Hubble detects a protective shield defending a pair of dwarf galaxies

For billions of years, the Milky Way’s largest satellite galaxies, the Large and Small Magellanic Clouds, have been on a perilous journey. Orbiting each other as they are drawn toward our home galaxy, they have begun to fall apart, leaving behind trails of gaseous debris. And yet, to the bewilderment of astronomers, these dwarf galaxies remain intact, with vigorous star formation underway.

“A lot of people were struggling to explain how these material flows could be there,” said Dhanesh Krishnarao, an assistant professor at Colorado College. “If this gas was removed from these galaxies, how come they are still forming stars?”

With the help of data from NASA’s Hubble Space Telescope and a retired satellite called the Far Ultraviolet Spectroscopic Explorer (FUSE), a team of astronomers led by Krishnarao finally found the answer: the Magellanic system is surrounded by a corona, a protective shield. supercharged hot gas. This engulfs the two galaxies, preventing the Milky Way from siphoning off their supplies of gas, and thus allowing them to continue forming new stars.

This discovery, which has just been published in Nature, deals with a novel aspect of the evolution of galaxies. “Galaxies wrap themselves in gaseous cocoons, 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 existence of the corona several years ago. “We found that if we included a corona in simulations of the Magellanic Clouds falling on the Milky Way, we could explain for the first time the mass of gas extracted,” explained Elena D’Onghia, a co-investigator at the University of Wisconsin–Madison. “We knew that the Large Magellanic Cloud should be massive enough to have a corona.”

But although the corona extends more than 100,000 light-years from the Magellanic clouds and covers much of the southern sky, it is practically invisible. Mapping it required combing through 30 years of archived data to get proper measurements.

Researchers believe that the corona of a galaxy is a remnant of the primordial cloud of gas that collapsed to form the galaxy billions of years ago. Although crowns have been seen around more distant dwarf galaxiesastronomers have never before been able to probe one in as much detail as this one.

“There are many predictions of computer simulations about how they should look like, how they should interact over billions of years, but in terms of observation we can’t really test most of them because dwarf galaxies are usually too hard to detect,” Krishnarao said. Because they are right in our threshold, the Magellanic Clouds provide an ideal opportunity to study how dwarf galaxies interact and evolve.

Searching for direct evidence of Magellan’s Corona, the team combed through the Hubble and FUSE archives for ultraviolet observations of quasars located billions of light-years behind it. Quasars are the extremely bright nuclei of galaxies that are home to huge, active black holes. The team reasoned that although the corona would be too faint to see on its own, it should be visible as a sort of fog that obscures and absorbs distinct patterns of bright light from quasars in the background. Hubble observations of quasars were used in the past to map the corona surrounding the Andromeda galaxy.

By analyzing patterns in ultraviolet light From 28 quasars, the team was able to detect and characterize the material surrounding the Large Magellanic Cloud and confirm that the corona exists. As predicted, the quasar’s spectra are imprinted with the distinct signatures of carbon, oxygen, and silicon that make up the halo of hot plasma that surrounds the galaxy.

The ability to detect the corona required extremely detailed ultraviolet spectra. “The resolution of Hubble and FUSE were crucial to this study,” explained Krishnarao. “The corona gas is so diffuse, it’s barely there.” Additionally, it mixes with other gases, including streams drawn from the Magellanic Clouds and material originating from the Milky Way.

By mapping the results, the team also found that the amount of gas decreases with distance from the center of the Large Magellanic Cloud. “It’s a perfect telltale signature that this crown is really there,” Krishnarao said. “It’s really engulfing the galaxy and protecting it.”

How can such a thin shell of gas protect a galaxy from destruction?

“Anything that tries to pass into the galaxy has to pass through this material first, so it can absorb some of that impact,” Krishnarao explained. “Also, the crown is the first material that can be mined. By giving up a bit of the crownyou are protecting the gas that is within the galaxy itself and capable of forming new stars.”