New Mexico astronomers make surprise discovery with VLA

2015 VLA radio image of the central region of Cygnus A.

| Photo: Credit: Perley, et al., NRAO/AUI/NSF

Stefan Dill

SOCORRO, N.M. — Pointing the Very Large Array (VLA) radio telescopes near Socorro, N.M. at a famous galaxy for the first time in two decades, a team of astronomers has discovered a bright new object near the galaxy’s core.

Rick Perley of the National Radio Astronomy Observatory said upgrades to the VLA in 2012 have enabled scientists to re-examine the Cygnus A galaxy.

Rick Perley, along with his son Daniel Perley, Vivek Dhawan, and Chris Carilli began the new observations in 2015 and continued them in 2016.

Artist’s conception of newly-discovered secondary supermassive black hole orbiting the main, central supermassive black hole of galaxy Cygnus A.

"To our surprise, we found a prominent new feature near the galaxy’s nucleus that did not appear in any previously 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."

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.

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