Using data from Hubble and FUSE, astronomers detected a protective shield surrounding the Small and Large Magellanic clouds.
An enormous shield of supercharged hot gas surrounds the Magellanic cloud system, a pair of satellite galaxies to the Milky Way. As a result, the Milky Way is prevented from siphoning off the gas from the two galaxies, meaning they can continue forming new stars. Their orbits are being pulled inward as they spiral in toward our home galaxy, leaving behind gaseous debris trails behind. This has puzzled astronomers for years; how do these galaxies remain intact with ongoing star formation, even though they are pulled towards the Milky Way?
According to Dhanesh Krishnarao, assistant professor at Colorado College, many people had trouble explaining the existence of these streams of material. “If this gas was removed from these galaxies, how are they still forming stars?”
Astronomers led by Krishnarao discovered that the Magellanic system is surrounded by a corona, a protective shield made from hot supercharged gas, using data from NASA’s Hubble Space Telescope and a retired satellite called the Far Ultraviolet Spectroscopic Explorer (FUSE). Thus, the Milky Way cannot siphon off gas from the two galaxies, which allows them to continue forming stars.
A novel aspect of galaxy evolution has been discovered in this new study published in Nature. In Baltimore, Maryland, Space Telescope Science Institute co-investigator Andrew Fox said that galaxy cocoons act as defensive shields against other galaxies.
The corona has been predicted by astronomers for several years. Elena D’Onghia, a co-investigator at the University of Wisconsin–Madison, explained that by including a corona in the simulations of Magellanic Clouds falling onto the Milky Way, they could explain the amount of gas extracted for the first time. “We knew that the Large Magellanic Cloud should be massive enough to have a corona,” said D’Onghia. In spite of covering a huge portion of the southern sky with more than 100,000 light-years, the corona is virtually invisible. Mapping it out required searching through archives for 30 years.
The corona of a galaxy is believed to be a remnant of the primordial gas cloud that collapsed billions of years ago to form the galaxy. Although coronas have been observed around more distant dwarf galaxies, astronomers hadn’t had a chance to probe one so closely before.
As Krishnarao pointed out, there are a lot of predictions about how dwarf galaxies will look and interact over billions of years. However, observationally most of them can’t really be tested because dwarf galaxies are usually too difficult to detect. Our proximity to the Magellanic Clouds makes them a great place to study dwarf galaxies and how they interact.
Using ultraviolet observations of quasars billions of light-years away, the team investigated Hubble and FUSE archives to uncover direct evidence for the Magellanic Corona. Usually, quasars are extremely bright cores of galaxies with active black holes at their centers. While the corona would be too dim for the team to see alone, it should be visible as a fog that obscures and absorbs quasar light. The corona surrounding the Andromeda galaxy has been mapped in the past based on Hubble observations of quasars.
Scientists discovered and characterized the corona of the Large Magellanic Cloud by analyzing ultraviolet light patterns from 28 quasars. The quasar spectrum is imprinted with the distinct signatures of carbon, oxygen, and silicon associated with the hot plasma surrounding the galaxy.
For the corona to be detected, extremely detailed ultraviolet spectra were needed. Krishnarao explained that Hubble’s and FUSE’s resolutions 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.
As the team mapped the results, they also discovered that the amount of gas decreases as one moves away from the center of the Large Magellanic Cloud. According to Krishnarao, it is the perfect telltale sign that this corona is actually there. “It really is cocooning the galaxy and protecting it.”
However, considering how thin the gas shroud is, how is it possible to protect a galaxy from destruction? As Krishnarao explained, it is necessary to pass through this material before something can pass into the galaxy. This allows it to absorb some of the impact. Coronas are the first materials that can be extracted. As you give up some of the corona, you’re protecting the gas inside the galaxy itself, which is capable of forming new stars, Krishnarao concluded.
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