Our home galaxy, the Milky Way, reached its present size partly by absorbing smaller galaxies. Now, new data obtained with NASA’s Hubble Space Telescope provide compelling evidence of a dwarf galaxy merging with the young Milky Way during the earliest stages of its evolution. This discovery extends our knowledge of the history of our galaxy 1.8 billion years further into the past than was previously known.
History of the Milky Way’s Formation
Today, the Milky Way is a massive spiral galaxy containing hundreds of billions of stars. However, our galaxy was not always so large; it grew through the formation of new stars from its own gas clouds and through the absorption of stars, gas, and dark matter from other galaxies as a result of mergers. This was reported by Phys.org.
The most recent major merger in the history of our galaxy occurred with the Sagittarius Dwarf Galaxy; it began more than 6 billion years ago and continues to this day. Looking even further into the past, researchers found that 10 billion years ago the Milky Way absorbed another dwarf galaxy known as Gaia-Sausage-Enceladus. This ancient merger had a significant impact on the structure of our galaxy’s stellar disk. Other, smaller mergers also occurred between these two events.
But the history of our galaxy does not end there. Both observations and simulations indicate that these two mergers were preceded by another major merger, although the details of that event remain the subject of intense debate. Now, Hubble has found compelling evidence of an earlier merger that occurred approximately 11.8 billion years ago, just 2 billion years after the Big Bang.
Hubble Looks into the Early History of Our Galaxy
Large-scale astronomical surveys and precise data obtained from spacecraft such as the European Space Agency’s (ESA) Gaia mission have played a crucial role in reconstructing the history of our galaxy. The further back into the Milky Way’s past scientists attempt to look, the more difficult it becomes to determine exactly what happened. When our galaxy was young, it was smaller and much closer in size to the galaxies with which it collided. It was also more chaotic, and it is entirely possible that traces of mergers were erased over billions of years.
It was into this obscure past that Hubble looked. Researchers used Hubble to study some of the Milky Way’s globular clusters: enormous, roughly spherical collections of tens of thousands to several million stars. Globular clusters contain some of the oldest stars in our galaxy and can serve as cosmic archaeological sites preserving stars from other galaxies that were collected by the Milky Way.
“Thanks to the high-resolution and deep imaging obtained with Hubble, we were able to determine the ages and metal content of these clusters with unprecedented precision,” said Chiara Zerbinati, a co-author of the study from the University of Bologna in Italy. “Combined with measurements from the Gaia mission, this allowed us to isolate a population of globular clusters that differ from the others. These are clusters that formed in the LKH galaxy, and they give us insight into when that galaxy was absorbed by ours and what its mass was.”
Ancient Globular Clusters — Traces of a Vanished Galaxy
The team analyzed Hubble observations of 39 globular clusters in the inner region of our galaxy within a radius of 20,000 light-years, where traces of the most ancient mergers should have been preserved. They expected this sample to include both globular clusters that formed in the young Milky Way and those captured from the Gaia-Sausage-Enceladus dwarf galaxy approximately 10 billion years ago.
Using Hubble’s highly sensitive observations to determine the precise age of each cluster and its associated metallicity — the abundance of elements heavier than helium — the researchers identified a third population of globular clusters in the inner regions of our galaxy. The team found that these clusters are older than the group formed as a result of the Gaia-Sausage-Enceladus merger, but younger than those that formed in the Milky Way itself, regardless of their metal content.
This indicates that these clusters originated from a separate and even earlier merger, during which the Milky Way absorbed a dwarf galaxy with a stellar mass of approximately 500 million times the mass of the Sun — a significant fraction of the mass of our galaxy at that time. They named this dwarf galaxy Low-energy-Kraken-Heracles, or LKH, in honor of three previous scientific papers that supported the idea of a merger during the early history of our galaxy.
Implications for Understanding the Evolution of the Milky Way
Such a major merger at such an early stage in the formation of the Milky Way has profound implications for the evolution of our galaxy.
“Some previous studies argued that the earliest stages of our galaxy’s evolution were defined by stars born exclusively within our galaxy,” said Davide Massari, lead author of the study. “In this work, we demonstrated that stars born in external galaxies must also be taken into account.”
The team plans to continue its work deciphering the history of the Milky Way by studying its globular clusters, with the goal of characterizing all of the major mergers our galaxy has experienced throughout cosmic history.
