Free Astronomy Magazine March-April 2023

49 MARCH-APRIL 2023 ing two black holes so close together in the nearby Uni- verse could pave the way for additional studies of the ex- citing phenomenon. “ALMA is unique in that it can see through large columns of gas and dust and achieve very high spatial resolution to see things very close together. Our study has identified one of the closest pairs of black holes in a galaxy merger, and because we know that galaxy mergers are much more com- mon in the distant Universe, these black hole binaries too may be much more common than previously thought.” If close-paired binary black hole pairs are indeed com- monplace, as Koss and the team posit, there could be significant implications for future detections of gravita- tional waves. Ezequiel Treister, an as- tronomer at Universidad Católica de Chile and a co-au- thor of the research said, “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 observ- ing frequent gravitational wave events caused by the mergers of these objects across the Universe.” Pairing ALMA data with multi-wave- length observations from other powerful telescopes like Chandra, Hubble, ESO’s Very Large Telescope, and Keck added fine details to an al- ready-compelling tale. “Each wave- length tells a different part of the story. While ground-based optical imaging showed us the whole merg- ing galaxy, Hubble showed us the nuclear regions at high resolutions. X-ray observations revealed that there was at least one active galactic S cientists using the At- acama Large Millimeter/sub- millimeter Array (ALMA) to look deep into the heart of the pair of merging galax- ies known as UGC 4211 dis- covered two black holes growing side by side, just 750 light-years apart. This artist’s concep- tion shows the late-stage gal- axy merger and its two central black holes. The binary black holes are the closest together ever observed in multiple wavelengths. [ALMA (ESO/ NAOJ/NRAO); M. Weiss (NRAO/ AUI/NSF)] galaxy mergers that create them may be surprisingly commonplace in the Universe. The results of the new research were published in The Astrophysical Jour- nal Letters, and presented in a press conferenceat the 241 st meeting of the American Astronomical Society (AAS) in Seattle, Washington. At just 500 million light-years away from Earth in the constellation Can- cer, UGC 4211 is an ideal candidate for studying the end stages of gal- axy mergers, which occur more fre- quently in the distant Universe, and as a result, can be difficult to ob- serve. When scientists used the highly sensitive 1.3mm receivers at ALMA to look 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 byprod- ucts 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 bina- ries in nearby galaxies would be in- active; much less common two growing black holes like we found,” said Michael Koss, a senior research scientist at Eureka Scientific and the lead author of the new research. Koss added that the use of ALMA was a game-changer, and that find-