Worried about the Andromeda galaxy crashing into our Milky Way in four billion years? Too bad, it's quite possibly already happening

Well, halo, halo

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Andromeda sports a ginormous halo of gas with a mass greater than 100 billion Suns that stretches from its outer edges up to two million light-years – a distance that reaches more than halfway to our own Milky Way galaxy.

If the structure was visible to the naked eye, it would appear three times the width of the Big Dipper, said NASA. The gas is made up of plasma and a large sprinkling of stardust shed from dead suns and supernovae. It’s split into two layers: the inner shell is lumpier and more dynamic, the outer shell is smoother and hotter.

“Understanding the huge halos of gas surrounding galaxies is immensely important,” said Samantha Berek, co-author of a paper into the thing [PDF] and an astrophysicist at Yale University.

“This reservoir of gas contains fuel for future star formation within the galaxy, as well as outflows from events such as supernovae. It’s full of clues regarding the past and future evolution of the galaxy, and we’re finally able to study it in great detail in our closest galactic neighbor.”

Andromeda is on a collision course with our Milky Way, and is due to meet us in about four billion years having swallowed others on its journey. Observations by the Hubble Space Telescope suggest the merger may already be underway as the alien galaxy's halo meets our galaxy's gas band.

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"It is quite plausible that halo gas from Andromeda and the Milky Way are already mixing in the outskirts of their halos,” Nicholas Lehner, first author of the paper and an astrophysics professor at the University of Notre Dame in the US, told The Register.

The team's study into Andromeda's halo, known as Project AMIGA aka the Absorption Map of Ionized Gas in Andromeda, is the first time scientists have been able to outline a galactic halo in such detail. To build the map, the team studied the light emitted from 43 quasars using the spectrograph instrument aboard the Hubble.

They traced how the light from the quasars was being absorbed as the rays passed through Andromeda's halo. Including more quasars produced a more accurate picture of the halo. It’s difficult to perform the same measurements for other galaxies, though, since they’re further away – or in our own case, because we're too close.

“Because we live inside the Milky Way, we cannot probe our own galaxy’s halo the same way," said Lehner. "It's a case of not being able to see the forest for the trees. So we will never be able to image the Milky Way’s halo as we can and will be able for other galaxies.”

Andromeda’s halo, therefore, is the probably the best model we have for estimating what the Milky Way’s halo looks like. Both galaxies have a similar mass and shape, and their globules of gas provide the matter to form new stars and influence how they evolve over time.

“Over the lifetime of a galaxy, there is an exchange of matter between the halo and disk of the galaxy via accretion of halo gas onto the galaxy and ejection of gas from the explosion of massive stars from the disk into the halo,” Lehner told us.

“Hence, halo gas is recycled in the disk of the galaxy, which can help to fuel new star formation and disk gas is recycled in the halo of the galaxy, enriching it with heavy elements that were formed in stars.” ®

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