The Galaxy.

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Astronomers were able to observe for the first time a distant galaxy in the process of "dying" after losing about half of the gas used to make stars, according to a study published on Monday.

Data collected by the Alma telescope at the European Southern Observatory (ESO) in Chile suggests that this phenomenon, usually attributed to the effect of a black hole, is the result here of the collision of this galaxy with a other.

“ID2299” is so distant that its light has taken 9 billion years to reach us.

It is observed when the Universe, 13 billion years old, was only 4.5.

This elliptical-shaped galaxy "is going through a rather extreme phenomenon, never observed at such a distance," said an astrophysicist at the Saclay Nuclear Research Center (which depends on the CEA), co-author of the study published in

Nature Astronomy

.

Sterile in a few million years

ID2299 "is expelling more than half of its gas, its fuel for star formation", at a phenomenal rate, equivalent to the mass of 10,000 suns per year, he explained.

And this while continuing to consume this same gas to produce stars at a very high rate, with a mass equivalent to about 550 times our sun.

By comparison, our galaxy, the Milky Way, produces the equivalent of three per year.

Under these conditions, the galaxy should become sterile in a few tens of millions of years, in no time on a cosmic scale.

The study recalls that so far, such a gas "leak" has been explained by the effect of winds caused by star formation or the activity of a supermassive black hole located in the galactic nucleus.

But "we have been able to show that another mechanism is at work, with a collision of galaxies (...) which has already taken place", according to the astrophysicist.

"Leak" of gas resulting from a fusion of galaxies

For a co-author of the study, quoted in a press release from ESO, the observation made with Alma "sheds new light on the mechanisms stopping the formation of stars in distant galaxies".

In astronomy, the observation of distant objects is like studying ancient times, when it is assumed that galaxies were formed.

The discovery was made by chance, during Alma's observation of a hundred distant galaxies to study the properties of the cold gas clouds they contain.

Using data collected in a matter of minutes, scientists concluded that the gas "leak" was the result of an ID2299 merger with a colleague.

This rate of ejection, which has already emptied the galaxy of 46% of its mass of molecular gas, is too fast, "even if it is measured in tens of millions of years", for enough gas to then fall back and that ID2299 begins to form stars again.

But after ?

"You never know what can happen a billion years later", explained the latter.

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