Revisiting the Two-Infall Model: How the Milky Way’s Bulge Formed in Two Acts
The study by Miller et al. (2025) explores how the Milky Way’s central bulge, the dense, star-packed region at the galaxy’s core, formed. Using a computational framework called OMEGA++, the authors test over 30,000 galactic chemical evolution (GCE) models combined with machine learning to uncover the bulge’s formation history. Their findings support a two-stage formation process: an early, rapid starburst followed by a slower, smaller wave of star formation billions of years later.
The Problem: A Complex Galactic Heart
The Milky Way’s bulge contains stars with very different ages and compositions, implying multiple formation pathways. Earlier theories suggested either a “classical” bulge, formed quickly through early galaxy mergers, or a “secular” bulge, built gradually by the inward flow of disk stars through bar-like structures. However, observations of the bulge’s metallicity distribution function (MDF), a measure of how metal-rich its stars are, show two distinct peaks. This bimodality implies at least two separate phases of star formation, motivating the authors to revisit the two-infall model, where two waves of gas collapse and star formation occur at different times.
Building the Model: Simulating Galactic Chemistry
Using OMEGA++, Miller and collaborators simulate the bulge as a single, well-mixed reservoir of gas that evolves chemically over time. Their “two-infall” setup assumes one rapid early inflow of gas (lasting less than 0.1 billion years) that triggers an intense starburst, followed by a second infall several billion years later. This later phase occurs more slowly and less efficiently, introducing younger, more metal-rich stars. The authors explore a vast parameter space, varying the timing, mass, and duration of gas inflows as well as supernova contributions and stellar yields. To find the best-fitting model, they use a hybrid genetic algorithm combined with Markov Chain Monte Carlo (MCMC), an approach that balances broad exploration and precise refinement of solutions.
The Best-Fit Scenario: Two Distinct Starbursts
Their optimized model reveals that the bulge formed in two key episodes. The first infall occurred around 0.1 billion years after the Big Bang, lasting only about 0.09 billion years, with a high star-formation efficiency (SFE ≈ 3 Gyr⁻¹). This early burst created the old, α-enhanced (oxygen and magnesium-rich) stars typical of rapid formation. The second infall began roughly 5 billion years later, with a slower gas inflow and lower efficiency (about 70% less than the first). This second wave reproduced the observed metal-rich peak of the bulge’s MDF and explained the decline in α-elements at higher metallicity. Together, these results suggest that the bulge’s stellar populations arose from a mix of classical collapse and later, bar-driven secular processes.
Understanding Degeneracy: Why Parameters Interact
A key finding of the study is that many parameters, like the timing of infall, star formation efficiency, and mass ratios, are strongly interdependent. Through statistical methods such as principal component analysis, the authors show that while the data constrain combinations of these parameters, no single variable can be isolated. For example, a slower second infall can be balanced by a higher efficiency of star formation, leading to the same final chemical makeup. This degeneracy explains why the bulge’s history has been so challenging to untangle.
Implications: A Composite Galactic Core
The results imply that the Milky Way’s bulge is a composite structure. The majority of its stars formed quickly during the galaxy’s earliest epochs, but a smaller, younger component formed several billion years later, possibly connected to bar formation or merger events like the Gaia–Sausage–Enceladus collision. The model aligns more closely with the Joyce et al. (2023) age–metallicity relation, favoring older bulge stars than earlier estimates by Bensby et al. (2017). In this view, the bulge’s metal-rich stars are the chemical “echo” of later accretion and star formation processes that reshaped the Milky Way’s center.
Limitations and Future Work
While successful, the model assumes the bulge is a single, well-mixed zone, meaning it cannot yet reproduce spatial metallicity gradients or the bulge’s complex 3D structure. Miller et al. note that future versions of their model, incorporating multi-zone simulations and radial mixing, could better capture the observed chemical and kinematic diversity of the Milky Way’s heart.
Source: Miller
