Galactic Archaeology

Our galaxy the Milky Way was originally thought to be the only galaxy in the universe, until the ground breaking observations of Edwin Hubble during the 1920’s. Now, we know that our galaxy is just one of billions. With this new perspective comes questions: Did our galaxy interact with any of its neighbours? How could this have effected the evolution of the Milky Way?

Two galaxies interacting

The two galaxies NGC 4038 and 4039 interacting.The galaxies are in the constellation Corvus at a distance of roughly 45 million light years. Star formation events are seen as clumpy pink regions throughout the merger system. The two nuclei of the galaxies, bright and yellow, are still separate in this image. Image: AAO/David Malin

When astronomers try to investigate how our Milky Way Galaxy has evolved during its 12 billion year history, they are faced with the problem that the Sun is embedded in the Galaxy's dusty disc, limiting our view of its spiral structure. With the aid of radio and infrared telescopes, which can overcome the visibility problem posed by the Sun, the Galaxy's spiral arms can be roughly mapped, giving an impression of how it would look to an outside observer.

In general, this matches what we would expect from advanced computer modelling of the Galaxy's evolution. However, we also know from these models that large galaxies like ours grow by swallowing many smaller galaxies that would have been formed in the same primordial cloud of gas. This process is often known as 'galactic cannibalism'.
Can we explore the star population of our Galaxy to find evidence of these swallowed-up smaller galaxies? The place to look is in the Galaxy's halo, a roughly spherical cloud of stars that encloses the disc. The view here is less clouded by dust, and traces of the digested remains of the captured galaxies should be detectable in populations of stars that have similar velocities through space, and similar chemical compositions.

This technique of exploring the Galaxy’s history by unpicking it star-by-star is known as Galactic Archaeology, and a major project called GALAH is underway at the Australian Astronomical Observatory that will reveal that history in more detail than ever before.

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