Hubble discovers a “shield” in the Milky Way’s largest satellite galaxies

For billions of years, the Milky Way’s largest satellite galaxies – the Small and Large Magellanic Clouds – have embarked on a perilous journey. Orbiting each other as they are pulled toward our galaxy, they have begun to break up, leaving trails of gaseous debris.

View of the Milky Way with its two largest satellite galaxies: the Large and Small Magellanic Clouds. Image: Peter Gudella –

However, these dwarf galaxies remain intact and vigorous star formation is still ongoing, leaving astronomers intrigued.

“A lot of people had a hard time explaining how these streams of matter could be there,” said Dhanesh Krishnarao, assistant scientist at NASA’s Goddard Space Flight Center. “If the gas was stripped from these galaxies, how are they still forming stars?”

Using data from the Hubble Space Telescope and a retired satellite called the Far Ultraviolet Spectroscopic Explorer (FUSE), an international collaborative project between NASA, the Canadian Space Agency and the French Space Agency that worked between 1999 and 2007), a team of astronomers led by Krishnarao has finally found the answer: the Magellanic system is surrounded by a corona, a protective shield of supercharged hot gases that “protects” the two galaxies, preventing their matter from being deflected by the Milky Way, thus allowing them to continue forming new stars .

This discovery, published on Wednesday (28) in the scientific journal nature, tackles a new aspect of galaxy evolution. “Galaxies wrap themselves in gaseous cocoons, which act as defensive shields against the impact of other galaxies,” said co-author Andrew Fox of NASA’s Space Telescope Science Institute (STScl).

According to a statement from the agency, astronomers predicted the existence of the corona several years ago. “We found that if we included a corona in the simulations of the Magellanic Clouds falling over the Milky Way, we could explain the mass of extracted gas,” explained Elena D’Onghia, a researcher at the University of Wisconsin-Madison and also co. – author of the article. “We knew that the Large Magellanic Cloud had to be massive enough to have a corona. »

Although the corona extends more than 100,000 light-years from the Magellanic system and covers much of the sky, it is invisible. “Mapping it required combing through 30 years of archived data for correct measurements,” the NASA statement said.

“There are many computer models that predict how dwarf galaxies will interact over billions of years, but we can’t test most of them because dwarf galaxies are usually very hard to detect,” Krishnarao said. “Since the Magellanic Clouds are on our doorstep, however, they provide an ideal opportunity to study the interaction and evolution of dwarf galaxies. »

Looking for direct evidence of Crown Magellan, the team searched the Hubble and FUSE archives for ultraviolet observations of quasars located billions of light years away from the system. Quasars are the extremely bright cores of galaxies that host active supermassive black holes.

The team explained that although the corona is too faint to observe, it can be seen as a kind of nebula that hides and absorbs distinct patterns of bright light from background quasars.

By analyzing the ultraviolet light patterns from 28 quasars, the team was able to detect and characterize the material surrounding the Large Magellanic Cloud and confirm the existence of the corona. As expected, the quasar spectra contain signatures of carbon, oxygen and silicon that make up the hot plasma halo surrounding the galaxy.

The researchers also discovered that the amount of gas decreases as one moves away from the center of the Large Magellanic Cloud. “It’s a perfect indication that this corona is really there,” Krishnarao said. “It really is a cocoon of the galaxy to protect her. »

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