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Galaxies could have their lives extended by supermassive black holes, which would provide them with “hearts and lungs” to help them “breathe” and prevent them from growing too large.
That’s the suggestion of new research that suggests the universe would age much faster today and be full of “zombie” galaxies containing dead or dying stars, were it not for the supermassive black holes thought to lie at the heart of all large galaxies. The astrophysicists behind the findings compare the jets of gas and radiation expelled from the poles of supermassive black holes to the airways that feed our lungs.
The University of Kent team thinks that pulses from each black hole “heart” cause shock fronts to oscillate back and forth across both jets. This is similar to how a part of our body called the thoracic diaphragm moves up and down inside our chest cavities to inflate and deflate our lungs.
In galaxies, this breathing-like action transfers the energy of jets ejected by a supermassive black hole into the surrounding medium, much like you might breathe warm air into cooler air on a cold winter morning. Stars form when clouds of interstellar gas cool and are allowed to condense. This means that this “breathing out” could slow down star formation and restrict the growth of galaxies.
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The team came to this conclusion after analysing simulations designed to mimic the effect that supersonic supermassive black hole-blowing jets might play in inhibiting galaxy growth. The simulations showed that the supermassive black hole heart could vibrate, creating high pressure in the jets, almost like a person suffering from high blood pressure, or “hypertension”.
When this happened, the team saw that the jets began to act like bellows, throwing out sound waves that traveled through the surrounding matter, which consisted of galactic gas and dust.
“We realised that there had to be some way for the jets to support the body – the ambient gas around the galaxy – and that’s what we discovered in our computer simulations,” team member Carl Richards, a PhD student at the University of Kent, said in a statement. “When we analysed the high-pressure computer simulations and allowed the heart to beat, the unexpected behaviour emerged.”
This sent a pulse of energy into the high-pressure jets, causing them to change shape as a result of the bellows-like action of the oscillating jet shock fronts. These jets expanded “like lungs full of air,” the researcher added. In doing so, they sent pressure waves into the galactic material around them, which stopped the galaxies in the simulations from growing.
There is other evidence of this phenomenon in real galaxies, away from the team’s simulations. For example, in the Perseus galaxy cluster, about 240 million light-years from Earth, astronomers saw evidence of large gas bubbles in this collection of thousands of galaxies embedded in a multimillion-degree cloud of gas. These are believed to be the result of sound waves passing through the galactic medium in this cluster.
The balance between black hole activity and the flow of gas into galaxies is extremely difficult to maintain, but supermassive black holes need a constant supply of gas and dust to create jets.
“Breathing too fast or too slow will not provide the life-giving vibrations needed to maintain the galactic environment and fuel the heart at the same time,” team member and University of Kent researcher Michael Smith said in the statement. “But this is not easy to do and we have constraints on the type of vibration, the size of the black hole and the quality of the lungs.”
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The team concluded that a galaxy’s lifetime could be extended with the help of its supermassive black hole “heart” and jet “lungs” blowing from the black hole’s core; these jets inhibit growth by limiting the amount of gas that collapses into stars at an early stage.
Without this mechanism, many galaxies in our 13.8 billion-year-old universe would have exhausted their fuel supply for star formation. As a result, they would have “gone extinct,” with most galaxies resembling so-called “red and dead” zombie galaxies at this point, full of old burnt-out stars.
The team’s research was published July 12 in the journal Monthly Notices of the Royal Astronomical Society.