Even today, our Universe makes stars every day. But when he was only three billion years old – the equivalent of 20% of his current age – he experienced the most prolific period in the field in all of its history. Yet, by turning the Hubble Space Telescope (Nasa) and Alma, the Atacama Large Millimeter / Sub-Millimeter Antenna Array, towards objects dating from this time, researchers have just made an astonishing discovery. Six massive galaxies that seemingly lacked the hydrogen needed to make stars.
“We thought that at that time all galaxies formed a lot of stars. And that the most massive ancient galaxies in the Universe had been made to slow down the rate at which they formed stars just billions of years after the Big Bang. But our work shows that in reality, these galaxies have not really slowed down. They were literally running on empty, ”said Kate Whitaker, an astronomer at the University of Massachusetts (United States), in a press release.
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It was a combination of observational techniques that enabled researchers to make the discovery. With the Hubble Space Telescope, they were able to locate stars in galaxies and thus ancient star formation sites. Together with Alma, they were able to detect traces of cold hydrogen that indicate where stars might still form in the future. Finally, thanks to a gravitational lensing effect produced by massive galaxy clusters in the foreground and offering amplified light in all wavelengths and unparalleled spatial resolution, they were able to study the details of these six mysterious galaxies.
“Galaxies that don’t make a lot of stars go out pretty quickly and they become difficult for us to observe. The gravitational lens effect allows us to study them anyway, ”says Justin Spilker, researcher at the University of Texas (United States). And to understand that these six galaxies have not stopped forming stars following a sudden inefficiency in the conversion of hydrogen. But rather because of a depletion of the cold gas tanks.