Inside-Out Chemistry: Unveiling the Metal Distribution in Simulated Milky Way-Mass Galaxies
In their recent study, Iza et al. investigate how different parts of galaxies like the Milky Way become enriched with elements heavier than hydrogen and helium, which astronomers refer to as “metals.” Using detailed computer simulations from the Auriga Project--a suite of realistic models of Milky Way-like galaxies--the team studies how metals are spread throughout galactic components like the disc, bulge, and halo. These simulations are run using the AREPO code, which captures the complex behavior of gas, stars, and magnetic fields in a cosmological setting.
How Galaxies Form and Evolve
Galaxies grow over time by accumulating gas and merging with smaller systems, all within the invisible framework of dark matter. As stars age and explode in supernovae, they release new elements into the surrounding gas, enriching future generations of stars. The authors note that this chemical history can vary greatly depending on where stars form. For instance, stars in central, spheroidal parts of a galaxy (like the bulge and halo) often formed earlier and contain more alpha-elements like oxygen, while younger stars in the disc tend to have more iron.
Simulated Galaxies and the Reference Case
To better understand these differences, Iza and collaborators first selected a sample of well-behaved galaxies from the Auriga simulations--those that haven't undergone recent dramatic mergers and have developed prominent disc structures. One galaxy in particular, Au9, serves as their reference case. To separate stars into different galactic components, they applied a method based on the stars’ orbits (how circular their paths are) and how tightly bound they are to the galaxy’s center. This allowed them to sort stars into the halo, bulge, and disc, which could then be studied individually.
Tracing Chemical Evolution Through Stellar Ages
The team then explored the age-metallicity relation (AMR) for these components. In Au9 and other galaxies, they found that older stars tend to have lower iron content ([Fe/H]), and younger stars are richer in iron. This trend is strongest in the disc, where recent star formation has enriched the gas over time. Meanwhile, the halo contains stars that are both old and metal-poor, consistent with early star formation followed by little chemical evolution. The bulge, lying between the disc and halo, contains stars of intermediate age and metallicity.
The Oxygen-Iron Clock
Another key result came from comparing two types of metals: iron and oxygen. Since oxygen is produced quickly in massive stars and iron takes longer to build up through supernovae Type Ia, the ratio of oxygen to iron ([O/Fe]) acts like a galactic clock. The authors show that stars in the halo have high [O/Fe] and low [Fe/H]--signatures of early formation--while disc stars show the opposite trend. Again, the bulge shows intermediate values, reinforcing the idea that these components formed on different timescales.
A Consistent Picture of Galaxy Formation
In summary, this study confirms that simulated Milky Way-mass galaxies develop in a way that reflects the “inside-out” formation scenario: older, less enriched stars form first in central regions (bulge and halo), while younger, more metal-rich stars form later in the extended disc. The careful analysis of metal abundances provides a powerful tool for tracing the history of galaxies. As the authors note, future work will explore how these patterns emerge over time, deepening our understanding of galactic evolution.
Source: Iza
