The Small and Large Magellanic Clouds — satellite galaxies of the Milky Way Galaxy — are surrounded by a diffuse halo of hot, supercharged gas named Magellanic Corona, according to an analysis of data from the Cosmic Origins Spectrograph aboard NASA’s Hubble Space Telescope and the archival data from the retired Far Ultraviolet Spectroscopic Explorer (FUSE) satellite.
Shown here in purple, the Magellanic Corona stretches more than 100,000 light-years from the main mass of stars, gas, and dust that make up the Magellanic Clouds, intermingling with the hotter and more extensive corona that surrounds the Milky Way. Image credit: Leah Hustak, STScI.
The Large Magellanic Cloud and the Small Magellanic Cloud are the closest massive satellite galaxies of the Milky Way.
They are probably on their first passage on an infalling orbit towards our Galaxy.
Recent measurements of a high mass for the Large Magellanic Cloud imply that this galaxy should host an ionized, warm-hot gaseous halo — a remnant of the primordial cloud of gas that collapsed 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 detail.
“Galaxies envelope themselves in gaseous cocoons, which act as defensive shields against other galaxies,” said Dr. Andrew Fox, an astronomer at the Space Telescope Science Institute.
“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,” added Dr. Elena D’Onghia, an astronomer at the University of Wisconsin-Madison.
“We knew that the Large Magellanic Cloud should be massive enough to have a corona.”
But although the Magellanic 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.
In search of direct evidence of this structure, the study authors combed through the Hubble and FUSE archives for ultraviolet observations of quasars located billions of light-years behind it.
They reasoned that although the corona would be too dim to see on its own, it should be visible as a sort of fog obscuring and absorbing 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 astronomers were able to detect and characterize the material surrounding the Large Magellanic Cloud and confirm that the corona exists.
As predicted, the quasar 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 for this study. The corona gas is so diffuse, it’s barely even there,” said Dr. Dhanesh Krishnarao, an astronomer at Colorado College.
“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, the team also discovered 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 really there. It really is cocooning the galaxy and protecting it,” Dr. Krishnarao said.
How can such a thin shroud 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,” Dr. Krishnarao said.
“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.”
A paper on the findings was published in the journal Nature.
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D. Krishnarao et al. 2022. Observations of a Magellanic Corona. Nature 609, 915-918; doi: 10.1038/s41586-022-05090-5
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