About 11.8 billion years ago, the Milky Way collided and merged with a dwarf galaxy, absorbing a significant amount of stellar material from it. This is the conclusion reached by astronomers who studied globular clusters using the Hubble Space Telescope and data from the Gaia mission. The results were published in the journal Nature Astronomy.

A third group of "foreign" clusters

The researchers identified a group of globular star clusters that, in terms of age, chemical composition, and stellar motion, do not resemble objects that formed within the Milky Way itself. They are also unrelated to the known merger with the Gaia‑Sausage‑Enceladus galaxy, which occurred about 10 billion years ago.

According to the scientists, these clusters originated in a dwarf galaxy they named LKH and were captured by the Milky Way about 11.8 billion years ago. In other words, this merger happened even earlier than the collision with Gaia‑Sausage‑Enceladus.

How large the "donor" galaxy was

By studying the number and mass of the captured globular clusters, the researchers estimated the total mass of LKH at roughly 500 million solar masses. For the young Milky Way, which at that time was noticeably smaller than it is today, such a galaxy could have made a substantial contribution to its growth.

According to the authors, the collision may have not only increased the mass of our galaxy. It could have also triggered a burst of star formation, and the stars arriving from LKH brought their own chemical "contribution" to the Milky Way's subsequent evolution.

What the discovery changes

Previously, it was thought that in the early stages of the Milky Way's development, the dominant role was played by stars formed inside the galaxy itself. The new data show that some of the building material came from outside—as a result of an ancient merger with another system.

In brief

Astronomers have found traces of a merger between the Milky Way and the dwarf galaxy LKH about 11.8 billion years ago—earlier than the known collision with Gaia‑Sausage‑Enceladus. The mass of the captured galaxy is estimated at roughly 500 million solar masses. The event could have accelerated the Milky Way's growth and influenced its chemical composition. The work was published in the journal Nature Astronomy.