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Now We Know How Galaxies Die

The galactic strangler remains at large.
Image: re-active, STScI and ESO

You may not be shocked to know that the universe is scattered with dead galaxies, but you may be shocked to know that Cambridge researchers have discovered evidence that those galaxies were strangled to death.

Just like the massive Ford sedans they share their names with, actual stellar galaxies run on gas. When there's a nice steady flow of cold gases like hydrogen, stars form, but when there isn't enough gas, star production stops and galaxies die.

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But astronomers have been wondering why the gas is missing in dead galaxies. Is it getting sucked out, or is the supply being cut off, effectively "strangling" the galaxy? In an article just published in the journal Nature, researchers explained that, even though it happens on a time scale that cannot be observed, they found evidence that galaxies die when the gas gets cut off.

Credit: Roberto Maiolino and Yingjie Peng

The researchers looked at the metallicity—the concentration of elements that are heavier than helium—of the stellar atmosphere in the galaxy. When cold gas is flowing and stars are forming, the galaxy's stellar mass grows, as does the metallicity of the stellar atmosphere.

If the gas was being sucked out, dead or "quiescent" galaxies should have the same metallicity and mass as lively gas-rich galaxies. If galaxies, instead, have their gas cut off and are "strangled" then the metallicity and mass of the quiescent galaxy that it becomes should be much higher, since there isn't more gas flowing in and diluting it, and also the mass should be higher.

"Galaxies living in environments more crowded with other galaxies tend to be suffocated more efficiently."

"Metals are produced by the formation of new stars, a fraction of which die by exploding as supernovae and release metals into the interstellar medium," the study's lead author Yingjie Peng told me in an email. "If a galaxy is 'strangulated' (i.e. the inflow of fresh gas is halted) the amount of gas still available in the galaxy allows star formation, hence metal production, to continue for a while, about 4 billion years, hence the final product—a quiescent old galaxy—is very much metal enriched."

The Sloan Digital Sky Survey's data collected on 26,000 nearby galaxies point overwhelmingly towards strangulation. The vast majority of dead galaxies just aren't metal poor, Peng said. But there's still a lot more detective work to do.

Peng told me that we still don't know what actually causes galactic strangulation, though the paper offers a few possible explanations. "One of these seems to be 'galaxy environment': galaxies living in environments more crowded with other galaxies tend to be suffocated more efficiently."

All physics, like politics being local, I had to ask how our own galaxy the Milky Way was doing. On this, Peng was very reassuring.

"The Milky Way is still in the process of accreting gas from the intergalactic medium, and does not seem to be at threat of being strangulated," he said, "at least for the moment being."