The universe, about a billion years old, was populated by many giant galaxies rich in gas, producing stars in abundance… According to the researchers’ terminology, the cold gas that condensed to form new suns has now worked its way into many of these giant galaxies. Haven’t… they have become “inactive”. Why ? Mystery. until a study published in Nature, and led by the University of Massachusetts Amherst (United States), finally provides an element of the answer: They may have simply run out of gas.

Galaxies that are very difficult to observe

This is the surprising conclusion of a study conducted on these six dormant galaxies, which are very difficult to observe by nature. First, because they are so far away from us: 10 to 12 billion light years. And especially because they shine less because they no longer form stars. So in order to obtain information about their composition, astronomers had to be clever. They chose galaxies whose image is distorted by the phenomenon of “gravitational lenses”. If very massive bodies, for example galaxies, are between a celestial body and us, then the object in question will seem very distorted to us. In question: the divergence of light rays by the gravitational field of galaxies. This distortion makes objects appear larger and brighter by focusing their light. The image provided by these true astronomical telescopes that are gravitational lenses was then observed jointly by the Hubble Space Telescope and ALMA, an array of antennas installed in Chile that observe the universe in a field of millimeter “radio” waves. The cold gas that astronomers are looking for is emitted precisely in this region of the electromagnetic spectrum.

an incomprehensible running out of fuel

Equipped in this way, the researchers were able to see what was happening in these dormant galaxies… their aim was to find out the classic mechanism of converting cold gas into a star, itself after the collapse of a gas cloud. Through, why was it out? count. Answer: It is not the mechanism that is in question, it is the gas that is lacking. In the six galaxies studied, the fraction of cold molecular gas represents between less than 1% and less than 5% of the total mass of stars in the galaxy, and is essentially concentrated in the center of these. This fuel exhaustion is a real mystery because the universe, young, is filled with this gas formed just after the Big Bang. It’s like a car running out of gas in the middle of an oil refinery. What stops galaxies from replenishing after the first batch of stars have formed? Are these newborn stars chasing gas out of the galaxy? Or should we be watching the action of a supermassive black hole at the center of the galaxy? In fact, the black hole’s surroundings emit a large amount of energy due to the acceleration of matter, before it ends up at the bottom of the hole. This energy can heat the ambient gas. However, it takes cold gas to form stars … To support these various hypotheses, investigations are ongoing. Next step: Study the residual gas at the center of galaxies, and try to understand why it is concentrated there.