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EI2GYB > ASTRO 13.10.25 14:39l 93 Lines 5574 Bytes #999 (0) @ WW
BID : 46534_EI2GYB
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Subj: Hunting for Pairs of Monster Black Holes
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Sent: 251013/1222Z @:EI2GYB.DGL.IRL.EURO #:46534 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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