Giant That Shaped the Early Days of the Milky Way Discovered

Artistic rendering of Enceladus being devoured by a Milky Way-like galaxy 

 

Credit: René van der Woude, Mixr.nl

In a previous paper, Helmi had already described a huge ‘blob’ of stars sharing a common origin1. Now, she shows that stars from this blob in the halo are the debris from the merging of the Milky Way with a galaxy which was slightly more massive than the Small Magellanic Cloud, some ten billion years ago. The galaxy is called Gaia-Enceladus, after the Giant Enceladus who in Greek mythology was born of Gaia (the Earth goddess) and Uranus (the Sky god).

The data on kinematics, chemistry, age and spatial distribution from the native Milky Way stars and the remnants of Gaia-Enceladus reminded Helmi of simulations performed by a former PhD student, some ten years ago. His simulations of the merging of a large disc-shaped galaxy with the young Milky Way produced a distribution of stars from both objects, which is totally in line with the Gaia data. “It was amazing to look at the new Gaia data and realize that I had seen it before!,” says the astronomer.

Computer simulation of the merger between a galaxy like the young Milky Way, whose stars are shown in cyan, and a smaller galaxy, indicated in red. According to a study based on the second data release of ESA’s Gaia mission, astronomers estimate that a significant merging event like the one shown in this animation has occurred during the Milky Way’s early formation stages, ten billion years ago. Such a merger has led to two important components of our Galaxy, the halo and the thick disc. Stars belonging to the accreted galaxy, which has been named Gaia-Enceladus, are interspersed with the Milky Way stars and can be seen across the entire sky, but could only be revealed thanks to Gaia’s extraordinary precision.

The movie shows an N-body simulation of the merger of a Milky Way-like galaxy (with its stars in blue) and a smaller disky galaxy resembling the Small Magellanic Cloud in mass (with it stars in red). At the beginning the two galaxies are clearly separated, but gravity pulls them together and this leads to the full accretion of the smaller one. Distinguishing the accreted stars from the rest is not easy by the final stage, but it is possible using the motions of the stars and their chemical  https://www.astro.rug.nl/~ahelmi/Villalobos-i60-final-v1.mp4

Contacts and sources: 
University of Groningen

Citation: . The merger that led to the formation of the Milky Way’s inner stellar halo and thick disk
Amina Helmi, Carine Babusiaux, Helmer H. Koppelman, Davide Massari, Jovan Veljanoski, Anthony G. A. Brown. Nature, 2018; 563 (7729): 85 DOI: 10.1038/s41586-018-0625-x    .