Hubble discovers SHIELD protecting Milky Way's two largest satellite galaxies

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"A lot of people were struggling to explain how these streams of material could be there," Dhanesh Krishnarao, assistant professor at Colorado College, said in a paper published in the journal Nature. "If this gas was removed from these galaxies, how are they still forming stars?" His team eventually found out the reason with the help of Hubble and a retired satellite called the Far Ultraviolet Spectroscopic Explorer (FUSE). "Galaxies envelop themselves in gaseous cocoons, which act as defensive shields against other galaxies," explained co-investigator Andrew Fox of the Space Telescope Science Institute in Baltimore, Maryland.

It's worth noting that astronomers predicted the existence of the corona several years ago. "We discovered that if we included a corona in the simulations of the Magellanic Clouds falling onto the Milky Way, we could explain the mass of extracted gas for the first time," said 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," she added.

Despite the corona stretching more than 100,000 light-years from the Magellanic clouds and covering a major portion of the southern sky, it is basically invisible. Astronomers, who had to scour through 30 years of archived data for mapping suitable measurements, believe that a galaxy's corona is a remnant of the original cloud of gas that collapsed to form the galaxy billions of years ago.

Coronas have been documented around more distant dwarf galaxies, but it's the first time astronomers were able to probe one in such detail. "There're lots of predictions from computer simulations about what they should look like, how they should interact over billions of years, but observationally we can't really test most of them because dwarf galaxies are typically just too hard to detect," Krishnarao stated.

The Magellanic Clouds offer a unique opportunity to study the behavior of dwarf galaxies considering they are right at our doorstep. The team of researchers looked through Hubble and FUSE archives for ultraviolet observations of quasars in search of direct evidence of the Magellanic Corona. Quasars can be described as extremely bright cores of galaxies that harbor supermassive active black holes.

Researchers concluded that the corona would be too dim to see on its own, but would be visible in the form of a fog obscuring and absorbing distinct patters of bright light from quasars in the background. It's important to note that Hubble observations of quasars were previously used to map the corona around the Andromeda galaxy. The team went on to analyze patterns in ultraviolet light from 28 quasars and were able to detect the material surrounding the Large Magellanic Cloud and confirm that the corona actually exists. 

The data proved astronomers' predictions right, showing how the quasar spectra are imprinted with unmistakable signatures of carbon, oxygen, and silicon that constitute the hot plasma that surrounds the galaxy. That said, the corona's detection required extremely detailed ultraviolet spectra. "The resolution of Hubble and FUSE were crucial for this study," said Krishnarao, adding, "The corona gas is so diffused, it's barely even there."

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The team mapped the results and also found that the amount of gas decreased with distance from the center of the Large Magellanic Cloud. "It's a perfect telltale signature that this corona is really there," Krishnarao explained. "It really is cocooning the galaxy and protecting it." One may wonder how such a thin veil of gas is protecting a galaxy from destruction, but according to the investigator, "Anything that tries to pass into the galaxy has to pass through this material first, so it can absorb some of that impact. In addition, the corona is the first material that can be extracted. While giving up a little bit of the corona, you're protecting the gas that's inside the galaxy itself and able to form new stars."