Hubble solves merger mystery from Milky Way's early years

Introduction

New observations from NASA's Hubble Space Telescope have uncovered evidence of a dwarf galaxy that collided with and merged into the young Milky Way about 12 billion years ago. The finding extends our knowledge of the galaxy's history 1.8 billion years farther back than previously known, and the results are published in the journal Nature Astronomy.

The Milky Way grew to its current size in part by consuming smaller galaxies. The most recent massive merger involved the Sagittarius dwarf galaxy, which began more than 6 billion years ago and is still ongoing. Earlier, researchers found that the Milky Way consumed a dwarf galaxy called Gaia-Sausage-Enceladus about 10 billion years ago. Both observations and simulations had suggested that another large merger preceded these, but the specifics were heavily debated.

The discovery

Working as part of the ARMA (Cluster Ages to Reconstruct the Milky Way Assembly) project, the team used dense conglomerations of ancient stars called globular clusters as tracers. The researchers analyzed Hubble observations of 39 globular clusters in the inner 20,000 light-years of the Milky Way. Using Hubble's high-resolution and deep imaging, they could measure the ages and metal content of these clusters with unprecedented precision.

They found that the clusters fell into three distinct groups: one linked to Gaia-Enceladus, one belonging to the original Milky Way, and a third distinct population. This third group, the team determined, were born in a dwarf galaxy that merged with the Milky Way around 12 billion years ago—just 2 billion years after the Big Bang.

The dwarf galaxy, named LKH, contained roughly 500 million times the mass of the sun in stars. That represents a significant fraction of the Milky Way's mass at the time, according to the researchers. The name LKH honors three earlier research papers that laid the groundwork for this work.

Significance and context

Chiara Zerbinati, of the Department of Physics and Astronomy at the University of Bologna, said in a statement: "This work tells us what happened to the Milky Way in its infancy. It sheds light on a particularly significant event that influenced the entire subsequent evolution of the galaxy. If one of humanity's great questions is 'where do we come from?', we offer at least a piece of the answer."

Davide Massari, from the Italian Institute for Astrophysics (INAF) and lead author of the study, added: "We started with the ambition of reconstructing the assembly history of the Milky Way, that is, all the galactic merger events that led to its current appearance. We used globular clusters as tracers..." He also noted, "We have shown that stars born in external galaxies also need to be considered."

Fernando Aguado-Agelet, a co-author, commented: "Hubble is observing globular clusters that have never been studied before, and this will help us characterize the merger events that are far back in time in the Milky Way's history."

The team's work relied on data from both Hubble and the European Space Agency's Gaia mission. The discovery of the merger helps astronomers better understand the early assembly of our galaxy, before the Gaia-Enceladus event that had previously marked the earliest known major collision.

The merger with LKH occurred about 11.8 billion years ago, or roughly 2 billion years after the Big Bang. At that time, the Milky Way was still in its infancy, and the addition of a galaxy of this size would have significantly influenced its structure and stellar content.

Perspectives

Researchers' perspective

The team behind the discovery views the finding as a crucial piece in understanding the Milky Way's early history. They emphasize the role of globular clusters as tracers and the importance of Hubble's precision in pushing back the timeline. As Zerbinati put it, the work "sheds light on a particularly significant event that influenced the entire subsequent evolution of the galaxy."

Ongoing investigation

The identification of the LKH merger opens new questions about the sequence of galactic collisions that shaped the Milky Way. While this merger is now the earliest known, astronomers continue to study globular clusters to uncover even earlier events, as Aguado-Agelet noted: "Hubble is observing globular clusters that have never been studied before."