Gargantuan Collision With The Giant Galaxy Gaia-Enceladus Fills The Milky Way With Stars

Imagine that you are trying to map your hometown using only the information you can collect from your window. Even with a pair of binoculars, the task would be difficult. Mapping our own galaxy, the Milky Way, is an equally daunting mission. Unlike other galaxies we can see from afar, we sit inside the Milky Way, about 26,000 light-years from its center. This means that when we try to look on the other side of the galaxy, much of our vision is obstructed by the stars and the dust that separates them.

But on April 25, our vision of the Milky Way became much clearer. The Gaia satellite of the European Space Agency has provided us with positions, distances and movements of 1.3 billion stars measured with higher precision than ever before. Now, these same data have helped researchers understand how our galaxy is filled with a considerable amount of stars. The results have just been published in Nature.

Gaia, which aims to trace the stars of the Milky Way with unprecedented detail, has a resolution comparable to being able to measure the width of a hair over the Empire State Building. But in this scenario, Gaia would not be in the streets of New York, narrowing her eyes to 443 meters at the top of the tower. That's tantamount to sitting on the roof of Buckingham Palace in London, watching over 5,500 kilometers across the Atlantic Ocean.

Funny moving stars

This illustration shows a planned fusion step between our Milky Way galaxy and the neighboring Andromeda galaxy, as it will unfold over the next billion years. In this image, which represents the night sky of the Earth since 3.75 billion years, Andromeda (left) fills the field of vision and begins to deform the Milky Way by pulling the tides. NASA / ESA / Z Levay / R van der Marel / STScI / T Hallas / A Mellinger

The team of astronomers of the new study, the Netherlands and France, discovered stars with retrograde movements, moving in a different direction than the majority of stars in the galaxy. They suspected that it could be debris from 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.

Galaxy collisions are dramatic events. They remodel large galaxies and can consume smaller galaxies entirely. In fact, this has already happened before, with the debris of satellite galaxies captured by the gravitational pull of the Milky Way, often seen as a stream of stars, tracing the orbit of the unhappy satellite. However, ten billion years is long (even for astronomy) – long enough to disperse the debris of a meltdown in the sky, rather than in a single stream.

Thus, although the discovery of a collection of stars strangely rotating, scattered all over the sky is interesting, scientists could not be certain that these stars were really associated with each other. To find out, they decided to compare the chemical properties of stars with the remaining properties of the Milky Way.

Chemical indices

The team used spectroscopic observations from the APOGEE-2 survey, which measure the amount of different elements in individual stars. The measurements of the heavy elements were essential because we know that they form when the stars explode in the form of supernovae, filling the interstellar medium of a galaxy. The element formed depends on the star. The alpha elements – formed by combining several helium atoms – are created by fast – moving massive star explosions, while iron forms in supernovae from star systems where two stars revolve around one of the 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 of retrograde stars with those of the Milky Way disk components, it was obvious that the retrograde stars had to be formed in a different chemical composition system. The addition of this extra piece of the puzzle clearly shows that it was acting at the origin of a separate galaxy.

The authors named the satellite galaxy "Gaia-Enceladus", which collapses, which fits perfectly. In Greek mythology, Enceladus was a child of the gods Gaia and Uranus. It was one of the giants, buried under Etna, where earthquakes disrupted the country.

Likewise, Gaia-Enceladus was a giant relative to most of the current Milky Way satellites – it would have had a mass about a quarter of that of the Milky Way at the time of fusion. He shook the Milky Way, changing the structure of the galactic disk, triggering bursts of star formation in his wake. Like the giant Encelade, Gaia-Encelade was buried, hidden deep in the galaxy. If Gaia can discover the remains of this event that occurred ten billion years ago, it is exciting to think about what he will discover again.

Victoria Scowcroft is a senior lecturer in physics at the University of Bath, UK.

This article is republished from The Conversation under a Creative Commons license. Read the original article.