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The mystery about how smaller galaxies emerge unscathed from their perilous journey in space has been solved by this Indian American assistant professor who, along with his team, has revealed that a protective shield made of supercharged gases keeps these dwarf galaxies from harm.
With the help of data from NASA’s Hubble Space Telescope and a retired satellite called the Far Ultraviolet Spectroscopic Explorer (FUSE), a team of nine astronomers led by Dr. Dhanesh Krishnarao, assistant professor of physics at Colorado College, has revealed that the Magellanic system is surrounded by a corona, a protective shield of hot supercharged gas. This cocoons the two galaxies, preventing their gas supplies from being siphoned off by the Milky Way, and therefore allowing them to continue forming new stars.
NASA explains – for billions of years, the Milky Way’s largest satellite galaxies – the Large and Small Magellanic Clouds – have followed a perilous journey. Orbiting one another as they are pulled in toward our home galaxy, they have begun to unravel, leaving behind trails of gaseous debris. And yet – to the puzzlement of astronomers – these dwarf galaxies remain intact, with ongoing vigorous star formation.
These dwarf satellite galaxies are protected by a pervasive shield that prevents the Milky Way from removing its essential star-forming gas. This Magellanic Corona, made of supercharged gas with temperatures of half a million degrees, acts as a cosmic crash zone keeping the stars and disk relatively unscathed during collisions.
Using a combination of the unique ultraviolet vision of the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer, along with the probing power of distant quasars, scientists have finally been able to detect and begin to map the Magellanic Corona. The discovery of this diffuse halo of hot gas, extending more than 100,000 light-years from the Large Magellanic Cloud and covering much of the southern sky, confirms the prediction and illuminates our understanding of how small galaxies can interact with larger galaxies without losing the fuel needed for future star formation.
“A lot of people were struggling to explain how these streams of material could be there,” said Krishnarao. “If this gas was removed from these galaxies, how are they still forming stars?”
“Recent simulations led by coauthors of this work had predicted the presence of the Magellanic Corona. They show how the Corona encompasses the galaxies and gives up some of its material instead of having to lose all the dense gas forming stars within the galaxies,” Krishnarao has explained while replying to a query on Twitter.
This discovery, which was published in Nature on September 28, addresses a novel aspect of galaxy evolution. Nature is a weekly international journal publishing the finest peer-reviewed research in all fields of science and technology.
“Wondering how the Magellanic Clouds stay intact while falling into the mighty Milky Way? These nearby little galaxies are cocooned within a massive, protective hot halo of gas around 300,000 K, extending at least 100,000 light years — The Magellanic Corona! Out today in Nature!,” said Krishnarao on Twitter.
“Galaxies envelop themselves in gaseous cocoons, which act as defensive shields against other galaxies,” said co-investigator Andrew Fox of the Space Telescope Science Institute in Baltimore, Maryland.
Astronomers had predicted the corona’s existence several years ago. But although the 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. Mapping it out required scouring through 30 years of archived data for suitable measurements.
Researchers think that a galaxy’s corona is 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 as much detail as this.
“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,” explained 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.”
Krishnarao added: “There are 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. Because they are right on our doorstep, the Magellanic Clouds provide an ideal opportunity to study how dwarf galaxies interact and evolve.”
In search of direct evidence of the Magellanic Corona, the team combed through the Hubble and FUSE archives for ultraviolet observations of quasars located billions of light-years behind it. Quasars are the extremely bright cores of galaxies harboring massive active black holes.
The team 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 team was 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 was crucial for this study,” explained Krishnarao. “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, 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,” said Krishnarao. “It really is cocooning the galaxy and protecting it.”
If you are wondering how can such a thin shroud of gas protect a galaxy from destruction, then here’s what Krishnarao has to say: “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 of the galaxy itself and able to form new stars.”
“This paper opens up an exciting new research path for me, moving beyond my initial interests of studying gas and dust within our own galaxy, and expanding to hotter gas that encompasses some of our closest companion galaxies and protects them as they fall in towards our mighty Milky Way,” Krishnarao said.
Krishnarao was recently awarded a $66,902 grant from the Space Telescope Science Institute, which is a research arm of the National Aeronautics and Space Administration, or NASA. He will be working with four other institutions on the three-year program, entitled “The LMC’s Galactic Wind through the Eyes of ULLYSES.”
“Soon, with the support of my recent grant from NASA/STScI, I’ll be working with students at CC and collaborators across the country to better study how our newly discovered Magellanic Corona relates to and interacts with gas being ejected out from the Large Magellanic Cloud in the form of high-speed winds blown out from some of the most massive, actively forming stars we know of,” Krishnarao added.
The Large Magellanic Cloud is a galaxy currently in an orbit falling into the Milky Way. The LMC is the closest major companion galaxy and provides the ideal venue to study the physics of how galaxies evolve over billions of years. The team from the five institutions will use observations of light from stars in the LMC from the Hubble Space Telescope to study the presence of gas between the LMC and the Milky Way, tracing the flow of gas in and out of the LMC as it forms new stars and triggers many exploding stars or supernovae.
With the grant, Krishnarao and his research students will map the properties of the LMC’s explosive wind of gas, allowing them to better understand how individual stars form and how their formation and lifetime can impact the large environment surrounding galaxies.
“For billions of years, the Magellanic Clouds have been traversing a perilous journey. The Large and Small Magellanic Clouds have been colliding with one another while also interacting with our Milky Way, leaving behind trails of gaseous debris. All this time, the Magellanic Clouds have also been actively creating new stars and trying to throw out additional debris in the form of massive ‘winds’ escaping from their hold,” says Krishnarao. “Using hundreds of observations from the Hubble Space Telescope, we will search for signatures of this powerful wind to better understand how galaxies evolve, form stars, and potentially fuel the formation of life in the universe.”
[Pic Caption: The maps are centered on the LMC and color-coded by column density (a) and velocity (b). Magellanic 21-cm H I emission is shown in a blue scale integrated into velocities to encompass the Magellanic system, with colored symbols showing HST/COS sightlines color-coded by C IV column densities. Upper limits are shown using open symbols. The grey background shows Galactic 21-cm H I emission from HI4PI11 integrated over −75 < vLSR < +75 km s−1. Panel b shows the mean velocity of H I with our sightlines color-coded by mean C IV absorption velocity. Dotted circles mark the LMC impact parameter and the South Galactic Pole (SGP) is marked with a white star.]