Decoding the Milky Way: How Galactic Discs and Chemical Fingerprints Form in the Cosmos

In a recent study led by Matthew D. A. Orkney, researchers dive deep into the chemical makeup of the Milky Way and its neighboring galaxies using computer simulations. The focus is on a specific pattern observed in stars—two distinct groups based on their magnesium and iron content, known as "α-sequences." These patterns, observed in the Milky Way's disc, are like chemical fingerprints that tell us how the Galaxy formed and evolved over time. The team uses the Auriga simulations, a set of thirty high-resolution models of Milky Way-like galaxies, to understand how these patterns came to be and whether they're shaped by cosmic events like galaxy mergers.

Stellar Chemistry and What It Tells Us

The paper begins with an overview of how our position inside the Milky Way allows astronomers to study individual stars in great detail. Observations show that stars fall into two main groups based on their [Mg/Fe] and [Fe/H] ratios—elements produced in different types of supernovae. One group, the "high-α" stars, is older and concentrated closer to the center of the Galaxy. The other, "low-α" stars, formed more recently and are found further out. This chemical divide may relate to different phases of star formation or cosmic events like the merger with the Gaia-Sausage Enceladus (GSE), an ancient dwarf galaxy that collided with the Milky Way.

Simulating the Formation of Galaxies

To test these ideas, the authors explore the Auriga simulations, which include realistic physics such as star formation, supernova feedback, and galaxy mergers. Each simulated galaxy is evolved from early times in the universe to the present day. By examining how stars in these galaxies behave chemically and dynamically, the team can compare the simulated "Milky Ways" to our own. They find a wide variety in the chemical structures of galactic discs. Some galaxies show clear bi-modal patterns like the Milky Way, while others have a smoother or more chaotic distribution of elements.

Mergers Aren’t the Only Story

One of the study’s key findings is that not all galaxies with a chemical bi-modality experienced a GSE-like merger. In fact, the presence of two chemical sequences seems to depend more on how the rate of star formation changed over time rather than on specific merger events. A burst in star formation, followed by a quiet period, can create a chemical gap between the high- and low-α stars. This happens because the explosions from massive stars enrich the galaxy in α-elements quickly, while iron from slower Type Ia supernovae accumulates later, lowering the [Mg/Fe] ratio in new stars.

How Gas Shapes Chemistry

The simulations also show that chemical mixing from galaxy mergers is not enough to explain the metallicity overlap between the two α-sequences. While mergers can bring in fresh, low-metal gas, they don’t consistently dilute the host galaxy’s gas enough to reset the chemical clock. Instead, long-term gas accretion from the galaxy’s surroundings—like the circum-galactic medium—is a more plausible source for forming the low-α stars. This insight challenges the idea that a single dramatic event like the GSE is necessary to explain the Milky Way’s chemical structure.

The Role of Star Formation Rates

A strong trend uncovered by the team is that changes in the star formation rate (SFR) are directly tied to how these chemical sequences form. When the SFR is high, many stars form rapidly and tend to have high [Mg/Fe] ratios. If the SFR slows down significantly, iron from older stars builds up, and newer stars start to form with lower [Mg/Fe]. This creates a natural separation in the chemical abundance of stars. In simulations with especially clear gaps between high- and low-α stars, such dips in SFR are pronounced and often linked to gas exhaustion or minor merger events.

Galactic Diversity, Galactic Insight

Overall, Orkney and collaborators demonstrate that there’s no one-size-fits-all pathway for galaxies to develop the kind of chemical fingerprints we see in the Milky Way. Their work suggests that internal processes, like how efficiently gas is turned into stars, may be just as important as cosmic events in shaping galactic discs. This study not only helps us understand the Milky Way’s past but also offers a broader context for how disc galaxies grow and evolve across the universe.

Source: Orkney