How A Catastrophic Cosmic Collision Changed the Milky Way Forever

The Andromeda galaxy which researchers think will collide with the Milky Way in billions of years. One new study shows how our galaxy was shaped by a merger with a dwarf galaxy 10 billion years ago

The Andromeda galaxy which researchers think will collide with the Milky Way in billions of years. One new study shows how our galaxy was shaped by a merger with a dwarf galaxy 10 billion years ago

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.

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.

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. Unlike other galaxies that we can view from a distance, we sit inside the Milky Way - around 26,000 light years from its centre. This study would not have been possible without the Gaia data.

Looking for traces of galactic mergers with the Milky Way's halo, the researchers found to their surprise that most of its stars were from the same immediate family.

Spiral galaxies like our own are made up of several parts: the central bulge, spiral arms, the disk and a surrounding halo. 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. Those measurements let scientists identify about 33,000 stars that live in our galaxy but were born elsewhere, carried here during a giant galactic collision.

Another piece of evidence was the composition of the stars themselves. Stars from different galaxies have their own kind of fingerprint.

This is a feature seen in some spiral galaxies, and astronomers don't really know how they got there.

"According to the legend, Enceladus was buried under Mount Etna, in Sicily, and responsible for local earthquakes". This merger probably contributed to the formation of the so-called thick disk of the Milky Way.

The evidence for Enceladus the galaxy is strong.

The Milky Way is believed to have formed more than 13 billion years ago. It was therefore clearly a major blow to our Galaxy. "We're seeing the inner workings of a galaxy". "It's fantastic. It's just so handsome and makes you feel so big and so small at the same time".

But astronomers have long argued as to whether the Milky Way bulked up on a diet of baby star clusters, or by merging with a single Big One. With the latest Gaia data release, there's much more to do.

The report was produced by a team of researchers, led by Dutch astronomer Amina Helmi, who used data the Gaia space telescope to reach their conclusions.

"We didn't expect to find that most halo stars have a shared origin", she added.

This is indeed what she found. By plotting both trajectory and chemical signature, the "invaders" stood out clearly. In fact, this has happened before, with the debris from satellite galaxies caught up in the Milky Way's gravitational pull often seen as a stream of stars, tracing out the orbit of the ill-fated satellite. "This means that the progenitor of this thick disk was already present when the fusion happened, and Gaia-Enceladus, because of its large size, shook it and puffed it up".

In a previous paper, Helmi had already described a huge "blob" of stars sharing a common origin. 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.

The galaxy that researchers believe collided into ours has been named Gaia-Enceladus, after the Greek goddess and her offspring.

The data on kinematics, chemistry, age and spatial distribution from the native Milky Way stars and the remnants of Gaia-Enceladus reminded Helmi of simulations performed by a former PhD student, some ten years ago.

Recent observations from the Gaia satellite mission revealed numerous stars in the Milky Way's halo are "invaders" - or remnants of another, younger galaxy. "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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