A Pair of Black Holes Dining Together in Nearby Galaxy Merger

the keck ii telescope’s adaptive optics paired with nirc2 captured a high-resolution near-infrared image of ugc 4211, unveiling the dual nuclei of the two merging galaxies.
Credit: M. Koss (Eureka Scientific) et al./W. M. Keck Observatory

Maunakea, Hawaiʻi While studying a nearby pair of merging galaxies, scientists discovered two supermassive black holes growing simultaneously near the center of the newly coalescing galaxy, dubbed UGC 4211. These super-hungry giants are the closest together that scientists have ever observed in multiple wavelengths. What’s more, the new research reveals that binary black holes and the galaxy mergers that create them may be surprisingly common in the universe.

The study, which includes data from W. M. Keck Observatory on Maunakea, Hawaiʻi, is published in today’s issue of The Astrophysical Journal Letters and presented in a press conference at the 241st meeting of the American Astronomical Society in Seattle, Washington.

At just 500 million light-years away from Earth in the constellation Cancer, UGC 4211 is an ideal candidate for studying the end stages of galaxy mergers, which occur more frequently in the distant universe, and as a result, can be difficult to observe.

When scientists looked deep into the merger’s active galactic nuclei— compact, highly luminous areas in galaxies caused by the accretion of matter around central black holes— they found not one, but two black holes gluttonously devouring the byproducts of the merger. Surprisingly, they were dining side-by-side with just 750 light-years between them.

“Simulations suggested that most of the population of black hole binaries in nearby galaxies would be inactive because they are more common, not two growing black holes like we found,” said Michael Koss, a senior research scientist at Eureka Scientific and the lead author of the study.

If close-paired binary black hole pairs are indeed common, as Koss and the team posit, there could be significant implications for future detections of gravitational waves.

“There might be many pairs of growing supermassive black holes in the centers of galaxies that we have not been able to identify so far. If this is the case, in the near future we will be observing frequent gravitational wave events caused by the mergers of these objects across the universe,” said Ezequiel Treister, an astronomer at Universidad Católica de Chile and a co-author of the research.

Artist’s conception of UGC 4211, a pair of merging galaxies with two central black holes growing side by side, just 750 light-years apart. The binary black holes are the closest together ever observed in multiple wavelengths. Credit: ALMA (ESO/NAOJ/NRAO); M. Weiss (NRAO/AUI/NSF)

The team observed UGC 4211 in multiple wavelengths using various telescopes around the world and in space; in addition to using Keck Observatory’s adaptive optics system paired with its Near-Infrared Camera, second generation (NIRC2) and OH-Suppressing Infrared Imaging Spectrograph (OSIRIS), the researchers also studied the galaxy merger with the Hubble Space Telescope, European Southern Observatory’s Very Large Telescope, Atacama Large Millimeter/submillimeter Array, NASA’s Chandra X-ray Observatory, and the Victor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory.

“All of these data together have given us a clearer picture of how galaxies such as our own turned out to be the way they are, and what they will become in the future,” said Treister.

So far, scientists have mostly studied only the earliest stages of galaxy mergers. The new research could have a profound impact on our understanding of the Milky Way’s own impending merger with the nearby Andromeda Galaxy.

“The Milky Way-Andromeda collision is in its very early stages and is predicted to occur in about 4.5 billion years. What we’ve just studied is a source in the very final stage of collision, so what we’re seeing presages that merger and also gives us insight into the connection between black holes merging and growing and eventually producing gravitational waves,” said Koss.


W. M. Keck Observatory is a distinguished leader in the field of adaptive optics (AO), a breakthrough technology that removes the distortions caused by the turbulence in the Earth’s atmosphere.  Keck Observatory pioneered the astronomical use of both natural guide star (NGS) and laser guide star adaptive optics (LGS AO) and current systems now deliver images three to four times sharper than the Hubble Space Telescope at near-infrared wavelengths. AO has imaged the four massive planets orbiting the star HR8799, measured the mass of the giant black hole at the center of our Milky Way Galaxy, discovered new supernovae in distant galaxies, and identified the specific stars that were their progenitors. Support for this technology was generously provided by the Bob and Renee Parsons Foundation, Change Happens Foundation, Gordon and Betty Moore Foundation, Mt. Cuba Astronomical Foundation, NASA, NSF, and W. M. Keck Foundation.


The Near-Infrared Camera, second generation (NIRC2) works in combination with the Keck II adaptive optics system to obtain very sharp images at near-infrared wavelengths, achieving spatial resolutions comparable to or better than those achieved by the Hubble Space Telescope at optical wavelengths. NIRC2 is probably best known for helping to provide definitive proof of a central massive black hole at the center of our galaxy. Astronomers also use NIRC2 to map surface features of solar system bodies, detect planets orbiting other stars, and study detailed morphology of distant galaxies.


The OH-Suppressing Infrared Imaging Spectrograph (OSIRIS) is one of W. M. Keck Observatory’s “integral field spectrographs.” The instrument works behind the adaptive optics system, and uses an array of lenslets to sample a small rectangular patch of the sky at resolutions approaching the diffraction limit of the 10-meter Keck Telescope. OSIRIS records an infrared spectrum at each point within the patch in a single exposure, greatly enhancing its efficiency and precision when observing small objects such as distant galaxies. It is used to characterize the dynamics and composition of early stages of galaxy formation. Support for this technology was generously provided by the Heising-Simons Foundation and the National Science Foundation.


The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.