Galaxy Collisions and the Milky Way

How did our galaxy form? The short answer is “galaxy collisions”. That keeps astronomers busy as they develop new tools to “look back” at the birth of our own galaxy. We know that the infant Milky Way began taking shape relatively early in the history of the universe. It probably existed for quite some time as a tiny dwarf galaxy. Then, it began to merge with others to “plump up” and grow.

Today, the Milky Way continues to dance the galaxy formation tango with other dwarf galaxies, such as the Sagittarius Dwarf. And, we know that it will eventually do an interactive dance with the Andromeda Galaxy. That series of galaxy collisions and interactions will happen several billion years from now. For now, it seems, that our galaxy is still forming through mergers. And, astronomers have some interesting tools to probe the details of the stars that got brought into the Milky Way.

The First Big Galaxy Collision

A couple of weeks ago, Ohio State University researchers released new information about the earliest stages of the Milky Way’s birth and growth. It seems the young Milky Way experienced a major merger with an orbiting satellite galaxy called Gaia-Enceladus. That happened about 10 billion years ago and influenced the distribution of stars in the galaxy.

Fiorenzo Vincenzo, who works at OSU’s Center for Cosmology and Astroparticle Physics, and a team of astronomers, wanted to know exactly what happened as a result of that merger. Specifically, what can the stars of our galaxy tell us about that ancient event?

They used several techniques to answer that, including asterochronology, asteroseismology, and a spectroscopic survey called APOGEE. Asterochronology studies the ages of stars to trace the basic “mergers and acquisitions” that helped form the Milky Way. Asteroseismology looks inside stars by studying their oscillations to get precise ages. The spectroscopic studies reveal the chemical compositions of stars. The combination of all three approaches gives amazingly good estimates for the ages and origins of stars in our galaxy.

“Our evidence suggests that when the merger occurred, the Milky Way had already formed a large population of its own stars,” said Vincenzo.

By calculating the ages of stars throughout our galaxy, the researchers determined something interesting. The stars captured from Gaia-Enceladus have similar or slightly younger ages than the majority of stars that already existed there.

During the merger, many of the infant Milky Way’s batch of “homemade” stars ended up in the “thick disk” that occupies the middle of the galaxy. The stars that were captured from Gaia-Enceladus during the merger exist mostly in the outer halo of our galaxy.

What Galaxy Collisions Change

Today, we know that most galaxies grow through mergers and acquisitions. So, it seems like a fairly normal occurrence in the universe. The merger of the baby Milky Way with Gaia-Enceladus was one of the most important in the Milky Way’s history. It shaped the galaxy we see today. It “shook things up”, by changing the orbits of stars already in the Milky Way.

Many stars were pushed into eccentric orbits. The stars from the long-disappeared Gaia-Enceladus, move in different ways than the “native” stars. Upon closer examination, stars that came from Gaia-Enceladus have different chemical compositions from those born inside the Milky Way.

Next Steps

Now that researchers peeked at the past collision and figured out what happened to the stars, what next? Their next step is to take the same approach and apply it to larger groups of Milky Way stars. The evolution of the Milky Way is, after all, an ongoing process. The end goal is to get a much clearer idea of that evolution and the changes mergers make to its stellar populations.

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