A time-resolved picture of our Milky Way's early formation history (2203.12110v1)

Abstract: The formation of our Milky Way can be parsed qualitatively into different phases that resulted in its structurally different stellar populations: the halo and the disk components. Revealing a quantitative overall picture of the Galactic assembly awaits a large sample of stars with very precise ages. Here we report an analysis of such a sample using subgiant stars. We find that the stellar age-metallicity distribution p(age, metallicity) splits into two almost disjoint parts, separated at 8 Gyr. The younger reflecting a late phase of quiescent Galactic disk formation with manifest evidence for stellar radial orbit migration; the other reflecting the earlier phase, when the stellar halo and the old alpha-process-enhanced (thick) disk formed. Our results indicate that the formation of the Galactic old (thick) disk started 13 Gyr ago, only 0.8 Gyr after the Big Bang, and two Gigayears earlier than the final assembly of the inner Galactic halo. Most of these stars formed 11 Gyr ago, when the Gaia-Sausage-Enceladus satellite merged with our Galaxy. Over the next 5--6 Gyr, the Galaxy experienced continuous chemical element enrichment, ultimately by a factor of 10, while the star-forming gas managed to stay well-mixed.

A Time-Resolved Picture of Our Milky Way's Early Formation History

The paper presented in this paper offers a detailed examination of the Milky Way's formation history by utilizing a substantial sample of subgiant stars with precise age determinations. This research delineates the evolutionary phases of our Galaxy and provides insights into the stellar populations contributing to the Milky Way's structural components: the halo and the disk. Utilizing advancements in astronomical data, particularly from the Gaia and LAMOST surveys, this paper contributes significantly to the field by interpreting the Galactic archeology through an expanded stellar sample.

Data and Methodology

The authors employ a large dataset comprising approximately 250,000 subgiant stars identified through Gaia's eDR3 and LAMOST DR7 data releases. The subgiant stars were selected based on their positions in the T_eff-log g diagram and were analyzed for their ages using Yonsei-Yale (YY) stellar isochrones coupled with a Bayesian approach for precise age estimation. The sample spans a broad range in both age (1.5 to 13.8 Gyr) and metallicity ([Fe/H] from -2.5 to 0.4).

The precision in age determination, with a median relative uncertainty of 7.5%, facilitates the exploration of the Galaxy’s age-metallicity distribution, $p(\tau, \mathrm{[Fe/H]})$. This distribution informs readers about the chemical enrichment and assembly history of Galactic structures, critical for understanding the formation processes of our Galaxy.

Results

The stellar age-metallicity distribution bifurcates into two distinct regimes, bifurcated at an age of approximately 8 Gyr. One regime corresponds to a younger, quiescent disk formation phase, characterized by significant radial orbit migration, while the other pertains to the older stellar populations associated with the halo and thick disk, with α-enhanced chemical signatures.

The analysis suggests that the thick disk commenced formation about 13 Gyr ago, closely following the Big Bang by only 0.8 Gyr. This epoch pre-dates the completion of the Galactic halo assembly by approximately 2 billion years. Notably, a major star formation event corresponded with the merging of the Gaia-Sausage-Enceladus with the Milky Way around 11 Gyr ago, enriching the star-forming gas while maintaining a well-mixed ISM.

Angular momentum and α-enhancement indicators allowed the authors to further distinguish between disk and halo populations. The "V-shape" and "Z-shape" structures observed in their respective metallicity-age diagrams enrich the understanding of radial stellar migration and chemical evolution dynamics.

Implications and Future Directions

This work underscores the complexity and multi-phase formation history of the Milky Way, highlighting the role of mergers and secular processes in shaping Galactic structures. The reported findings provide a clearer picture of the Milky Way's chemical evolution and suggest that large-scale spectroscopic and astrometric surveys can offer unprecedented insights into Galactic history.

Future research in astrophysics can build on these results by exploring the connections between stellar dynamics and chemical evolution. Further integrating spectroscopy with new and upcoming astrometric data sets, such as Gaia’s future data releases, will refine our comprehension of galaxy formation processes. Moreover, comparative studies with external galaxies using similarly precise datasets could contextualize the Milky Way's formation in the broader universe.

In conclusion, this paper represents a significant contribution to our understanding of the Milky Way, offering valuable perspectives on its early formation and subsequent evolutionary pathways.