(ORDO NEWS) — For billions of years, the Milky Way’s largest satellite galaxies, the Large and Small Magellanic Clouds, have performed dangerous tricks.
Rotating around each other as they were pulled towards our galaxy, they began to unravel, leaving trails of gaseous debris in their wake.
And yet, to the surprise of astronomers, these dwarf galaxies remain intact and star formation continues.
“Many people have struggled to explain how the material flows could have ended up there,” said Dhanesh Krishnarao, an assistant professor at Colorado College. “If gas has been removed from these galaxies, how are they still forming stars?”
Using data from NASA‘s Hubble Space Telescope and the former Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, a team of astronomers led by Krishnarao finally found the answer: The Magellanic system is surrounded by a corona, a protective shield of hot gas.
It cocoons the two galaxies, preventing their gas reserves from leaking into the Milky Way and therefore allowing them to continue forming new stars.
This discovery, published in the journal Nature, concerns a new aspect of galaxy evolution. “Galaxies wrap themselves in gaseous cocoons that act as protective shields from other galaxies,” said Andrew Fox of the Space Telescope Science Institute in Baltimore, Maryland.
Astronomers predicted the existence of the corona years ago. “We found that if we include the corona in simulations of the Magellanic Clouds falling into the Milky Way, we can for the first time explain the mass of the extracted gas,” explained Elena D’Ongia, co-investigator at the University of Wisconsin-Madison.
“We knew that the Large Magellanic Cloud would have to be massive enough to ‘wear’ the crown.”
Although the corona extends over 100,000 light-years from the Magellanic Clouds and covers a vast portion of the southern sky, it is virtually invisible. To map it, it was necessary to look through archived data for 30 years in search of suitable measurements.
Researchers believe that the corona of the galaxy is the remnant of a primordial gas cloud that collapsed billions of years ago to form a galaxy.
Although coronas have been seen around more distant dwarf galaxies, astronomers have never been able to study them in as much detail as they do now.
“With computer simulations, you can make a lot of predictions about what they should look like, how they should interact over billions of years, but in terms of observations, we can’t test most of them because dwarf galaxies tend to be too difficult to detect. Krishnarao said.
“Because they are right on the doorstep, the Magellanic Clouds provide an ideal opportunity to study how dwarf galaxies interact and evolve.
Looking for direct evidence of the existence of the Magellanic Corona, the team combed the Hubble and FUSE archives for ultraviolet observations of quasars billions of light-years behind them.
Quasars are extremely bright galactic nuclei that contain massive active black holes. The team concluded that while the corona would be too faint to be seen, it should be visible as a kind of haze, obscuring and absorbing distinct patterns of bright light from quasars in the background.
Hubble’s observations of quasars have been 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 material surrounding the Large Magellanic Cloud and confirm the existence of a corona.
As predicted, the spectra of quasars contain distinct signatures of carbon, oxygen and silicon, which make up the halo of hot plasma that surrounds the galaxy.
Extremely detailed ultraviolet spectra were required to detect the corona. “The resolution of Hubble and FUSE was critical to this study,” Krishnarao explained.
“The corona gas is so dispersed that it’s almost non-existent.” In addition, it mixes with other gases, including the Magellanic Cloud flows and material originating from the Milky Way.
Comparing the results, the team also found that the amount of gas decreases with distance from the center of the Large Magellanic Cloud.
“This is the perfect tell-tale sign that the crown really exists,” Krishnarao said. “It’s really the cocoon of the galaxy and its protection.”
How can such a thin veil of gas protect the galaxy from destruction?
“Anything that tries to enter the galaxy must first pass through this material so that it can absorb some of the impact,” Krishnarao explained.
“In addition, the crown is the first material that can be extracted. By giving away some of the crown, you are protecting the gas that is inside the galaxy itself and is capable of forming new stars.”
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