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EI2GYB > ASTRO 13.10.25 21:32l 45 Lines 5575 Bytes #28 (0) @ WW
BID : 46534_EI2GYB
Subj: Hunting for Pairs of Monster Black Holes
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Sent: 251013/1222Z 46534@EI2GYB.DGL.IRL.EURO LinBPQ6.0.25
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Hunting for Pairs of Monster Black Holes
When galaxies collide, it's not a gentle affair but it does take millions of years. Over this time the two massive star systems slowly merge together, their gravitational pull drawing them closer. At the heart of each galaxy lies a supermassive black hole, an object containing millions or even billions of times the mass of our Sun. After the galaxies merge, these two black holes should eventually find each other, settling into orbit around their shared centre of gravity. The result is one of the universe's most extreme phenomena, a supermassive black hole binary. But to date, none have been found.
Despite decades of theoretical predictions, proof of these binary systems remains frustratingly elusive. A recent review by an international team of astronomers and led by Martin G. H. Krause, examines the current state of evidence for these elusive pairs, bringing together observations from across the electromagnetic spectrum and exploring what we might expect to see as these systems spiral toward their ultimate merger.
Theory says that the formation of supermassive black hole binaries follows a predictable path. After two galaxies merge, the black holes from each progenitor galaxy sink toward the centre of the new combined galaxy through a process called dynamical friction. As each black hole moves through the galaxy, it transfers energy to nearby stars and dark matter, gradually losing momentum. Eventually, the two black holes find themselves orbiting each other at distances measured in thousands of light years, then hundreds, then potentially down to just a few light years apart.
Beyond their formation, these binary systems should leave distinctive fingerprints across multiple wavelengths of light. When gas falls onto the orbiting black holes, it can create characteristic double peaked emission lines in their spectra. If one of the black holes launches a jet, a powerful beam of particles and radiation shooting out at nearly the speed of light, the orbital motion of the binary can cause that jet to wobble or precess, creating distinctive S-shaped or curved structures visible in radio observations. Astronomers have identified several promising candidates showing exactly these features, including examples from the LOFAR radio telescope.
The review highlights observations of dual active galactic nuclei, systems where both black holes may be actively feeding on surrounding gas and shining brightly. These potential candidates have been spotted at scales ranging from thousands of light years down to just a few light years of separation. At larger separations, astronomers can sometimes directly image both black holes as distinct objects. At closer distances, the evidence becomes more indirect but no less compelling, relying on those telltale spectroscopic signatures and peculiar jet alignments.
What makes supermassive black hole binaries particularly exciting is their connection to gravitational wave astronomy. As these binary systems get closer, they begin radiating gravitational waves, ripples in the fabric of spacetime itself. Unlike the stellar mass black hole mergers detected by LIGO, which produce high frequency gravitational waves lasting just a few seconds, supermassive black hole binaries emit low frequency waves that could be detected by pulsar timing arrays or future space based detectors like the Laser Interferometer Space Antenna (LISA.) These observations would provide an entirely new window into the merger history of galaxies.
The review makes clear that while the evidence is compelling and diverse, significant uncertainties remain. One major puzzle is the "final parsec problem", the question of how binary black holes manage to shrink from separations of a few light years down to the final merger. At these distances, there may not be enough surrounding matter to extract energy from the orbit efficiently. Various solutions have been proposed, from the influence of massive stars passing nearby to the effects of gas disks or even a third supermassive black hole from another galactic collision.
Source : Evidence for Supermassive Black Hole Binaries
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