Our Milky Way holds a galactic secret

The Milky Way Has a Gigantic Skeleton in Its Closet

The evidence is littered across the sky all around us, but it has taken Gaia and its extraordinary precision to show us what has been hiding in plain sight all along. Most recently, an worldwide team of scientists led by Amina Helmi of the University of Groningen identified one of the major galactic collisions (or "mergers") that shaped the Milky Way as we know it. The observed stars in particular are now passing by our solar neighborhood.

We are so deeply embedded in this collection that its stars surround us nearly completely, and so can be seen across most of the sky.

Stars on the outskirts of our galaxy was "aliens".

If those stars had been born in this galaxy, they would be marching around the core in the same direction as the rest of us. Although all these studies blamed a past collision for such oddities, they disagreed over when the collision occurred, the mass of the satellite galaxy and whether the event involved a single large dwarf galaxy or several smaller ones. This study would not have been possible without the Gaia data.

Find out more in the video above. "The youngest stars from Gaia-Enceladus are actually younger than the native Milky Way stars in what is now the thick disk region", said Helmi.

Recent observations from the Gaia satellite mission revealed numerous stars in the Milky Way's halo are "invaders" - or remnants of another, younger galaxy. And there is the thick disc, which is a few thousand light years deep. It contains about 10-20 percent of the Galaxy's stars yet its origins have been hard to determine.

The team's modelling suggests that a collision with a satellite dwarf galaxy roughly the size of the Small Magellanic Cloud would have both populated the Milky Way's halo with stars, and heated the pre-existing thin disc, expanding it into a thicker one.

Stars that form in different galaxies have unique chemical compositions that match the conditions of the home galaxy.

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

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A newly discovered star is thought to be one of the oldest in the Milky Way.

There's now more work to be done, trying to locate the stars left over from other collisions - and learning more about the Enceladus event.

Large galaxies like our Milky Way are the result of mergers of smaller galaxies.

The Andromeda galaxy, which researchers think will collide with the Milky Way in billions of years. 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. That vision is paying off. This is what Gaia was built for added Amina. "It's fantastic. It's just so attractive and makes you feel so big and so small at the same time".

Co-authored by Amina Helmi, an astronomer at the Kapteyn Astronomical Institute in the Netherlands, and her colleagues, the paper is one of a torrent following the long-awaited second data release from Gaia-a spacecraft launched in 2013 by the European Space Agency to chart the heavens in unprecedented detail. She has now used the data to look for traces of mergers in the halo: "We expected stars from fused satellites in the halo. It has eaten many dwarf galaxies in the past, and we've just found a major one that it ate in the past", she says. And that was the case with the 30,000 odd-moving stars they discovered in the Milky Way's halo.

'And they are a fairly homogenous group, which indicates they share a common origin, ' Helmi said. By plotting both trajectory and chemical signature, the "invaders" stood out clearly. This merger probably contributed to the formation of the so-called thick disk of the Milky Way.

The now consumed galaxy has been given the name Gaia-Enceladus after one of the Giants in ancient Greek mythology, who was the offspring of Gaia, the Earth, and Uranus, the Sky. By comparing the trends for the retrograde stars with stars in the Milky Way's disk components, it was clear that the retrograde stars must have formed in a system with a different chemical composition. Initially, the two galaxies are clearly separated, but gravity pulls them together and they merge.

"The other thing we'd like to do is go beyond this 10 billion years to earlier and earlier and see if we can find evidence of mergers that took place early on and what those mergers looked like", Helmi said.

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