- 3799 - BLACKHOLES - merging? Black Holes shouldn’t be able to merge, but dozens of mergers have been detected. “How Do They Do It?' Who knows what lurks in the hearts of some globular clusters?
--------------------- 3799
- BLACKHOLES -
merging?
-
- Astronomers using a collection of
gravitational wave observatories found evidence of collections of smaller black
holes dancing together as binaries in the hearts of globulars.
-
- What’s more, they’ve detected an increased
number of gravitational wave events when some of these stellar-mass black holes
crashed together.
The globular cluster NGC 6397
contains many stellar-mass black holes among its 400,000 stars.
-
-NGC 6397 orbits the Milky Way
at a distance of about 8,000 light-years from Earth. It has undergone core
collapse, with a tightly packed core. Not only does that core contain stars,
but also white dwarfs and neutron stars, indicating the aging stellar
population.
-
- Black holes shouldn’t be able to merge. Once
black holes get fairly close together in binary pairs, they can settle into
stable orbits with each other. The situation changes, however, if they’re
dancing together in a crowded environment. That actually describes globular
clusters.
- Those stellar agglomerations contain tens of
thousands or even millions of stars packed together. Those stars are tightly
gravitationally bound together, which creates a gravity “gradient” from the
outside into the core.
-
- As aging supermassive stars in a
globular die, some end up as stellar-mass black holes. Eventually, they sink to
the core of the cluster. That’s called “mass segregation”. Eventually, they
create a sort of “invisible dark core”.
-
- Black holes in binary pairs in the cluster
are most likely to merge. Any nearby massive objects can remove orbital energy
from the binary pair. Astronomers call these “dynamical interactions”. The loss
of energy pushes them close together and affects the shape of the orbit to make
it more elongated. That takes the black hole pair out of the stable orbit
they’ve enjoyed.
-
- If this is actually what’s happening, then
the black holes pass closer and closer together under the effect of the
gravitational interaction. Eventually, a merger occurs. That sets off
gravitational waves that we can detect here on Earth. When two black holes are
in such an elongated orbit, their gravitational wave signal has characteristic
“fingerprints”.
-
- Since 2015, at least 85 pairs of black holes
have crashed into each other and been detected by the LIGO-Virgo-KAGRA
Collaboration. Gravitational wave research into these kinds of mergers requires
worldwide cooperation. That’s because multiple gravitational wave detectors can
make it easier for verified events to be studied.
-
- The twin LIGO observatories in the
United States work together with the Virgo facility in Italy and the KAGRA
observatory in Japan. They carry out joint observations and analysis of
resulting data and have worked together since 2010.
- The research team now expects to sense more
mergers of binaries in globular clusters during the next LIGO-Virgo-KAGRA
observing run, which begins in 2023.
-
December 24, 2022 BLACKHOLES -
merging? 3799
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--------------------- --- Friday, December 30,
2022 ---------------------------
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