Galactic collision ripples across eons to shape our view of the cosmos

Nov 8, 2018 by

Why We Wrote This

Looking up at the heavens from below, it can be easy to think of the universe as a fixed constellation of stars. But, as a new study illustrates this week, the cosmos is ever changing – as is our understanding.

If you look at a map of the Milky Way today, you might think our galaxy looks calm and constant. But that’s just a snapshot of this moment in celestial time. Some 10 billion years ago, scientists say, it was anything but.

At the time, our galaxy was much smaller than it is today. So when gravity drew it together with another galaxy about a fourth its size, the Milky Way churned. The two smashed together and merged. Scientists think this collision could explain the mysterious inner halo and thick disc we see in the Milky Way’s structure today.

It’s difficult to discern the remnants of the other galaxy from the original Milky Way material at a glance. But with an influx of new data, that’s just what an international team of scientists did. In a paper published Wednesday in the journal Nature, researchers reconstruct the tale of this mega-merger between the Milky Way and the now-defunct galaxy they have dubbed Gaia-Enceladus.

Such a formative collision in our galaxy’s past doesn’t come as a surprise to modern cosmologists. There have been clues of such a merger previously. And it’s now broadly accepted that structures and objects of all sizes in space move around, collide, and merge.

But we haven’t always seen the cosmos as so dynamic. Over the centuries of pondering the heavens, we have shifted from seeing the universe as fairly static and simple, to something in swirling motion and constant upheaval.

“We’re seeing that the universe is a lot more active,” says Eric Chaisson, an astrophysicist and research associate at the Harvard-Smithsonian Center for Astrophysics, “as opposed to the view that ancient astronomers had that things were a whole lot more passive, a whole lot more inactive.”

A revolution in perspective

Throughout much of history, people viewed the cosmos as something that moved in a uniform, circular manner, with Earth at the center of it all. Models of a fairly static universe held sway into the early 20th century. But by the late 1920s, Edwin Hubble and others had begun to amass evidence that hinted the universe was actually expanding. The groundwork was laid for models such as the Big Bang theory.

From there, cosmologists have only added to the picture of the cosmos as being in constant, varied motion. The discovery of quasars in 1963, for example, further cemented the idea that the universe is roiling, says Dr. Chaisson. “We began to realize that at great distances, there were objects that were quite violent and, at the time, seeming to have energy emissions beyond what the laws of physics could explain,” he says. And now we know that there’s chaos around a black hole at the center of our galaxy, too.

Still, it’s difficult to observe motion in the cosmos because of the sheer scale. If something millions of light-years away moves a mile in a day, it won’t have shifted all that much from the human perspective.

“We don’t see things changing from day to day or even over a lifetime,” says Anthony Brown, a co-author on the Nature paper about the galactic merger and a senior researcher at the Leiden Observatory in the Netherlands. So for our cosmically interested ancestors, “it was a natural thing to say we were living in a fairly static universe.”

But now that we know that isn’t the case, astronomers are looking to stars’ movement to elucidate the structure of our galaxy and piece together its history. To do that, scientists are employing what has been called “galactic archaeology,” says Gurtina Besla, a theoretical astrophysicist at the University of Arizona in Tucson. They look at characteristics of the stars to figure out where they formed and under what conditions.

That’s what Dr. Brown and colleagues did to discover the Milky Way’s dramatic prehistoric collision. Using data gathered by the European Space Agency’s Gaia mission mapping the stars in our galaxy, the team led by Amina Helmi at the University of Groningen in the Netherlands examined the velocity, composition, and other characteristics of seven million stars. Some 30,000 stars were moving in elongated trajectories opposite to the vast majority of other stars in the Milky Way (including our sun). Other characteristics of the stars, like their composition, are also consistent with the merger model.

Galactic archaeology

Scientists have only just begun to scratch the surface in understanding how galaxies form and influence their environs. And motion has increasingly become a valuable tool in the cosmologist’s toolkit to resolve these questions. By measuring the motions of galaxies, scientists can study the relationships between them. For example, Dr. Besla and colleagues used this approach to determined that two small galaxies near our own, the Large and Small Magellanic Clouds, collided cosmically recently.

Motion has opened up new ways of looking at the cosmos. Before we understood just how much stars and galaxies move around, researchers would surmise that a group of stars were related because they were near each other and had similar physical characteristics. But now, scientists can group stars together based on which direction they’re moving, too.

And that’s key. The stars from Gaia-Enceladus, for example, are intermingled with the other stars in the Milky Way. In fact, many of them are currently passing through our own stellar neighborhood. So determining that they were moving in a different direction was important to discovering they came from elsewhere.

“We’ve known that galaxies are in perpetual motion” for a while, Besla says, but measuring the speeds at which they and the stars within them are moving has proved tricky because those motions of distant objects appear very small across our line of sight, and the atmosphere can obscure such small movements. Detecting movement using ground-based telescopes takes comparing images over decades so that the movement is significant enough to rule out atmospheric distortion.

Astronomy has long been the realm of snapshots of the universe in a moment of time and theory building on those snapshots. But by taking telescopes above the atmosphere, like NASA’s Hubble Space Telescope and ESA’s Gaia, researchers have been able to capture the data necessary to piece together movies rather than simply snapshots.

For now, that has been limited to our galaxy, and there’s still a lot left to learn about the history of the Milky Way from future data from Gaia. But hopes are high that when NASA’s embattled James Webb Space Telescope finally launches, we will be able to look even farther.

Already, scientists have looked to the rest of our galactic neighborhood and projected the trajectory of our neighbors. And it looks like more dramatic collisions lie in the Milky Way’s future, according to some of Besla’s research. Our neighbor Andromeda is on track to slam into our galaxy in perhaps 4 billion years. And before that, the Milky Way is expected to eat the Large Magellanic Cloud, a nearby small galaxy.

“The fact that galaxies are moving has been known in astronomy for a long time,” Besla says. “But the difference is that now we can measure exactly in which direction they’re moving, and that is a very powerful new piece of information.”

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