Free Astronomy Magazine January-February 2019

49 JANUARY-FEBRUARY 2019 SPACE CHRONICLES Way. The stars now form most of our Galaxy’s inner halo – a diffuse compo- nent of old stars that were born at early times and now surround the main bulk of the Milky Way known as the central bulge and disc. The galactic disc itself is composed of two parts. There is the thin disc, which is a few hundred light years deep and con- tains the pattern of spiral arms made by bright stars. And there is the thick disc, which is a few thou- sand light years deep. It contains about 10–20 per- cent of the Galaxy’s stars yet its origins have been difficult to determine. According to the team’s simulations, as well as supplying the halo stars, the accreted galaxy could also have disturbed the Milky Way’s pre-exist- ing stars to help form the thick disc. “We became only certain about our interpretation after complementing the Gaia data with additional infor- mation about the chemical composi- tion of stars, supplied by the ground- based APOGEE survey,” says Carine Babusiaux, Université Grenoble Alpes, France, and second author of the paper. Stars that form in different galaxies have unique chemical compositions that match the conditions of the home galaxy. If this star collection was indeed the remains of a galaxy that merged with our own, the stars should show an imprint of this in their composition. And they did. The astronomers called this galaxy Gaia-Enceladus after one of the Gi- ants in ancient Greek mythology, who was the offspring of Gaia, the Earth, and Uranus, the Sky. “According to the legend, Enceladus was buried because they all move along elongated tra- jectories in the oppo- site direction to the majority of the Gal- axy’s other hundred billion stars, including the Sun. They also stood out in the so-called Hertz- sprung-Russell diagram – which is used to com- pare the colour and brightness of stars – indicating that they belong to a clearly dis- tinct stellar population. The sheer number of odd-moving stars in- volved intrigued Ami- na and her colleagues, who suspected they might have something to do with the Milky Way’s formation history and set to work to un- derstand their origins. In the past, Amina and her research group had used computer simula- tions to study what happens to stars when two large galaxies merge. When she compared those to the Gaia data, the simulated results matched the observations. “The col- lection of stars we found with Gaia has all the properties of what you would expect from the debris of a galactic merger,” says Amina. In other words, the collection is what they expected from stars that were once part of another galaxy and have been consumed by the Milky T wo artist’s impression of the merger between the Gaia-Ence- ladus galaxy and our Milky Way, which took place during our Galaxy’s early formation stages, 10 billion years ago. [ESA (artist’s impression and composition); Koppelman, Villa- lobos and Helmi (simulation); NASA/ESA/Hubble (galaxy image)]