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| Using spectroscopic images astronomers have mapped the Magnellic Corona that may prevent dwarf galaxies of the Milky Way from having their star-forming gas ripped away. (Image credit: NASA, ESA, Leah Hustak (STScI)) |
An odd shield of supercharged gas protects dwarf galaxies from being ripped
apart by the gravitational pull of the Milky Way, 30 years' worth of
observations by the Hubble Space Telescope reveal.
For years, astronomers have struggled to explain why the Large and Small
Magellanic Clouds, two tiny galaxies orbiting the Milky Way, still form
stars. The two dwarf galaxies have been orbiting our galactic home for
billions of years, research suggests, during which the gravitational force
of the much more massive Milky Way has been stripping gas from the two
smaller galaxies, creating a trail in their wake. In theory, this gas loss
should quench star birth, but the small satellite galaxies have still
managed to maintain vigorous star formation.
"A lot of people were struggling to explain how these streams of material
could be there," assistant professor at Colorado College, Dhanesh
Krishnarao, said in a statement. "If this gas was removed from these
galaxies, how are they still forming stars?"
This has led astronomers to theorize that the Large and Small Magellanic
Clouds may be protected by a cosmic "shield" — called the Magellanic Corona
— made up of supercharged gas with temperatures of half a million degrees.
Thus far, observational evidence of the Magellanic Corona has been elusive,
but new research based on 30 years' worth of observations by the Hubble
Space Telescope and the now retired Far Ultraviolet Spectroscopic Explorer
(FUSE) satellite may have changed that situation.
When analyzing the data, Krishnarao and his colleagues found that the Large
and Small Magellanic Clouds — collectively known as the Magellanic system —
are indeed surrounded by a shield of hot supercharged gas which cocoons the
dwarf galaxies preventing their star-forming gas from being siphoned away by
the Milky Way. This, in turn, helps them continue forming stars.
"Galaxies enveloped themselves in gaseous cocoons, which act as defensive
shields against other galaxies," Andrew Fox, an astronomer at the Space
Telescope Science Institute in Maryland, who was part of the team, said in
the statement.
This cosmic shield has been difficult to detect because despite stretching
from the Magellanic system for 100,000 light-years, the Magellanic Corona is
almost invisible. If it weren't it would cover a huge portion of the
southern sky.
The team thinks that galactic coronas like the freshly discovered Magellanic
Corona are the remains of primordial clouds of gas that undergo
gravitational collapse and form galaxies. These coronas have been spotted
around other galaxies, but have never been seen around one so close to the
Milky Way and thus in such detail.
"There're lots of predictions from computer simulations about what they
should look like, how they should interact over billions of years," said
Krishnarao. "Observationally we can't really test most of them because dwarf
galaxies are typically just too hard to detect."
Searching through the Hubble/Fuse data, the astronomers were specifically
looking for ultraviolet observations of quasars located billions of
light-years behind the Magellanic Corona.
At the heart of many galaxies are active galactic nuclei (AGNs), some of
which are quasars powered by supermassive black holes feeding on gas. As
they swallow this material the black holes emit so much energy that they
shine brighter than all the stars in their respective galaxy combined.
The team therefore reasoned that the invisible corona shielding the galaxy
in the foreground (from the observer's point of view) should make itself
apparent by the effect it has on the quasar's light shining through it,
appearing as a distorting fog obscuring and absorbing distinct patterns of
bright light from the background quasars.
The astronomers looked at patterns in the ultraviolet light from 28
different quasars which allowed them to characterize the material
surrounding the Large Magellanic Cloud.
The light spectrum from the quasars was found to contain the elemental
'fingerprints' of carbon, oxygen, and silicon in a halo of hot plasma
surrounding the Large Magellanic Cloud indicating the presence of the
Magellanic Corona.
"It's a perfect telltale signature that this corona is really there. It
really is cocooning the galaxy and protecting it," Krishnarao said.
The astronomer also explained how a thin cloud of gas could shield a
galaxy.
"Anything that tries to pass into the galaxy has to pass through this
material first, so it can absorb some of that impact," he 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 the galaxy itself and able to form new stars."
The team's research is published in the Sept 28 edition of the journal
Nature
Reference:
Krishnarao, D., Fox, A.J., D’Onghia, E. et al. Observations of a Magellanic
Corona. Nature 609, 915–918 (2022).
DOI: 10.1038/s41586-022-05090-5
Tags:
Space & Astrophysics