Free Astronomy Magazine November-December 2022

28 NOVEMBER-DECEMBER 2022 F or billions of years, the Milky Way’s largest satellite galaxies – the Large and Small Magel- lanic Clouds – have followed a per- ilous journey. Orbiting one another as they are pulled in toward our home galaxy, they have begun to unravel, leaving behind trails of gaseous debris. And yet – to the puzzlement of astronomers – these dwarf galaxies remain intact, with ongoing vigorous star formation. “A lot of people were struggling to explain how these streams of mate- rial could be there,” said Dhanesh Krishnarao (pictured above), assis- tant professor at Colorado College. “If this gas was removed from these galaxies, how are they still forming stars?” With the help of data from NASA’s Hubble Space Telescope and a re- tired satellite called the Far Ultravi- olet Spectroscopic Explorer (FUSE), by NASA/ESA Margaret W. Carruthers a team of astronomers led by Krish- narao has finally found the answer: the Magellanic system is sur- rounded by a corona, a protective shield of hot supercharged gas. This cocoons the two galaxies, prevent- ing their gas supplies from being si- phoned off by the Milky Way, and therefore allowing them to con- tinue forming new stars. This discovery, recently published in Nature , addresses a novel aspect of galaxy evolution. “Galaxies enve- lope themselves in gaseous co- coons, which act as defensive shields against other galaxies,” said co-investigator Andrew Fox of the Space Telescope Science Institute in Baltimore, Maryland. Astronomers predicted the corona’s existence several years ago. “We dis- covered that if we included a co- rona in the simulations of the Mag- ellanic Clouds falling onto the Milky The Magellanic Corona should exist Way, we could explain the mass of extracted gas for the first time,” ex- plained Elena D’Onghia, a co-inves- tigator at the University of Wis- consin–Madison. “We knew that the Large Magellanic Cloud should be massive enough to have a co- rona.” But although the corona stretches more than 100,000 light-years from the Magellanic clouds and covers a huge portion of the southern sky, it is effectively invisible. Mapping it required scouring through 30 years of archived data for suitable meas- urements. Researchers think that a galaxy’s corona is a remnant of the primordial cloud of gas that col- lapsed to form the galaxy billions of years ago. Although coronas have been seen around more distant dwarf galaxies, astronomers had never before been able to probe one in as much detail as this.