The Large and Small Magellanic Clouds are the two largest satellite galaxies of the Milky Way. As they are drawn in toward our galaxy, they have started to unravel, leaving behind trails of gaseous debris as they orbit one another. These dwarf galaxies are still intact and undergoing active star formation, which baffles astronomers.
If this gas was removed from these galaxies, how are they still forming stars?
Using data from NASA‘s Hubble Space Telescope and a retired satellite called the Far Ultraviolet Spectroscopic Explorer (FUSE), astronomers finally found the answer: a protective shield of hot gas called corona is defending the Magellanic system. This envelops the two galaxies and stops the Milky Way from stealing their gas resources, allowing them to continue forming new stars.
Co-investigator Andrew Fox of the Space Telescope Science Institute in Baltimore, Maryland, said, “This discovery, which was just published in Nature, addresses a novel aspect of galaxy evolution. Galaxies envelope themselves in gaseous cocoons, which act as defensive shields against other galaxies.”
Elena D’Onghia, a co-investigator at the University of Wisconsin–Madison, said, “Astronomers predicted the corona’s existence 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. We knew the Large Magellanic Cloud should be massive enough to have a corona.”
However, while covering a significant southern sky area and extending more than 100,000 light-years from the Magellanic clouds, the corona is essentially undetectable. It required searching through 30 years of old data for appropriate metrics to map it out.
According to scientists, a galaxy’s corona is a piece of the original gas cloud that collapsed billions of years ago to create the galaxy. Despite the fact that coronas have been observed surrounding more distant dwarf galaxies, scientists have never been able to study one in such great detail.
Krishnarao said, “There’re many predictions from computer simulations about what they should look like, how they should interact over billions of years, but observationally we can’t test most of them because dwarf galaxies are typically just too hard to detect. Because they are right on our doorstep, the Magellanic Clouds provide an ideal opportunity to study how dwarf galaxies interact and evolve.”
The team could identify and define the material encircling the Large Magellanic Cloud and establish that the corona exists by examining patterns in ultraviolet light from 28 quasars. As expected, the distinctive traces of silicon, oxygen, and carbon that make up the halo of hot plasma surrounding the galaxy are etched on the quasar spectra.
Krishnarao said, “The ability to detect the corona required extremely detailed ultraviolet spectra. The resolution of Hubble and FUSE were crucial for this study. The corona gas is so diffuse it’s barely even there. In addition, it is mixed with other gases, including the streams pulled from the Magellanic Clouds and material originating in the Milky Way.”
“By mapping the results, we 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 corona is there. It is cocooning the galaxy and protecting it.”
“Anything that tries to pass into the galaxy has to pass through this material first so that 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 of the galaxy itself and able to form new stars.”