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

James Marshall
November 2, 2018

The team has named that galaxy Enceladus, and the below video shows a simulation of its collision with the Milky Way. "It's a dead galaxy, so that makes it kind of fun".

Some of these then became integrated into the Milky Way.

An worldwide team led by an astronomer from the University of Groningen used data from the second data release of the Gaia satellite mission last April to spot signs of an ancient merger in the Milky Way's halo.

The research is described in a paper published today (Oct. 31) in the journal Nature.

Brown: Gaia provided the highly precise 3D positions and motions of the stars which were needed to isolate the population of stars that were brought into the Milky Way by the merger. But in this scenario, Gaia wouldn't be down on the NY streets, squinting up 443 metres to the top of the tower.

Researchers hope the star, known as J0815+4729, which is in line with the Lynx constellation, will help them learn more about the Big Bang, the popular theory about the galaxy's evolution.

10 billion years ago, our own galaxy collided with a smaller dwarf galaxy in a crash so intense that it ripped the smaller body apart and forever changed the structure of the Milky Way. This second galaxy would have been a smaller "satellite" companion of the Milky Way, travelling around it.

An image of galaxy UGC 12158, which is thought to resemble the Milky Way in appearance. That means that while this is hardly the only galactic collision scientists have pinpointed, it's a comparatively huge one, the scientists said. They reshape large galaxies and can entirely consume smaller galaxies.

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. 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.

The team used spectroscopic observations from the APOGEE-2 survey - which measures the amounts of different elements in individual stars. 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.

The merger would have produced brilliant stellar explosions - supernovas - and the rapid birth of stars. "We can now say this is the way the Galaxy formed in those early epochs".

The researchers found that the chemical signatures of many halo stars in the Milky Way were not the same as its "native" stars.

Globular clusters are groups of up to millions of stars, held together by their mutual gravity and orbiting the centre of a galaxy. 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. The galaxy is called Gaia-Enceladus, after the Giant Enceladus who in Greek mythology was born of Gaia (the Earth goddess) and Uranus (the Sky god).

"According to the legend, Enceladus was buried under Mount Etna, in Sicily, and responsible for local earthquakes".

Further analysis revealed that this galaxy was about the size of one of the Magellanic Clouds - two satellite galaxies roughly ten times smaller than the current size of the Milky Way.

The Milky Way's halo is not like the ones of other spiral galaxies. Like Enceladus the giant, Gaia-Enceladus was buried, hidden deep inside the galaxy.

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