Galaxy ‘mega-merger’ 10 billion years ago forged Milky Way

The Milky Way Has a Gigantic Skeleton in Its Closet

Astronomers using the European Space Agency's Gaia space telescope have discovered that our galaxy was involved in a cosmic merger 10 billion years ago.

By looking at the chemical signatures and trajectories of the stars, Helmi's team was able to easily tell the Gaia-Enceladus stars from the "native" ones.

We often think of space as peaceful and serene. Studies like this are a detailed portrait of one individual from that mass. Instead, massive galaxies bulk up by catching and consuming smaller galaxies. The fact that so many clusters could be linked to Gaia-Enceladus is another indication that this must have once been a big galaxy in its own right, with its own entourage of globular clusters. Measurements of heavy elements were essential, as we know that these are formed when stars explode as supernovae, filling the interstellar medium of a galaxy. "When stars move the opposite way, that already tells you that they basically didn't form in the same place as the majority of the stars in our galaxy". In the Milky Way's case, the galactic disc can clearly be classified as two distinct parts: the thin disc, which is about 400 light-years thick, containing gas, dust and stars; and the thick disc, which extends to 1,000 light-years, containing only stars older than 10 billion years, like the inner halo stars. "It's like uncovering a fossil or an archaeological piece of evidence for how the galaxy got started", says James Bullock, an astronomer at the University of California, Irvine, who is unaffiliated with the new research.

The team also studied the chemistry of almost 600 of those stars using ground-based telescope data, which confirmed that these stars had come from somewhere beyond the Milky Way.

The sheer number of odd-moving stars involved intrigued Amina and her colleagues, who suspected they might have something to do with the Milky Way's formation history and set to work to understand their origins.

Then, Helmi and her colleagues noticed something else.

Helmi: This is hard to say. But how many and when was these clashes is unknown.

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The Milky Way (white) gobbles up the dwarf galaxy Gaia-Enceladus (red) in scenes from a computer simulation of the ancient event.

The experts discovered that some 10 billion years ago, the Milky Way merged with a large galaxy.

The researchers named the galaxy that merged with ours Gaia-Enceladus, after both the telescope and the mythical Greek figure who was the son of Gaia, the mother of all life.

"This paper is suggesting that the stellar halo is dominated by the cannibalisation of at least one fairly large dwarf galaxy", says astrophysicist Gurtina Besla of the University of Arizona.

The collision reordered the heavens more than five billion years before the Earth was formed. This second galaxy would have been a smaller "satellite" companion of the Milky Way, travelling around it.Collisions of galaxies are dramatic events. The Milky Way, researchers determined, is now sipping gas from the Small Magellanic Cloud, using the material to produce new stars and planets. Its partner will suffer the same fate. It shook the Milky Way, changing the structure of the galactic disk, setting off bursts of star formation in its wake. However, ten billion years is a long time (even for astronomy)-long enough to scatter the debris from a merger all over the sky, rather than just in a clear stream.So while the discovery of a collection of oddly rotating stars scattered over the whole sky is interesting, the scientists couldn't be sure these stars were actually associated with each other. Now hurtling toward one another at 402,336 kilometres an hour, these galaxies will merge and change our night sky forever-that is, if there are any sentient beings left clinging to this rock we call home.

A HUNDRED MILLION STARS IN 3 MINUTES In January 2015, NASA released the largest image ever of the Andromeda galaxy, taken by the Hubble telescope.

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