Gaia-Enceladus (2018): The Merger That Shaped the Milky Way

Gaia-Enceladus is a galaxy that merged with the young Milky Way about 10 billion years ago. Amina Helmi and colleagues identified it in 2018 from star motions, chemistry and ages measured by ESA’s Gaia mission. Its debris dominates our galaxy’s inner halo and helped form the thick disk.

The question

The Milky Way’s stellar halo, a sparse cloud of old stars surrounding the disk, has long been thought to be built partly from smaller galaxies that the Milky Way swallowed. But a galaxy that merged billions of years ago leaves no obvious shape behind; its stars are scattered across the sky. The only lasting trace is in how the stars move and what they are made of. Until Gaia, nobody had measurements precise enough, for enough stars, to read that record and tell which halo stars came from where.

What the paper found

• The inner stellar halo is dominated by debris from a single galaxy, which the authors named Gaia-Enceladus.
• At the time it fell in, Gaia-Enceladus was slightly more massive than today’s Small Magellanic Cloud.
• The merger had an estimated mass ratio of about 4 to 1, a major event for the young Milky Way.
• It dynamically heated the Milky Way’s early disk and contributed to forming the thick disk about 10 billion years ago.
• Hundreds of RR Lyrae variable stars and 13 globular clusters can be linked to Gaia-Enceladus through their orbits.

How they did it

The team combined kinematics, the positions and velocities of stars, from Gaia’s second data release with measurements of the stars’ chemistry and ages. Stars that arrived together in a merger keep similar orbital properties, such as their energy and their angular momentum, long after they are spread across the sky. In Gaia’s data a large group of halo stars, spread over nearly the whole sky, shared slightly retrograde, elongated orbits: they circle the galaxy a little backward compared with the disk. Their chemistry follows a distinct sequence, the signature of stars born in a different, smaller galaxy, and their ages fit a merger about 10 billion years ago. The result also matches simulations predicting that the inner halo should be built from just a few massive progenitors.

Why it mattered

Gaia turned galactic archaeology, reconstructing a galaxy’s history from its stars, into a precise science. This merger is now considered a defining event in the Milky Way’s history: the moment the young galaxy absorbed a large neighbor and its disk was puffed up into the thick component we see today. It is a real, local example of the galaxy mergers that Toomre and Toomre modeled in 1972.

What we know now

Gaia-Enceladus (also called Gaia-Sausage-Enceladus) is now one of several known mergers in our galaxy’s past. In 2026 a Hubble study of globular clusters found an even earlier merger, with a dwarf galaxy named LKH, about 11.8 billion years ago. Looking forward, the Milky Way’s encounter with Andromeda is less certain than once thought: a 2025 study gives it only about a 50% chance within 10 billion years. In September 2026 the Kavli Prize in Astrophysics went to Vasily Belokurov, Amina Helmi and Rodrigo Ibata for Gaia-based work on the Milky Way’s merger history, as reported by the Gaia mission. See also our galaxies guide.

Who wrote it

Amina Helmi led the study with five co-authors, including Carine Babusiaux, Helmer Koppelman, Davide Massari, Jovan Veljanoski and Anthony Brown. Helmi works at the Kapteyn Astronomical Institute of the University of Groningen.

Read the original

Helmi, A., Babusiaux, C., Koppelman, H. H., Massari, D., Veljanoski, J. and Brown, A. G. A. (2018). “The merger that led to the formation of the Milky Way’s inner stellar halo and thick disk.” Nature 563, 85-88. doi:10.1038/s41586-018-0625-x

Free to read: the arXiv preprint.