# Rediscovering the Milky Way with orbit superposition approach and APOGEE data II. Chrono-chemo-kinematics of the disc

**Authors:** Sergey Khoperskov, Matthias Steinmetz, Misha Haywood, Glenn van de Ven, Davor Krajnovic, Bridget Ratcliffe, Ivan Minchev, Paola Di Matteo, Nikolay Kacharov, Léa Marques, Marica Valentini, Roelof S. de Jong  
**arXiv:** [2411.16866](https://arxiv.org/abs/2411.16866)  
**Date:** 2024-11-25  
**Summary page:** https://www.summarizepaper.com/en/paper/2411.16866v1/

## Key points

- Authors delve deep into the structure of the Milky Way (MW) disc, leveraging a novel orbit superposition approach and data from APOGEE DR 17
- Detection of periodic azimuthal metallicity variations within 6-8 kpc range, peaking along the bar major axis
- Radial metallicity profile shows decline outside solar radius and nearly flat profile in inner region due to old, metal-poor high-alpha populations
- Coexistence of thick disc and high-alpha populations with comparable mass fractions
- Reconstructed 3D model quantifies differences in stellar population content between components, providing insights into galactic evolution
- Nuanced analysis reveals subtle variations in alpha/Fe-bimodality across different galactic radii (5-9 kpc)
- Proposal of inner/outer disc dichotomy tied to evolutionary stages in Milky Way history, suggesting transition zones with characteristics of both inner and outer discs

## Summary

In this groundbreaking study, the authors delve deep into the structure of the Milky Way (MW) disc, shedding light on its chrono-chemo-kinematic properties with unprecedented detail. Leveraging a novel orbit superposition approach and harnessing data from APOGEE DR 17, they unveil a comprehensive view of the MW disc that has previously been obscured by selection biases and incomplete observational coverage. One key discovery is the detection of periodic azimuthal metallicity variations within a specific range (6-8 kpc), exhibiting an amplitude of 0.05-0.1 dex and peaking along the bar major axis. This finding not only provides insights into the distribution of metals within the MW disc but also hints at underlying dynamical processes shaping its structure. Furthermore, the radial metallicity profile of the MW disc showcases intriguing patterns reminiscent of other spiral galaxies. Outside the solar radius, there is a discernible decline in metallicity, while in the inner region, a nearly flat profile emerges. This phenomenon is attributed to the presence of old, metal-poor high-alpha populations that contribute significantly to the total stellar mass. The authors also highlight the coexistence of geometrically defined thick disc and high-alpha populations within comparable mass fractions. By leveraging their reconstructed 3D model of the MW structure, they quantify differences in stellar population content between these components, providing valuable insights into their respective contributions to galactic evolution. Moreover, by examining the well-known alpha/Fe-bimodality in the MW disc and considering stellar mass weighting effects, they reveal subtle variations across different galactic radii (5-9 kpc). This nuanced analysis helps explain why this bimodality may appear less prominent in external galaxies and simulations of galaxy formation. Building on these findings, a compelling scenario for MW disc formation emerges from analyzing a distinct double age-abundance sequence. The authors propose an inner/outer disc dichotomy closely tied to evolutionary stages in Milky Way history. They suggest that regions like the extended solar vicinity serve as transition zones sharing chemical properties characteristic of both inner (old age-metallicity sequence) and outer discs (young age-metallicity sequence). Overall, this study represents a significant leap forward in our understanding of the intricate dynamics governing our own galaxy's disc structure and offers valuable insights that can inform future research on galactic evolution and formation processes.