Peering back in time some 3.5 billion years, a team of astronomers says it has spotted evidence for a pair of supermassive black holes in the final throes of merging – a cosmic clash of titans whose final union would send tremors rippling through the fabric of space-time itself.
Mergers represent important steps in the evolution of large galaxies and the supermassive black holes that lie at their centers. These black holes, millions to billions of times more massive than the sun, are thought to play a crucial role in regulating and ultimately stifling star formation in a galaxy.
If the team's interpretation of its evidence holds up, the observation would represent the first time anyone has detected a pair of supermassive black holes at such an advanced stage of merging – a crucial stage to observe because it's poorly understood.
Astronomers have found plenty of binary supermassive black holes separated by distances ranging from a few light-years to several thousand light-years, said George Djorgovski, an astronomer at the California Institute of Technology in Pasadena, who leads a project called the Catalina Real-Time Transient Survey, which provided the evidence that led to the discovery.
This new pair is separated by a relatively scant 2,000 astronomical units, or 2,000 times the distance between Earth and the sun, he said during a briefing Thursday at the American Astronomical Society's winter meeting in Seattle. That's roughly the distance from the sun to the start of the Oort Cloud, a distant source of comets that flit through the solar system.
The merger of the galaxies hosting the black holes probably began perhaps 100 million to 1 billion years before the stage that the team observed, Dr. Djorgovski said. The combined mass of the two black holes is on the order of hundreds of millions of times the mass of the sun.
Theory suggests that as seen from Earth, the two black holes will merge sometime in the next 100,000 to 1 million years. Such a merger would release as much energy as 10 billion supernovas, mainly in the form of ripples in space-time known as gravity waves.
The evidence for this binary supermassive black hole emerged as the team was studying variations in light coming from quasars as a window on the physics behind them.
Quasars are galaxies with extremely active supermassive black holes at their centers. As a central black hole's gravity draws matter toward it in a spiraling disk, the matter heats up, releasing energy that forms into jets extending outward along the black hole's spin axis. Seen from Earth, quasars are at their brightest when viewed end-on.
Typically, quasars brighten, then dim at random intervals, Djorgovski explained. As the team sifted through data on some 247,000 quasars, 20 stood out because they dimmed and brightened at regular intervals. This was their first hint that they had found supermassive black holes that were orbiting each other in a binary system.
One in particular, PG 1302-102, was the brightest of the 20, so the researchers focused their attention on it.
PG 1302-102, some 3.5 billion light-years away, appeared to brighten once every four years, but the survey's data on it went back only nine years. That wasn't long enough to build confidence in the observation. Fortunately, other teams had previously observed the same quasar, allowing the team to build a 20-year record of changes in its brightness. The pattern appeared throughout the 20-year span.
Evidence also came from the quasar's spectra.
The researchers acknowledge that other explanations could produce regular changes in brightness displayed in the observations, which also were published in Thursday's issue of the journal Nature. The paper's lead author is Matthew Graham, a senior computational scientist at Caltech.
The changes in brightness could come from a wobble in the spin of a single black hole as the wobble alters the viewing angle on the quasar's jets. But theory predicts that a single change from dim to bright and back via this mechanism would occur over hundreds to millions of years.
Regular changes, at least over the short term, could be imparted by a warp in the disk of dust and gas falling into the black hole. Astronomers have seen this in other supermassive black holes, but the pattern of change in the light from those doesn’t match the pattern from PG 1302-102.
In the end, "we do not know the exact mechanism by which the brightness variations occur, not yet anyway," Djorgovski acknowledged. "But anything we can possibly think of requires the presence of a second massive object. We're talking gravitational fields. If you want to perturb processes near a supermassive black hole, you really need some comparable mass to do that."