Huge Halo of Warm Gas around Magellanic Clouds is Key to Formation of Magellanic Stream

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A team of astronomers from Australia and the United States has found that a halo of warm ionized gas surrounding the Large and Small Magellanic Clouds acts as a protective cocoon, shielding the dwarf galaxies from the Milky Way Galaxy’s own halo and contributing most of the mass of the Magellanic Stream.

The Magellanic Stream in zenithal equal-area coordinates: (a) observed H I data for the Magellanic Stream, with the line-of-sight velocity indicated by the color scale and the relative gas column density indicated by the brightness; the points represent the sightlines with ultraviolet-absorption-line observations from the NASA/ESA Hubble Space Telescope, colored by their line-of-sight velocity; these points show the extent of the ionized gas associated with the stream; (b) the results of the model including the Magellanic Corona and the Milky Way’s hot corona; gas originating in the disks of the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) is shown in the model, without separating neutral gas from ionized gas; this affects the morphology of the stream, causing the model to appear smoother and less fragmented than the data; however, the model reproduces the current spatial location and velocity of both clouds, and the velocity gradient of the gas along the stream; the Milky Way disk and background are extracted from real H I images. Image credit: Lucchini et al, doi: 10.1038/s41586-020-2663-4.

The Magellanic Stream in zenithal equal-area coordinates: (a) observed H I data for the Magellanic Stream, with the line-of-sight velocity indicated by the color scale and the relative gas column density indicated by the brightness; the points represent the sightlines with ultraviolet-absorption-line observations from the NASA/ESA Hubble Space Telescope, colored by their line-of-sight velocity; these points show the extent of the ionized gas associated with the stream; (b) the results of the model including the Magellanic Corona and the Milky Way’s hot corona; gas originating in the disks of the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) is shown in the model, without separating neutral gas from ionized gas; this affects the morphology of the stream, causing the model to appear smoother and less fragmented than the data; however, the model reproduces the current spatial location and velocity of both clouds, and the velocity gradient of the gas along the stream; the Milky Way disk and background are extracted from real H I images. Image credit: Lucchini et al, doi: 10.1038/s41586-020-2663-4.

The dominant gaseous structure in the Milky Way’s halo is the Magellanic Stream.

This extended network of neutral and ionized filaments surrounds the Large Magellanic Cloud and the Small Magellanic Cloud, the two most massive satellite galaxies of the Milky Way.

“The existing models of the formation of the Magellanic Stream are outdated because they can’t account for its mass,” said first author Scott Lucchini, a graduate student in the Department of Physics at the University of Wisconsin-Madison.

“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,” said lead author Professor Elena D’Onghia, a researcher in the Department of Physics and Department of Astronomy at the University of Wisconsin-Madison and the Center for Computational Astrophysics at Flatiron Institute.

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.

Lucchini, Professor D’Onghia and their colleagues realized that a halo, or corona, of warm gas enveloping the Magellanic Clouds would dramatically alter how the stream formed.

In new simulations, 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% 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 halo.

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,” said co-author Dr. Andrew Fox, an astronomer at the Space Telescope Science Institute.

“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 team’s paper was published in the September 9, 2020 issue of the journal Nature.

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S. Lucchini et al. 2020. The Magellanic Corona as the key to the formation of the Magellanic Stream. Nature 585, 203-206; doi: 10.1038/s41586-020-2663-4

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