Astronomers find peers at the centers of merging galaxies

Led by University of Maryland alumnus Michael Koss (MS '07, Ph.D. '11, astronomy), a research scientist at Eureka Scientific, Inc., with contributions from UMD astronomers, the team surveyed hundreds of nearby galaxies using imagery from the WM Keck Observatory in Hawaii and NASA's Hubble Space Telescope. The Hubble observations represent more than 20 years worth of images from the telescope's lengthy archive. The team describes their findings in a paper published on November 8, 2018, in the journal Nature.

"Seeing the peers 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 are taken in. You can not argue with it; it's a very 'clean' result, which does not depend on interpretation."

The high-resolution images of astronomers were more common in the early days when galaxy mergers were more frequent. When the black holes finally do collide, they will be unleashed powerful energy in the form of gravitational waves-ripples in space-time by the twin laser Interferometer Gravitational-wave Observatory (LIGO) detectors.

The images also presage what will happen in a few billion years, when our Milky Way galaxy merges with the Andromeda galaxy neighbors. Both galaxies host supermassive black holes at their center, which will eventually be smashed together and merge into a larger black hole.

The team was inspired by a Hubble image of two interacting galaxies collectively called NGC 6240, which later served as a prototype for the study. The team first looked for visually obscured, active black holes by sifting through 10 years' worth of X-ray data from the Burst Alert Telescope (BAT) aboard NASA's Neil Gehrels Swift Observatory.

"The advantage to using Swift's BAT is that it observes high-energy, 'hard' X-rays," said study co-author Richard Mushotzky, a professor of astronomy at UMD and a fellow of the Joint Space-Science Institute (JSI) . "These X-rays penetrate active galaxies, allowing the BAT to see things that are literally invisible in other wavelengths."

The researchers then combed through the Hubble archive, zeroing in on the merging galaxies they spotted in the X-ray data. They then used the Keck telescope's super-sharp, near-infrared vision to observe a larger sample of the X-ray-producing black holes found in the Hubble archive.

The team targeted galaxies located an average of 330 million light-years Earth-related close by in cosmic terms. Many of the galaxies are similar in size to the Milky Way and Andromeda galaxies. In total, the team analyzed 96 galaxies with the Keck telescope and 385 galaxies from the Hubble archive.

Their results suggest that more than 17 percent of these galaxies host a pair of black holes at their center, which are locked in the late stages of spiraling ever closer together before merging into a single, ultra-massive black hole. The researchers were surprised at such a high degree of late-stage mergers, because most simulations suggest that black hole pairs spend very little time in this phase.

To check their results, the researchers compared galaxies with a control group of other galaxies from the Hubble. In this group, only about one percent of the surveyed galaxies were suspected to host peers in the later stages of merging together.

This last step helped the researchers confirm that Galaxies are actually a signature of rapidly-growing black hole peers headed for a collision. According to the researchers, this finding is consistent with theoretical predictions, but until now, had not been verified by direct observations.

"Explained how to make it happen before, but what really enabled this particular study were the X-rays that can break through the cocoon of dust," explained Koss. "We also looked at the universe in a larger sense of space, giving us a greater chance of finding more luminous, rapidly-growing black holes."

It is not easy to find galactic nuclei so close together. Most prior observations of merging galaxies have caught coalescing black holes at earlier stages, when they were about 10 times farther away. The late stage of the merger process is so elusive because the galaxy interactions are encased in dense dust and gas, requiring very high-resolution observations that can be seen through the clouds and pinpoint the two merging nuclei.

"Computer simulations of galaxy smashups show us that black holes grow fastest during the final stages of mergers," said Laura Blecha, an assistant professor of physics at the University of Florida and co-author of the study. Blecha was a JSI Prize Postdoctoral Fellow in the UMD Department of Astronomy Prior to Joining UF's faculty in 2017. "The fact that black holes grow faster and faster as they grow up. be so monstrously big. "

Future infrared telescopes such as NASA's highly anticipated James Webb Space Telescope (JWST), slated for launch in 2021, will provide an even better view of mergers in dusty, heavily obscured galaxies. For nearby black hole peers, JWST should also be able to measure the masses, growth rates and other physical parameters for each black hole.

"Even with Hubble, many nearby galaxies at low redshift can not be resolved-the two nuclei just merge into one," said study co-author Sylvain Veilleux, a professor of astronomy at UMD and a JSI Fellow. "With JWST's higher angular resolution and sensitivity to the infrared, which can pass through the dusty cores of these galaxies, searches for these nearby objects should be easy to do." With these observations, we can begin to explore the subject of the most distant regions of the world-which should be fairly frequent. "

Explore further:
Tumultuous galaxy mergers better at switching on black holes

More information:
A population of luminous accreting black holes with hidden mergers, Nature (2018). DOI: 10.1038 / s41586-018-0652-7, https://www.nature.com/articles/s41586-018-0652-7