Milky Way collided with another galaxy

A 10 billion-year-old collision filled our galaxy’s inner halo and thickened its disc.

By looking at recently released data from the Gaia space telescope, researchers found that the Milky Way’s inner halo is largely made up of stars from a 10 billion-year-old impact with another galaxy. The age, chemistry, distribution, and motion of individual stars helped researchers determine the precise nature of the collision with the newly named galaxy “Gaia-Enceladus”. Previous models on the Milky Way's development suggested that its halo was made from mergers with other galaxies, but no one knew the timing or the number of mergers. We spoke with two of the study’s authors Anthony Brown, Leiden Observatory and Amina Helmi, University of Groningen about the study.

ResearchGate: Can you tell us briefly what you discovered?

Brown: We discovered the remnants of a significant merger between the Milky Way and another smaller galaxy which happened some 10 billion years ago in the Gaia DR2 data. This merger probably contributed to the formation of the so-called thick disk of the Milky Way. We also see the stars of the smaller galaxy which merged with the Milky Way scattered all over the sky.

RG: How did this collision affect the Milky Way?

Brown: This collision deposited a significant number of stars in the inner regions of the Milky Way's halo and contributed to the formation of the thick disk. In addition, the stars from the smaller galaxy that fell into the Milky Way 10 billion years ago still form a population of stars that are rotating around the Milky Way center, in a sense contrary to the majority of the stars.

RG: How did you discover this? What role did the Gaia telescope play in the discovery?

Brown: Gaia provided the highly precise 3D positions and motions of the stars which were needed to isolate the population of stars that were brought into the Milky Way by the merger. This study would not have been possible without the Gaia data.

RG: What does this mean for our understanding of the universe 10 billion years ago?

Amina Helmi: This is an interesting question that we are just starting to explore. Using the debris from merged galaxies to understand what galaxies looked like in the early universe is a very new avenue of research because previously we could not disentangle which stars had been newly brought together. So, the first lesson we learned was that the Milky Way had a disk at that time and that this disk was strongly perturbed by the merger with Gaia-Enceladus. In the near future we plan to better characterize both systems.

RG: Are galaxies still colliding today? If so, is this a common occurrence?

Helmi: Not so often anymore. The Universe was smaller in the past and so collisions were more frequent. We believe, from cosmological models, that all galaxies have experienced mergers. But proving this is not easy and it is only now possible with Gaia for the Milky Way, or for events that have happened relatively recently.

RG: What do you make of the predicted collision between the Andromeda and the Milky Way in four billion years?

Helmi: That it will lead to “fireworks” in the sky. Andromeda is much larger, so the collision will likely completely re-shape the Milky Way, and the new galaxy will likely end up being an elliptical system.

RG: If the Milky Way were to collide with another galaxy, what would that mean for life on Earth?

Helmi: This is hard to say. If the merger was significant, and there was a lot of gas involved, then there will be many new stars born, and some of these might explode as supernovae. These explosions might have an impact on life, but I am not an expert in this.

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