Hot winds from supermassive black holes stunt star growth

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An artist’s rendition of galaxies Akira (right) and Tetsuo (left) in action. Akira’s gravity pulls Tetsuo’s gas into its central supermassive black hole. But warm winds, generated by the black hole, dampen star formation in Akira.

Why some gas-gobbling galaxies, churning out stars at a prodigious rate, suddenly quash stellar formation and become featureless deserts has been a cosmic conundrum.

Now scientists have identified a possible culprit – the supermassive black hole lying in their centre.

An international team led by Edmond Cheung from the University of Tokyo, Japan discovered “red geysers”, a common phenomenon where supermassive black holes blast their host galaxy with hot winds. These gusts dampen star formation.

“Galaxies start out as star-making machines with a simple recipe: gas plus gravity equals stars,” says Kevin Bundy, also from the University of Tokyo and co-author of the study.

And the immense gravitational pull of a supermassive black hole lurking in the centre of a galaxy should provide the force to trigger more star formation.

But often, it doesn’t – around 10% of galaxies with masses 20 billion times that of the Sun become dormant right when they should be pumping out stars.

So Cheung and colleagues used the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey to peer at the near-dormant but distant galaxy nicknamed Akira, after the famous Japanese manga comic character.

MaNGA allows scientists to see galaxies in 3-D, along with the movement of stars and gas within.

Akira pulled gas from its companion galaxy Tetsuo, named after another character in the same manga comic. (Aptly, in the comic, Akira absorbs Tetsuo.)

Tetsuo’s gas fuelled Akira’s supermassive black hole winds. Mechanical energy from the winds, which travelled as fast as 200 kilometres each second, was enough to heat the surrounding gas in Akira, preventing star formation.

Galaxies such as Akira are dubbed “red geysers” because of the lack of young, “blue” stars.

As stars are hot balls of gas, it seems paradoxical that heating gas stops stars forming.

But gas needs to first clump together to create a star, “a bit like the drops of rain condense from the water vapour”, says Oxford University’s Michele Cappellari, who was part of the research team. The hot wind stops that happening.

The work was published in Nature.

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