"Black Holes: Guardians of the Galaxy, Not Agents of Destruction"

An artist’s impression showing bi-polar jets of gas originating from a supermassive black hole at the center of a galaxy. Credit: ESA/Hubble, L. Calçada (ESO)

A study has revealed that galaxies possess a regulatory mechanism similar to a heart and lungs, which controls their growth by limiting gas absorption.

This mechanism, involving a supermassive black hole and its jet emissions, prevents galaxies from expanding too rapidly, ensuring their longevity and preventing premature aging into “zombie” galaxies.

Galaxies avoid an early death because they have a “heart and lungs” which effectively regulate their “breathing” and prevent them from growing out of control, a new study suggests.

If they didn’t, the Universe would have aged much faster than it has and all we would see today is huge “zombie” galaxies teeming with dead and dying stars.

That’s according to a new study published in the Monthly Notices of the Royal Astronomical Society, which investigates one of the great mysteries of the Universe – why galaxies are not as large as astronomers would expect.

Something appears to be stifling their enormous potential by limiting the amount of gas they absorb to convert into stars, meaning that instead of endlessly growing, something inside resists what was thought to be the inevitable pull of gravity.

This clip shows a supersonic jet generating a “bellows-like action,” by receiving pulses from its black hole “heart,” causing it to expand and contract “like an air-filled lung,” “breathing out warm air” (pressure ripples) into its surroundings. The graph axes are non-dimensional distance scales. Credit: C Richards/MD Smith/University of Kent

Now, astrophysicists at the University of Kent think they may have uncovered the secret. They suggest that galaxies could in fact control the rate at which they grow through how they “breathe.”

In their analogy, the researchers compared the supermassive black hole at the center of a galaxy to its heart and the two bi-polar supersonic jets of gas and radiation they emit to airways feeding a pair of lungs.

Pulses from the black hole — or “heart” — can lead to jet shock fronts oscillating back and forth along both jet axes, much like the thoracic diaphragm in the human body moves up and down inside a chest cavity to inflate and deflate both lungs.

This can result in jet energy being transmitted widely into the surrounding medium, just as we breathe out warm air, resulting in slowing galaxy gas-accretion and growth.

Two different examples of the simulation of one side of symmetric bi-polar jets, where pressure ripples spread out across the extra-galactic medium. Shown here are pressure variations using a red-temperature colour scale (dark=low pressure, light=high pressure). Each jet enters from the left with a pressure that rapidly falls as it pushes against the ambient medium. The axes are non-dimensional distance scales. Credit: C Richards/MD Smith/University of Kent

PhD student Carl Richards came up with the theory after creating new, never-before-tried simulations to investigate the role supersonic jets might play in inhibiting galaxy growth.

These involved allowing the black hole “heart” to pulse and the jets to be at high pressure – much like a form of hypertension, if extending the comparison to the human body.

This caused the jets to “act like bellows,” he said, by sending out sound waves “like ripples on a pond surface.”

The phenomena are similar to the terrestrial equivalent of sound and shock waves produced when opening a bottle of champagne, the screech of a car, rocket exhausts, and the puncture of pressurized enclosures.

The sound waves (ripples) in the hot gas that fills the Perseus cluster are shown in this artist’s impression. They are thought to have been generated by cavities blown out by jets from a supermassive black hole (bright white spot) at the center of the galaxy. Credit: NASA/NASA/CXC/M.Weiss

“We realized that there would have to be some means for the jets to support the body — the galaxy’s surrounding ambient gas — and that is what we discovered in our computer simulations,” Richards said.

“The unexpected behavior was revealed when we analyzed the computer simulations of high pressure and allowed the heart to pulse.

“This sent a stream of pulses 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 overpressured jets effectively expanded “like air-filled lungs,” the researchers said.

In doing so, they transmitted sound waves into the surrounding galaxy in the form of a series of pressure ripples, which were then shown to suppress the galaxy’s growth.