A Galaxy in Transition: Tracing the Milky Way's Disc Evolution After the Gaia-Sausage-Enceladus Merger
Funakoshi et al. explore how our home galaxy, the Milky Way, evolved structurally and dynamically after a major cosmic event, the Gaia-Sausage-Enceladus (GSE) merger. Using advanced models and a large sample of stars, the authors trace how the disc of the Milky Way transitioned from a thick, compact structure to the broad, thin disc we see today. Their analysis not only identifies when this transformation happened but also hints at a temporary shrinking of the galaxy’s gas disc during that period.
Data and Methodology: Watching Stars Move to Learn the Past
The Milky Way’s disc has two main components: the thick disc and the thin disc. The thick disc is older, with stars that move more randomly and sit higher above the galaxy’s midplane. The thin disc, by contrast, is younger, flatter, and more orderly. The authors investigate when and how the galaxy switched from forming thick disc stars to thin disc stars. They focus on a timeline about 8–12 billion years ago, when a dwarf galaxy called GSE merged with the Milky Way. This merger is thought to have dumped large amounts of gas into the galaxy, sparking intense star formation and setting the stage for the thin disc to form.
Sample Selection and Modelling: A Kinematic Portrait of the Disc
To track this transformation, Funakoshi and colleagues used a sample of 16,617 red giant stars from the APOGEE and Gaia surveys. They used machine learning (via the BINGO model) to estimate the ages of these stars and applied a sophisticated statistical method to model their motion through space. Rather than analyzing where stars are located, they modeled how stars move, an approach that avoids many biases from incomplete data. The main parameters they studied included how stars’ speeds vary across the galaxy and how spread out they are, both of which change as the galaxy evolves.
Results: Catching the Moment of Transition
One of the key discoveries was a sharp transition in the structure of the disc around 10 billion years ago. Before this time, stars tended to form in a compact, dense region (with a scale length around 1.7 kiloparsecs), characteristic of the thick disc. Afterward, new stars began forming in a disc that grew steadily larger over time, a hallmark of thin disc growth. The team found that right at the transition point, stars had unusually short scale lengths, indicating a brief period when the galaxy's star-forming gas disc shrank before expanding again. This feature had been predicted in simulations but was now supported by observational data.
Validation Through Simulation: Lessons from Auriga
To test this interpretation, the authors applied their method to a simulated galaxy from the Auriga project that had experienced a similar transition. In the simulation, they found a clear dip in disc scale length during the thick-to-thin disc transition, matching the behavior seen in the Milky Way. This dip was associated with a pause in gas inflow and a switch from cold-mode to hot-mode gas accretion, two different ways galaxies pull in gas from their surroundings. During this switch, the central gas supply temporarily shrank, leading to a more compact disc before resuming growth in the thin disc phase.
Implications: Shrinking Discs and Accretion Modes
Finally, the authors place this transformation within a broader picture of galactic evolution. They suggest that the GSE merger, by bringing in fresh gas and increasing the Milky Way’s mass, may have triggered both a starburst and a change in how gas entered the galaxy. As a result, the Milky Way briefly became less able to form stars across its full disc, causing the observed dip in scale length. This transition also aligns with chemical changes in the stars forming at that time, linking the structural, dynamical, and chemical evolution of the galaxy into one coherent timeline.
Conclusion: A Galactic Pivotal Moment Uncovered
Overall, this study sheds light on a key moment in the Milky Way’s history, when a violent merger helped reshape the galaxy and laid the groundwork for the thin disc we see today. By combining stellar motion, age, and simulations, the authors provide a compelling case for how galaxies evolve through cosmic time.
Source: Funakoshi
