Free Astronomy Magazine May-June 2024

36 MAY-JUNE 2024 ASTRO PUBLISHING mass of a supermassive bi- nary black hole plays a key role in stalling a potential merger. “The data archive serving the International Gemini Observatory holds a gold mine of untapped scientific discovery,” says Martin Still, NSF program director for the International Gemini Observatory. “Mass meas- urements for this extreme supermassive binary black hole are an awe-inspiring example of the potential impact from new research that explores that rich archive.” Understanding how this bi- nary formed can help pre- dict if and when it will merge — and a handful of clues point to the pair form- ing via multiple galaxy mergers. The first is that B2 0402+379 is a ‘fossil cluster,’ meaning it is the re- sult of an entire galaxy cluster’s worth of stars and gas merging into one single massive galaxy. Addition- ally, the presence of two supermas- sive black holes, coupled with their large combined mass, suggests they resulted from the amalgamation of multiple smaller black holes from multiple galaxies. Following a galactic merger, super- massive black holes don’t collide head-on. Instead they begin sling- shotting past each other as they set- tle into a bound orbit. With each pass they make, energy is trans- ferred from the black holes to the surrounding stars. As they lose en- ergy, the pair is dragged down closer and closer until they are just light-years apart, where gravita- tional radiation takes over and they merge. This process has been di- rectly observed in pairs of stellar- mass black holes — the first ever recorded instance being in 2015 via U sing archival data from the Gemini North telescope, one half of the International Gem- ini Observatory, operated by NSF’s NOIRLab, a team of astronomers have measured the heaviest pair of supermassive black holes ever found. This video highlights the discov- ery. [Images and Videos: NOIRLab/NSF/AURA/J. daSilva/M. Zamani, ESA/Hubble (M. Korn- messer), N. Bartmann. Music: zero project - The Lower Dungeons (zero-project.gr) ] the detection of gravitational waves — but never in a binary of the supermassive variety. With new knowledge of the sys- tem’s extremely large mass, the team concluded that an exception- ally large number of stars would have been needed to slow the bi- nary’s orbit enough to bring them this close. In the process, the black holes seem to have flung out nearly all the matter in their vicinity, leav- ing the core of the galaxy starved of stars and gas. With no more ma- terial available to further slow the pair’s orbit, their merger has stalled in its final stages. “Normally it seems that galaxies with lighter black hole pairs have enough stars and mass to drive the two together quickly,” said Ro- mani. “Since this pair is so heavy it required lots of stars and gas to get the job done. But the binary has scoured the central galaxy of such matter, leaving it stalled and acces- sible for our study.” Whether the pair will overcome their stagnation and eventually merge on timescales of millions of years, or continue in orbital limbo forever, is yet to be determined. If they do merge, the resulting gravi- tational waves would be a hundred million times more powerful than those produced by stellar-mass black hole mergers. It’s possible the pair could conquer that final dis- tance via another galaxy merger, which would inject the system with additional material, or potentially a third black hole, to slow the pair’s orbit enough to merge. However, given B2 0402+379’s status as a fos- sil cluster, another galactic merger is unlikely. “We’re looking forward to follow- up investigations of B2 0402+379’s core where we’ll look at how much gas is present,” says Tirth Surti, Stan- ford undergraduate and the lead author on the paper. “This should give us more insight into whether the supermassive black holes can eventually merge or if they will stay stranded as a binary.” !