A new study helps reveal what happens to supermassive black holes when three galaxies merge, as reported in our latest press release. This result, which used data from NASA's Chandra X-ray Observatory and several other telescopes, tells astronomers more about how galaxies and the giant black holes in their centers grow over cosmic time.

While there have been previous studies of mergers between two galaxies, this is one of the first to systematically look at the consequences for supermassive black holes when three galaxies come together. This panel of images contains data from two of seven galactic collisions in the new study containing two supermassive black holes left growing after the collision. The pair of mergers are seen in X-rays from Chandra (left in purple) and optical data (right) from NASA's Hubble Space Telescope and the Sloan Digital Sky Survey (SDSS). Circles in a labeled version of the Chandra image show X-rays from hot gas falling towards each black hole.

These triple galaxy mergers were first identified by sifting through data from the SDSS and NASA's WISE mission and then comparing the results to X-ray data in the Chandra archive. This method identified seven triple galaxy mergers located between 370 million and one billion light years from Earth.

Using specialized software, the team went through Chandra data targeting these systems to detect X-ray sources marking the location of growing supermassive black holes. As material falls toward a black hole, it gets heated to millions of degrees and produces X-rays. The combination of the new software and Chandra's sharp X-ray vision enabled the researchers to identify the black holes despite their close proximity in the images.

Out of seven triple galaxy mergers, there results are: one with a single growing supermassive black hole, four with double growing supermassive black holes (two of which are shown in the main graphics), and one that is a triple. The final merger of three galaxies they studied seems to have no X-ray emission detected from the supermassive black holes. This means that none of the supermassive black holes were left rapidly pulling in matter. In the systems with multiple black holes, the separations between them range between about 10,000 and 30,000 light years.

Once they found evidence for bright X-ray sources as candidates for growing supermassive black holes in the Chandra data, the researchers incorporated archival data from other telescopes such as WISE mission, the Infrared Astronomical Satellite, and the Two Micron All Sky Telescope as another check in the process.

Studies of triple mergers can help scientists understand whether pairs of supermassive black holes can approach so close to each other that they make ripples in spacetime called gravitational waves. The energy lost by these waves will inevitably cause the black holes to merge.

Adi Foord presented the new study, which she worked on as part of her Ph.D. at the University of Michigan, at the 237th meeting of the American Astronomical Society, which is being held virtually from January 11-15, 2021. Two papers describing this work have recently been accepted for publication in The Astrophysical Journal and preprints are available here and here.

NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science from Cambridge Massachusetts and flight operations from Burlington, Massachusetts.