Tuesday, 13 November, 2018

Galaxy 'mega-merger' 10 billion years ago forged Milky Way

The Milky Way Has a Gigantic Skeleton in Its Closet Dramatic galaxy collision filled the Milky Way with stars, astronomers discover
Cary Marshall | 02 November, 2018, 23:40

Astronomers, who have long thought the Milky Way likely grew from a vast number of merging dwarf galaxies, have struggled to uncover the signs of the largest mergers-until today. NASA/ESA/Z Levay/R van der Marel/STScI/T Hallas/A Mellinger The team of astronomers behind the new study, from the Netherlands and France, discovered stars with retrograde motions-moving in a different direction to the majority of the galaxy's stars.

Spiral galaxies like our own are made up of several parts: the central bulge, spiral arms, the disk and a surrounding halo.

"The youngest stars of Gaia-Encelado are actually younger than the native Milky Way stars in what is now the thick disc region". What the data shows is that a group of about 30,000 of those stars aren't rotating around the galactic center like they should. His simulations of the merging of a large disc-shaped galaxy with the young Milky Way produced a distribution of stars from both objects, which is totally in line with the Gaia data.

Before this discovery, the team had run simulations of galactic mergers.

"It has eaten many dwarf galaxies in the past, and we've just found a major one that it ate in the past".

Instead of meandering around the galaxy the same way as the hundred billion or so other stars in the Milky Way, these stars, although completely interspersed in the inner halo, all moved along elongated trajectories in the opposite direction.

The globular cluster Omega Centauri - with as many as ten million stars - is seen in this image captured from the European Southern Observatory's La Silla Observatory.

"We became only certain about our interpretation after complementing the Gaia data with additional information about the chemical composition of stars, supplied by the ground-based APOGEE survey", says Carine Babusiaux, Université Grenoble Alpes, France, and second author of the paper. Stars from different galaxies have their own kind of fingerprint.

However another thing also set these stars apart; aside from moving differently, they are also chemically different.

"It's very cool that stars that formed in another galaxy could be lurking right next door to us", said Kim Venn, a professor at the University of Victoria's department of physics and astronomy, who was not involved in the study.

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"If you were there ... you would see bright, blue young stars".

Helmi: That it will lead to "fireworks" in the sky.

If that sounds extreme, don't forget that in roughly five billion years, the Milky Way will collide with our nearest neighbour, the giant Andromeda galaxy.

Ten billion years ago, however, when the merger with Gaia-Enceladus took place, the Milky Way itself was much smaller, so the ratio between the two was more like four to one.

Helmi and her colleagues named the dwarf galaxy Gaia-Enceladus, after Gaia, the Greek mother of all life, and Enceladus, the son she had with Uranus, the sky god.

Massive galaxies such as our Milky Way galaxy are the outcome of mergers with other smaller galaxies. Now, she shows that stars from this blob in the halo are the debris from the merging of the Milky Way with a galaxy which was slightly more massive than the Small Magellanic Cloud, some ten billion years ago.

This Hubble image of the Antennae galaxies is the sharpest yet of their merger.

The Milky Way's halo is not like the ones of other spiral galaxies. "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. Now a new paper published in Nature provides proof-or something close to it. With the latest Gaia data release, there's much more to do.

And that galactic sleuthing is exactly what's so enticing to Helmi.

"The Milky Way is our home, and people like to know their origins; they like to know their own history. And to me, that's what's fascinating".

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