Galactic mergers involving two or more galaxies have been observed throughout the Universe. Like a car crash playing out in slow motion, galaxies can lock onto each other with the pull of their gravities. Our own Milky Way is believed to have consumed a dwarf galaxy about 10 billion years ago and is presently heading towards a head-on collision with the Andromeda galaxy (M31).
Astrophysicist Neil DeGrasse Tyson explained on an episode of his 2104 series Cosmos: A Spacetime Oddysey how this merger will play out.
He said: “Using nothing more than Newton’s first laws of gravitation, we astronomers can confidently predict that several billion years from now, our home galaxy the Milky Way, will merge with our neighbouring galaxy, Andromeda.
“Because the distances between the stars are so great compared to their sizes, few if any stars in either galaxy will actually collide.
“Any life on the worlds of that far-off future should be safe, but they will be treated to an amazing, billion-year long light show.”
But the collision may not be so far off after all, according to a new study suggesting the two galaxies are already touching.
The Milky Way and Andromeda are separated by about 2.5 million light-years, flying towards each other at several hundreds of thousands of miles per hour.
But a study published this summer in the Astrophysical Journal has found a dark region of space surrounding the Andromeda galaxy extends far out enough to touch the Milky Way.
This area is known as the galactic halo, which is a region of gas, stars and dust that is fairly hard to detect.
Astronomers have charted the halo around the galaxy thanks to NASA’s Hubble Space Telescope and Project AMIGA – Absorption Map of Ionized Gas in Andromeda – which has been dubbed the “the most comprehensive study of a halo in a surrounding galaxy”.
The study found Andromeda’s faint halo appears to be much bigger than previously thought, reaching up to 1.3 million light-years towards the Milky Way.
And if the same can be assumed about the Milky Way, which is about the same size and shape as Andromeda, it stands to reason the galaxies have already made contact.
Study co-investigator Samantha Berek of Yale University, US, said: “Understanding the huge halos of gas surrounding galaxies is immensely important.
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“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 neighbour.”
However, the galactic centres of the Milky Way and Andromeda are not going to meet for billions of years.
And if humanity is still around five billion years from now, the night sky view will be quite spectacular.
This is because Andromeda will grow gradually bigger and brighter in our night skies.
And the two galaxies will eventually form a single, elliptical entity.
Gurtina Besla of Columbia University in New York said in 2012: “In the worst-case-scenario simulation, M31 slams into the Milky Way head-on and the stars are all scattered into different orbits.
“The stellar populations of both galaxies are jostled, and the Milky Way loses its flattened pancake shape with most of the stars on nearly circular orbits.
“The galaxies’ cores merge, and the stars settle into randomized orbits to create an elliptical-shaped galaxy.”
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