Merging galactic black holes focus of new survey

These images show galactic nuclei in the process of merging, with supermassive black holes in the cores of each pair of galaxies gorging on stellar debris and rapidly growing in size. In the top image of NGC 6240, the approaching black holes are just 3,000 light years apart. Image: NASA, ESA, and M. Koss (Eureka Scientific, Inc.)

Astronomers are getting their best view yet of close pairs of supermassive black holes in the cores of colliding galaxies as the holes move closer and closer together, feasting on stellar debris and rapidly growing to enormous size as they near coalescence.

Michael Koss of Eureka Scientific Inc. used high-resolution images infrared images from the Hubble Space Telescope and the W.M. Keck Observatory to carry out the largest survey yet of nearby galactic cores, peering through thick clouds of gas and dust that shroud the growing supermassive black holes.

“Seeing the pairs of merging galaxy nuclei associated with these huge black holes so close together was pretty amazing,” Koss said. “In our study, we see two galaxy nuclei right when the images were taken. You can’t argue with it; it’s a very ‘clean’ result, which doesn’t rely on interpretation.”

Laura Blecha of the University of Florida said computer simulations of galactic collisions “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.”

“The fact that black holes grow faster and 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 team’s observations and results were posted in the journal Nature.

The images provide a preview of sorts, showing the eventual fates of the supermassive black holes in the nuclei of the Milky Way and the Andromeda galaxies, which are expected to crash together in five billion years or so. They also illustrate a phenomenon that was more common in the early universe when galactic mergers were more frequent.

Galaxy mergers take a billion years or more to run their course, kicking up huge quantities of gas and dust as the slow-motion gravitational encounter proceeds. The ejected material can form a thick “curtain” around the centres of the merging galaxies, providing a ready source of cosmic food for the central black holes to feast on.

The fastest growth occurs during the last 10 million to 20 million years of the merger. The Hubble and Keck Observatory images show the best views yet of this final stage, when the rapidly growing black holes in the galaxy merger known as NGC 6240 are just 3,000 light years apart.

“Gas falling onto the black holes emits X-rays, and the brightness of the X-rays tells you how quickly the black hole is growing,” Koss said. “I didn’t know if we would find hidden mergers, but we suspected, based on computer simulations, that they would be in heavily shrouded galaxies.Therefore we tried to peer through the dust with the sharpest images possible, in hopes of finding coalescing black holes.”

It wasn’t easy. The team first sifted through 10 years of X-ray data from the Neil Gehrels Swift Telescope and then worked through the Hubble archive, identifying the merging galaxies found in the X-ray data. They then used Keck for infrared observations of X-ray-producing black holes not found in the Hubble archive.

“People had conducted studies to look for these close interacting black holes before, but what really enabled this particular study were the X-rays that can break through the cocoon of dust,” Koss said. “We also looked a bit farther in the universe so that we could survey a larger volume of space, giving us a greater chance of finding more luminous, rapidly growing black holes.”

Future infrared telescopes will provide more detailed views of dusty galaxy collisions, allowing astronomers to measure the masses, growth rates and dynamics of close black hole pairs.