Massive halo finally explains stream of gas swirling around the Milky
Way

Date:
September 9, 2020
Source:
University of Wisconsin-Madison
Summary:
Astronomers have discovered that a halo of warm gas surrounding
the Magellanic Clouds likely acts as a protective cocoon, shielding
the dwarf galaxies from the Milky Way's own halo and contributing
most of the Magellanic Stream's mass.



FULL STORY ==========================================================================
The Milky Way is not alone in its neighborhood. It has captured smaller galaxies in its orbit, and the two largest are known as the Small and
Large Magellanic Clouds, visible as twin dusty smears in the Southern Hemisphere.


==========================================================================
As the Magellanic Clouds began circling the Milky Way billions of years
ago, an enormous stream of gas known as the Magellanic Stream was ripped
from them. The stream now stretches across more than half of the night
sky. But astronomers have been at a loss to explain how the stream became
as massive at it is, over a billion times the mass of the sun.

Now, astronomers at the University of Wisconsin-Madison and their
colleagues have discovered that a halo of warm gas surrounding the
Magellanic Clouds likely acts as a protective cocoon, shielding the
dwarf galaxies from the Milky Way's own halo and contributing most
of the Magellanic Stream's mass. As the smaller galaxies entered the
sphere of the Milky Way's influence, parts of this halo were stretched
and dispersed to form the Magellanic Stream. The researchers published
their findings Sept. 9 in the journal Nature.

"The existing models of the formation of the Magellanic Stream are
outdated because they can't account for its mass," says Scott Lucchini,
a graduate student in the UW-Madison physics department, first author
of the paper.

"That's why we came out with a new solution that is excellent at
explaining the mass of the stream, which is the most urgent question to
solve," adds Elena D'Onghia, a professor of astronomy at UW-Madison who supervised the research.

D'Onghia collaborated with physicists and astronomers at UW-Madison,
the Space Telescope Science Institute in Baltimore, and the University of Sydney. She completed the work while a scholar at the Flatiron Institute's Center for Computational Astrophysics in New York City.



========================================================================== Older models suggested that gravitational tides and the force of the
galaxies pushing against one another formed the Magellanic Stream out
of the Magellanic Clouds as the dwarf galaxies came into orbit around
the Milky Way. While these models could largely explain the stream's
size and shape, they accounted for just a tenth of its mass.

Recently, astronomers discovered that the Magellanic Clouds are massive
enough to have their own halo, or corona, of warm gas enveloping
them. D'Onghia and her team realized this corona would dramatically
alter how the stream formed.

In new simulations carried out by Lucchini, the creation of the Magellanic Stream is divided into two periods. While the Magellanic Clouds were
still far away from the Milky Way, the Large Magellanic Cloud stripped
gas away from its smaller partner over billions of years. This stolen gas ultimately contributed 10 to 20 percent of the final mass of the stream.

Later, as the clouds fell into the Milky Way's orbit, the corona gave
up a fifth of its own mass to form the Magellanic Stream, which was
stretched across an enormous arc of the sky by interactions with the
Milky Way's gravity and its own corona.

The new model is the first to explain the full mass of the Magellanic
Stream and the vast majority that comes from ionized gas, which is
more energetic than non-ionized gas. It also better explains how the
stream adopted its filamentous shape and why it lacks stars -- because
it was formed largely from the star- free corona, not the dwarf galaxies themselves.

"The stream is a 50-year puzzle," says Andrew Fox, one of the
co-authors of the study and an astronomer at the Space Telescope Science Institute, which operates the Hubble Space Telescope. "We never had a
good explanation of where it came from. What's really exciting is that
we're closing in on an explanation now." The researchers' proposal can
now be directly tested. The Hubble should be able to see the telltale signatures of the corona of gas surrounding the Magellanic Clouds.

In the 1990s, a group of astronomers at UW-Madison found the first hints
that the Magellanic Clouds might have an extensive corona. Now with a
better understanding of the corona's influence on the Magellanic Stream
and a clear- cut test for its existence, there's a chance to explain a half-century mystery about the origin of the stream, offering a fuller
picture of our galactic neighborhood.

"This work redefines our understanding of how gas accretes onto the
Milky Way and forms the reservoir for future star formation," says Joss Bland-Hawthorn, a co-author of the paper and director of the Sydney
Institute for Astronomy in Australia.


========================================================================== Story Source: Materials provided
by University_of_Wisconsin-Madison. Original written by Eric
Hamilton. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Lucchini, S., D'Onghia, E., Fox, A.J. et al. The Magellanic Corona
as the
key to the formation of the Magellanic Stream. Nature, 2020 DOI:
10.1038/ s41586-020-2663-4 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200909114841.htm

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