Merging Black Holes Trapped in Galactic Death Spiral
We now know that the vast majority of galaxies have supermassive black hole behemoths living in their cores. We also suspect that they grow when their host galaxies merge with other galaxies, eventually spawning the mother of all growth spurts: black hole mergers — when bigger black holes with masses millions or even billions of times the mass of the sun are created. Now, with the help of NASA’s Wide-field Infrared Survey Explorer (WISE), two supermassive black holes have been discovered, caught in the middle of a merging dance in the center of a galaxy 3.8 billion light-years away.
WISE’s primary mission came to an end in 2011 after its coolant ran dry, but a new batch of data has just been released and an oddity was discovered. Assumed to be a lively star-forming region, astronomers soon realized the infrared object known as WISE J233237.05-505643.5 had some weird properties. With the help of followup studies by the Australian Telescope Compact Array (ATCA) near Narrabri, Australia, and the Gemini South observatory in Chile, the real nature of the object was unraveled.
“At first we thought this galaxy’s unusual properties seen by WISE might mean it was forming new stars at a furious rate,” said WISE project manager Peter Eisenhardt, at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “But on closer inspection, it looks more like the death spiral of merging giant black holes.”
When active black holes “feed” on surrounding matter in galactic hubs, a superheated plasma forms around the black hole’s event horizon. Through complex physics in the intense relativistic environment that are not yet fully understood, jets blast from the poles of the spinning black hole, generating powerful emissions. In the case of WISE J233237.05-505643.5, it appears that two black holes are orbiting one another, separated by only a few light-years (which is very close considering the gigantic masses of these huge multi-million solar mass black holes), and one of the black hole’s jets are being “wiggled” by the gravitational interactions with its merging partner.
“We think the jet of one black hole is being wiggled by the other, like a dance with ribbons,” said Chao-Wei Tsai, of NASA’s Jet Propulsion Laboratory, lead author of this research set to appear in a paper to be published in the Dec. 10 issue of Astrophysical Journal. “If so, it is likely the two black holes are fairly close and gravitationally entwined.”
There’s only one way this extreme cosmic dance will end — both black holes will eventually spiral together, losing energy and momentum to gravitational waves rippling from their spacetime warping, collide and merge to form an even bigger black hole.
Black hole mergers are a rare event to observe and only a few candidates have been found. Some black hole merger candidates have been identified, but this example is the most distant discovered to date. Following the WISE discovery of the black hole pair, radio emission data from the ATCA spotted the strange zig-zag pattern one of the black holes’ jets seemed to be producing. Then, from infrared/optical data gathered by Gemini South, the anomalous jet wiggle was confirmed. Clumpiness in material surrounding one of the suspected black holes also indicates the local region is being perturbed by the gravitational presence of another black hole.
According to a JPL news release, astronomers are not sure how close the merging black holes are, only that this event will inevitably provide a privileged window into supermassive black hole evolution at a time just before their masses catastrophically combine.
“We note some caution in interpreting this mysterious system,” said JPL’s Daniel Stern, a co-author of the study. “There are several extremely unusual properties to this system, from the multiple radio jets to the Gemini data, which indicate a highly perturbed disk of accreting material around the black hole, or holes. Two merging black holes, which should be a common event in the universe, would appear to be simplest explanation to explain all the current observations.”