A team led by Eureka Scientific’s Dr Michael Koss used the WM Keck Observatory in Hawaii, the Hubble Space Telescope, and the Burst Alert Telescope (BAT) aboard NASA’s Neil Gehrels Swift Observatory, to find and photograph several pairs of merging black holes.
Coalescing galaxies have been spotted before, but only in their early stages. The final stage of the process, in which their respective black holes merge, has never been imaged before because it occurs behind the enormous, dense cloud of gas and dust that is produced as each of them consumes the surrounding material.
The team was able to overcome this obstacle with the help of X-ray data gathered by the BAT. “Gas falling onto the black holes emits X-rays [that penetrate the surrounding cloud] and the brightness of the X-rays tells you how quickly the black hole is growing,” Koss explained.
The BAT data pointed to the locations of possible black hole mergers and was cross-referenced against an archive of Hubble images of coalescing galaxies. Koss’s team then turned to the infrared telescopes at the WM Keck Observatory that could peer through the dust cloud to spot the black holes coming together.
It’s long been hypothesised that coalescing galaxies play a key role in the formation of supermassive black holes, which lie at the core of galaxies like our own Milky Way. But, until now, there has been no observational evidence to support the idea.
“Computer simulations of galaxy smash-ups show us that black holes grow fastest during the final stages of mergers, near the time when the black holes interact, and that’s what we have found in our survey,” said study team member Laura Blecha of the University of Florida, in Gainesville. “The fact that black holes grow faster as mergers progress tells us galaxy encounters are really important for our understanding of how these objects got to be so monstrously big.”
The images presage what could happen in a few billion years, when our Milky Way merges with the Andromeda Galaxy. Both galaxies host supermassive black holes at their cores, which will eventually smash together into one larger black hole.