Gargantuan Collision With Giant Gaia Galaxy-Enceladus Filled Milky Way With Stars

Imagine trying to get to know your home town. Even with a pair of binoculars you'd find it a difficult task. Mapping out our own galaxy, the Milky Way, is a similarly daunting mission. Unlike other galaxies that we can view from a distance, we sit inside the Milky Way-around 26,000 light years from its center. This means that when we try to look at the opposite side of the galaxy, we are left behind by the stars and dust in between.

But on April 25, the Milky Way has become much clearer. The European Space Agency 's Gaia satellite provided with positions, distances and motions of 1.3 billion. Nowadays this same data has been produced by our galaxy with a huge amount of stars. The results have just been published in Nature.

Gaia, which aims to chart the stars in the Milky Way in unprecedented detail, has a comparable resolution to being able to measure the size of a Empire State Building. But in this scenario, Gaia would not be down on the New York streets, squinting up 443 meters to the top of the tower. It is the equivalent of being on the roof of Buckingham Palace in London, staring out over 5,500 kilometers (3,400 miles) across the Atlantic Ocean.

Funny moving stars

This illustration shows a stage in the predicted merger between our Andromeda Galaxy, Andromeda galaxy, as it will unfold over the next several billion years. In this image, representing Earth's night sky in 3.75 billion years, Andromeda left the field of view and begins to distort the Milky Way with tidal sweater. 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. They suspected that this could be the result of a collision between the Milky Way and another galaxy ten billion years ago. This second galaxy would have been a smaller "satellite" companion of the Milky Way, traveling around it.

Collisions of galaxies are dramatic events. They reshape large galaxies and can only consume smaller galaxies. In fact, this has happened before, with the debris of 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. 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 is interesting, the scientists could not be sure these stars were actually associated with each other. To find out, they decided to compare the chemical properties of the stars with the remaining ones in the Milky Way.

Chemical clues

The team used spectroscopic observations from the APOGEE-2 survey-which measures the amounts of different elements in individual stars. Measurements of heavy elements are essential, as we know that they are as exploded as supernovae, filling the interstellar medium of a galaxy. Exactly which element is formed depends on the star. 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.

We know that the fraction of alpha elements in a galaxy decreases with time, while the fraction of iron increases. 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. Adding in this extra piece of the puzzle makes it clear that they were originally separate galaxy.

The authors have named the satellite galaxy crash "Gaia-Enceladus," which is very fitting. In Greek mythology, Enceladus was a child of gods Gaia and Uranus. It was one of the giants, buried under Mount Etna, where it caused earthquakes disrupting the land.

Similarly, Gaia-Enceladus was a giant compared to most of the Milky Way's current satellites-it would have been a part of the Milky Way's at the time of the merger. It shook the Milky Way, changing the structure of the galactic disk, setting off bursts of star training in its wake. Like Enceladus the giant, Gaia-Enceladus was buried, hidden deep inside the galaxy. If Gaia could dig out the remains of this event that happened ten years ago, it is exciting to think what else it will uncover.

Victoria Scowcroft is a Lecturer of Physics at the University of Bath, U.K.

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