have seen it in the
earlier images if nothing had changed,” said Rick Perley. “That means it
must have turned on sometime between 1996 and now,” he added.
The scientists then 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.
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,” Dhawan said.
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,” Carilli said. “These two would be one of
the closest pairs of supermassive black holes ever discovered, likely
themselves to merge in the future.”
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, they said, 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.
Also at the time of those first VLA observations of Cygnus A, Carilli and Dhawan were office mates as graduate students at MIT.
Carilli, now NRAO’s Chief Scientist, was Rick’s graduate student
while working as a predoctoral fellow at NRAO. His doctoral dissertation
was on detailed analysis of 1980s VLA images of Cygnus A.
The National Radio Astronomy Observatory is a facility of the
National Science Foundation, operated under cooperative agreement by
Associated Universities, Inc.
Media Contact:
Dave Finley, Public Information Officer
(575) 835-7302
dfinley@nrao.edu
