Free Astronomy Magazine November-December 2022

37 NOVEMBER-DECEMBER 2022 ASTRO PUBLISHING age, has nearly completed merging with another galaxy. During ob- servations with the HST and ALMA scientists discovered tidal tails con- taining roughly half of the entire system’s cold, star-forming gas. The discovery of the forcefully discarded material — equal to 10 billion times the mass of Earth’s Sun — indicated that the merger might be responsi- ble for snuffing out star formation, which scientists didn’t expect. “What initially made this massive galaxy interesting was that, for some reason, it suddenly stopped forming stars about 70 million years ago, immediately following a burst of star-forming activity. Most galax- ies are happy just to keep forming stars,” said Justin Spilker, an as- tronomer at Texas A&M University and the paper’s lead author. “Our observations with ALMA and Hub- ble proved that the real reason the galaxy stopped forming stars is that the merger process ejected about half the gas fuel for star formation into intergalactic space. With no fuel, the galaxy couldn’t keep form- ing stars.” The discovery is shedding light on the processes by which galaxies live or die and helping scientists to un- derstand their evolution better. “When we look out at the Universe, we see some galaxies actively form- ing new stars, like our own Milky Way, and some that aren’t. But those ‘dead’ galaxies have many old stars in them, so they must have formed all of those stars at some point and then stopped making new