Galactic crash gave Milky Way its cream

The Milky Way is believed to have formed more than 13 billion years ago. It shook the Milky Way, changing the structure of the galactic disk, setting off bursts of star formation in its wake.

The researchers found evidence of the merger by studying the movement of seven million stars in the the Milky Way's inner halo, a region around the galaxy's thick disk of stars.

They discovered the bulk of these objects came from a single source - and that source was another galaxy. We know our own Milky Way has likely smashed up with other galaxies in the distant past; now astronomers have revealed that one particular collision resulted in a mysterious feature called the "thick disc".

Globular clusters are groups of up to millions of stars, held together by their mutual gravity and orbiting the centre of a galaxy.

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

'We expected stars from fused satellites in the halo, ' said University of Groningen's Professor of Astronomy, Amina Helmi.

Even though no more evidence was really needed, the team also found hundreds of variable stars and 13 globular clusters in the Milky Way that follow similar trajectories as the stars from Gaia-Enceladus, indicating that they were originally part of that system.

"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. "It's a dead galaxy, so that makes it kind of fun".

"The chemical signature was clearly different from the "native" Milky Way stars", Helmi said.

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"Playing these videos backwards allows astronomers to study how our galaxy was assembled, and how it has evolved", noted Kim Venn, an astronomer at the University of Victoria in Canada, commenting on the study. Alpha elements - formed by combining multiple helium atoms - are created in explosions of massive, rapidly evolving stars, while iron is formed in supernovae from star systems where two stars orbit each other.

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

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

'What we didn't expect to find was that most halo stars actually have a shared origin in one very large merger'.

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. They reshape large galaxies and can entirely consume smaller galaxies. In addition, the stars from the smaller galaxy that fell into the Milky Way 10 billion years ago still form a population of stars that are rotating around the Milky Way center, in a sense contrary to the majority of the stars.

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

The researchers hope that by understanding the collision in the Milky Way, they can better understand the process in other galaxies as well. One of these mergers will be with the Andromeda galaxy, in at least 5 billion years, Helmi said.

Helmi and her colleagues drew on data from a European project called Gaia, which is mapping the location of 1 billion stars in the Milky Way with unprecedented precision. "It was incredible to look at the new Gaia data and realize that I had seen it before!", says the astronomer.

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