very long baseline array



Pointing the 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 to appear in the Astrophysical Journal [, preprint will appear on by tomorrow morning] 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 1,500 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.

TOP IMAGE….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

UPPER IMAGE….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

CENTRE IMAGE….VLA radio image (orange) of central region of Cygnus A, overlaid on Hubble Space Telescope image, from 1989. edit: Perley, et al., NRAO/AUI/NSF, NASA

LOWER IMAGE….2015 VLA radio image (orange) of Cygnus A, overlaid on Hubble Space Telescope image. Credit: Perley, et al., NRAO/AUI/NSF, NASA

BOTTOM IMAGE….1989 VLA radio image of the central region of Cygnus A.
Credit: Perley, et al., NRAO/AUI/NSF

LAST IMAGE….2015 VLA radio image of the central region of Cygnus A.
Credit: Perley, et al., NRAO


This composite NASA image of the spiral galaxy M81, located about 12 million light years away, includes X-ray data from the Chandra X-ray Observatory (blue), optical data from the Hubble Space Telescope (green), infrared data from the Spitzer Space Telescope (pink) and ultraviolet data from GALEX (purple). The inset shows a close-up of the Chandra image. At the center of M81 is a supermassive black hole that is about 70 million times more massive than the Sun.

A new study using data from Chandra and ground-based telescopes, combined with detailed theoretical models, shows that the supermassive black hole in M81 feeds just like stellar mass black holes, with masses of only about ten times that of the Sun. This discovery supports the implication of Einstein’s relativity theory that black holes of all sizes have similar properties, and will be useful for predicting the properties of a conjectured new class of black holes.

In addition to Chandra, three radio arrays (the Giant Meterwave Radio Telescope, the Very Large Array and the Very Long Baseline Array), two millimeter telescopes (the Plateau de Bure Interferometer and the Submillimeter Array), and Lick Observatory in the optical were used to monitor M81. These observations were made simultaneously to ensure that brightness variations because of changes in feeding rates did not confuse the results. Chandra is the only X-ray satellite able to isolate the faint X-rays of the black hole from the emission of the rest of the galaxy.

The supermassive black hole in M81 generates energy and radiation as it pulls gas in the central region of the galaxy inwards at high speed. Therefore, the model that Markoff and her colleagues used to study the black holes includes a faint disk of material spinning around the black hole. This structure would mainly produce X-rays and optical light. A region of hot gas around the black hole would be seen largely in ultraviolet and X-ray light. A large contribution to both the radio and X-ray light comes from jets generated by the black hole. Multiwavelength data is needed to disentangle these overlapping sources of light.

Astronomers detect orbital motion in pair of supermassive black holes

Using the supersharp radio “vision” of the National Science Foundation’s Very Long Baseline Array (VLBA), astronomers have made the first detection of orbital motion in a pair of supermassive black holes in a galaxy some 750 million light-years from Earth.

The two black holes, with a combined mass 15 billion times that of the Sun, are likely separated by only about 24 light-years, extremely close for such a system.

“This is the first pair of black holes to be seen as separate objects that are moving with respect to each other, and thus makes this the first black-hole ‘visual binary,’” said Greg Taylor, of the University of New Mexico (UNM).

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The Far Side of the Milky Way

Mapping Spiral Structure for an Improved Picture of our Home Galaxy

Astronomers from the Max Planck Institute for Radio Astronomy in Bonn, Germany, and the Harvard-Smithsonian Center for Astrophysics have directly measured the distance to a star-forming region on the opposite side of our Milky Way Galaxy from the Sun, using the Very Long Baseline Array.

Their achievement reaches deep into the Milky Way’s terra incognita and nearly doubles the previous record for distance measurement within our Galaxy.

Their results are published in the 13 October issue of the journal Science.

Distance measurements are crucial for an understanding of the structure of the Milky Way. Most of our Galaxy’s material, consisting principally of stars, gas, and dust, lies within a flattened disk, in which our Solar System is embedded. Because we cannot see our Galaxy face-on, its structure, including the shape of its spiral arms, can only be mapped by measuring distances to objects elsewhere in the Galaxy.

The astronomers used a technique called trigonometric parallax, first applied by Friedrich Wilhelm Bessel in 1838 to measure the distance to the star 61 Cygni in the constellation of the Swan. This technique measures the apparent shift in the sky position of a celestial object as seen from opposite sides of the Earth’s orbit around the Sun. This effect can be demonstrated by holding a finger in front of one’s nose and alternately closing each eye – the finger appears to jump from side to side.

Measuring the angle of an object’s apparent shift in position this way allows astronomers to use simple trigonometry to directly calculate the distance to that object. The smaller the measured angle, the greater the distance is. In the framework of the Bar and Spiral Structure Legacy (BeSSeL) Survey, it is now possible to measure parallaxes a thousand times more accurate than Friedrich Bessel. The Very Long Baseline Array (VLBA), a continent-wide radio telescope system, with ten dish antennas distributed across North America, Hawaii, and the Caribbean, can measure the minuscule angles associated with great distances. In this case, the measurement was roughly equal to the angular size of a baseball on the Moon.

“Using the VLBA, we now can accurately map the whole extent of our Galaxy,” says Alberto Sanna, of the Max Planck Institute for Radio Astronomy in Germany (MPIfR).

The new VLBA observations, made in 2014 and 2015, measured a distance of more than 66,000 light-years to the star-forming region G007.47+00.05 on the opposite side of the Milky Way from the Sun, well past the Galaxy’s center in a distance of 27,000 light-years. The previous record for a parallax measurement was about 36,000 light-years.

“Most of the stars and gas in our Galaxy are within this newly-measured distance from the Sun. With the VLBA, we now have the capability to measure enough distances to accurately trace the Galaxy’s spiral arms and learn their true shapes,” Sanna explains.

The VLBA observations measured the distance to a region where new stars are being formed. Such regions include areas where molecules of water and methanol act as natural amplifiers of radio signals – masers, the radio-wave equivalent of lasers for light waves. This effect makes the radio signals bright and readily observable with radio telescopes.

The Milky Way has hundreds of such star-forming regions that include masers. “So we have plenty of ‘mileposts’ to use for our mapping project. But this one is special: Looking all the way through the Milky Way, past its center, way out into the other side”, says the MPIfR’s Karl Menten.

The astronomers’ goal is to finally reveal what our own Galaxy looks like if we could leave it, travel outward perhaps a million light-years, and view it face-on, rather than along the plane of its disk. This task will require many more observations and much painstaking work, but, the scientists say, the tools for the job now are in hand. How long will it take?

“Within the next 10 years, we should have a fairly complete picture,” predicts Mark Reid of the Harvard-Smithsonian Center for Astrophysics.

TOP IMAGE….Artist’s view of the Milky Way with the location of the Sun and the star forming region (maser source G007.47+00.05) at the opposite side in the Scutum-Centaurus spiral arm.
© Bill Saxton, NRAO/AUI/NSF; Robert Hurt, NASA.

LOWER IMAGE….Distance determination by measuring the angle of apparent shift in an object’s position, as seen from opposite sides of Earth’s orbit around the Sun (trigonometric parallax technique).
© Bill Saxton, NRAO/AUI/NSF; Robert Hurt, NASA.


Ask Ethan: Can We See Our Galaxy’s Supermassive Black Hole?

I understand that our galaxy has a massive black hole in its center, but how close would you have to be to actually see it? I’m guessing you wouldn’t have to be close to the event horizon, but given all the stars surrounding it, and the dust and debris being sucked into it, it seems unlikely that you could see it from any appreciable distance, even if you were directly above or below the plane of the galaxy.

One of the great discoveries of the past few decades was that of a supermassive black hole at the center of our Milky Way. No longer was it mere conjecture or unverified theory; observations in the X-ray, infrared, radio, and of stars orbiting a central, non-luminous point all indicate the presence of a 4 million solar mass black hole at a location known as Sagittarius A*. At a distance of 26,000 light years, an object as small as this black hole’s event horizon – even at 23 million km in diameter – would be unresolvable to a telescope the size of an entire country. But thanks to the very clever technique of very long baseline interferometry, the proposed Event Horizon Telescope has the capabilities, for the first time, of imaging a black hole’s event horizon directly.