Unveiling the Chemical Map of the Milky Way’s Thin Disc

The study explores the metallicity gradients of the Milky Way's thin disc through main-sequence stars, providing insights into the Galaxy's chemical and dynamical evolution. Led by Furkan Akbaba, the team utilized data from the GALAH DR3 and Gaia DR3 surveys, emphasizing the dynamic behaviors of stars over their lifetimes. Metallicity gradients reveal how elements like iron (Fe) and magnesium (Mg) are distributed across the Galaxy, reflecting its formation and evolution.

Data and Methods

The research uses spectroscopic data from GALAH DR3 and astrometric data from Gaia DR3 to analyze over 66,000 thin-disc main-sequence stars. These stars were selected based on strict quality controls, ensuring accurate chemical and orbital measurements. Key metrics like metallicity ([Fe/H]), alpha-element ratios ([α/Fe]), and magnesium ([Mg/H]) were examined alongside dynamic properties like guiding radii (average orbital distances) and traceback radii (approximations of earlier orbits).

To model stellar orbits and calculate metallicity gradients, the study applied the GALPY framework, which simulates stellar dynamics using an axisymmetric Galactic potential. Radial orbital variations — changes in star positions due to Galactic forces — were considered to refine the metallicity measurements.

Results

1. Metallicity Gradients:

   1. The study identified a negative radial metallicity gradient for [Fe/H] of approximately -0.074 dex/kpc, indicating that stars closer to the Galactic center are generally more metal-rich.

   2. For [Mg/H], the gradient was similarly negative, whereas [α/Fe] showed a slight positive trend, suggesting a stable abundance of alpha elements across radii.

2. Role of Radial Orbital Variations:

   1. The team found that orbital shifts over time minimally impacted metallicity gradients, with a maximum effect of 6%. Older stars were more likely to exhibit these variations due to longer exposure to Galactic forces.

3. Time Evolution of Gradients:

   1. Observations revealed that metallicity gradients persist throughout the Galaxy's history, consistent with models suggesting an inside-out formation process. This means the Milky Way’s disc likely grew outward from a dense, metal-rich core.

4. Age Dependency:

   1. Younger stars showed steeper gradients, reflecting recent chemical enrichment, while older stars exhibited flatter gradients, shaped by billions of years of orbital drift.

Discussion and Comparison

The results align with prior studies and theoretical models of Galactic chemical evolution. Notably, comparisons with Spitoni et al.'s chemical evolution model highlighted the utility of combining observational and simulated data to decode the Milky Way's history. The study emphasized that radial migration—movement of stars across the disc—complicates direct interpretations but has minimal statistical effect on the overall gradient.

Conclusion

This work advances our understanding of the Milky Way's chemical structure, confirming that metallicity gradients serve as a robust tool for probing Galactic evolution. The findings underscore the importance of considering dynamic factors like radial orbital variations, especially in older stellar populations. Future research aims to expand the sample to other Galactic regions, refining the models that map our Galaxy’s intricate past.

Source: Akbaba