Free Astronomy Magazine May-June 2021

24 MAY-JUNE 2021 ASTRO PUBLISHING ID2299. Because the galaxy is also forming stars very rapidly, hundreds of times faster than our Milky Way, the remaining gas will be quickly consumed, shutting down ID2299 in just a few tens of million years. The event responsible for the spec- tacular gas loss, the team believes, is a collision between two galaxies, which eventually merged to form ID2299. The elusive clue that pointed the sci- entists towards this scenario was the association of the ejected gas with a “tidal tail”. Tidal tails are elongated streams of stars and gas extending into interstellar space that result when two galaxies merge, and they are usually too faint to see in distant galaxies. However, the team man- aged to observe the relatively bright feature just as it was launching into space, and were able to identify it as a tidal tail. Most astronomers be- lieve that winds caused by star formation and the activity of black holes at the centres of massive gal- axies are responsible for launching star-forming material into space, A distant galaxy dies as astronomers watch G alaxies begin to “die” when they stop forming stars, but until now astronomers had never clearly glimpsed the start of this process in a far-away galaxy. Using the Atacama Large Millime- ter/submillimeter Array (ALMA), in which the European Southern Observatory (ESO) is a partner, as- tronomers have seen a galaxy eject- ing nearly half of its star-forming gas. “This is the first time we have observed a typical massive star-form- ing galaxy in the distant Universe about to ‘die’ because of a massive cold gas ejection,” says Annagrazia Puglisi, lead researcher on the new study, from the Durham University, UK, and the Saclay Nuclear Research Centre (CEA-Saclay), France. The galaxy, ID2299, is distant enough that its light takes some 9 billion years to reach us; we see it when the Universe was just 4.5 bil- lion years old. The gas ejection is happening at a rate equivalent to 10,000 Suns per year, and is removing an astonishing 46% of the total cold gas from by ESO - Bárbara Ferreira