From studying the exquisite images collected by ground- and space-based telescopes over the past century, astronomers have learnt that galaxies can collide. In a paper in Nature, Helmi et al.1 use data from the Gaia space observatory to determine that the Milky Way was hit by a satellite galaxy roughly ten billion years ago. Stars from this galaxy are still around us today to tell the story.
Gaia was launched in 2013 by the European Space Agency as the successor to Hipparcos — a satellite that in 1997 produced the first high-precision catalogue of nearby stars2. Gaia was designed to conduct ongoing observations of the visual characteristics and positions of more than one billion objects in the sky3 (Fig. 1). Such map-making might seem like tedious work, but repeated measurements made by Gaia can also be used to determine precise distances and velocities across the sky for about 1% of all the stars in our Galaxy3.
The information from Gaia can be combined with spectroscopic measurements of velocities along the observer’s line of sight to make videos that show the precise motions of the stars (see, for example, go.nature.com/2atris8). Playing these videos backwards allows astronomers to study how our Galaxy was assembled and how it has evolved.
Helmi and colleagues used the Gaia mission’s second data release, which was published earlier this year4, to analyse the motion of stars near the Sun (within a distance of about 10 kiloparsecs). The authors compared these observations with predictions from simulations in which the Milky Way and a satellite galaxy with 20% of the mass of our Galaxy merged in the past5. The similarities are striking, particularly the detailed motions of some high-velocity stars that orbit the Galactic Centre in the opposite direction to the Sun.
Using astronomical-data catalogues that provide the ages and chemical compositions of stars6, Helmi et al. determined that the Milky Way’s inner halo — a region that surrounds the thick stellar disk — is made up mainly of stars from the satellite galaxy. These stars provide a record of the galactic collision, which the authors estimate took place approximately ten billion years ago.
Several other research groups working with the Gaia data have reached a similar conclusion to that of Helmi and colleagues, using other analytical methods or data catalogues7–9. However, there are small differences between the results of Helmi et al. and those of the other groups, such as the mass of the satellite galaxy, when the collision occurred and whether the event involved a single satellite galaxy or a few smaller galaxies.
One conclusion on which all of the groups agree is that the event might have contributed to the formation of the Milky Way’s thick stellar disk. If a thin disk of stars encircled the Galactic Centre at the time of the merger, the orbits of the stars would have been disrupted. Originally, the stars would have had a specific chemical composition that reflected the young age of the Milky Way. But today, they would be old and relatively poor in metals (elements heavier than helium), and on orbits in a thickened disk. All the research groups reported the possible identification of these old stars in the Gaia data.
Astronomers have speculated for several decades that an ancient satellite galaxy merged with the Milky Way in the past, because such an event could explain differences in the motions and chemical compositions of stars in the neighbourhood of the Sun. For example, one of the most unusual objects in our Galaxy is Omega Centauri — a cluster of stars so distinctive that it is thought to be the core of a satellite galaxy that was disrupted and absorbed by the Milky Way. Researchers have suggested that some of the stars found in the Gaia data might be debris from this event10,11.
Obtaining proof that some stars are associated with a merger required the high precision and large survey area of Gaia, in combination with large databases of the spectral and chemical properties of stars. Over the next decade, several international observatories will carry out massive surveys of the spectra of stars throughout the Milky Way. These surveys will provide new data to identify the characteristics of more stars from the satellite galaxy.
The Gaia mission will continue for another few years, sharpening our vision of the Milky Way. With Gaia’s detection of even more stars that originated in the satellite galaxy, astronomers will be better able to determine the mass of this galaxy, and when the merger occurred. It might even be possible to learn about the star-formation history of the satellite galaxy before it collided with the Milky Way.
Helmi and colleagues named the satellite galaxy Gaia–Enceladus, in honour of the space observatory that provided the crucial data and after one of the Giants of Greek mythology. Enceladus was the offspring of Gaia (Earth) and Uranus (the sky). He was said to be buried under Mount Etna in Italy and responsible for earthquakes in the region. The authors suggest that this is an appropriate name because Gaia–Enceladus was a giant compared with other past and present satellite galaxies of the Milky Way. Furthermore, it shook our Galaxy, leading to the formation of the thick stellar disk. Regardless of the name, it is clear that the history of the merging event is written in the stars.
Nature 563, 43-44 (2018)