The Age-Thickness Relation of the Milky Way Disk: A Tracer of Galactic Merging History (2412.12304v1)

Abstract: The prevailing model of galaxy formation proposes that galaxies like the Milky Way are built through a series of mergers with smaller galaxies over time. However, the exact details of the Milky Way's assembly history remain uncertain. In this study, we show that the Milky Way's merger history is uniquely encoded in the vertical thickness of its stellar disk. By leveraging age estimates from the value-added LAMOST DR8 catalog and the StarHorse ages from SDSS-IV DR12 data, we investigate the relationship between disk thickness and stellar ages in the Milky Way using a sample comprising Red Giants (RG), Red Clump Giants (RCG), and metal-poor stars (MPS). Guided by the IllustrisTNG50 simulations, we show that an increase in the dispersion of the vertical displacement of stars in the disk traces its merger history. This analysis reveals the epoch of a major merger event that assembled the Milky Way approximately 11.13 billion years ago, as indicated by the abrupt increase in disk thickness among stars of that age, likely corresponding to the Gaia-Sausage Enceladus (GSE) event. The data do not exclude an earlier major merger, which may have occurred about 1.3 billion years after the Big Bang. Furthermore, the analysis suggests that the geometric thick disk of the Milky Way was formed around 11.13 billion years ago, followed by a transition period of approximately 2.6 billion years leading to the formation of the geometric thin disk, illustrating the galaxy's structural evolution. Additionally, we identified three more recent events -- 5.20 billion, 2.02 billion, and 0.22 billion years ago -- potentially linked to multiple passages of the Sagittarius dwarf galaxy. Our study not only elucidates the complex mass assembly history of the Milky Way and highlights its past interactions but also introduces a refined method for examining the merger histories of external galaxies.

• The paper identifies a major merger event 11.13 billion years ago, evidenced by a significant increase in disk thickness.
• It utilizes LAMOST and SDSS data alongside IllustrisTNG50 simulations to analyze the vertical dispersion of stars across age groups.
• The study traces the evolution from a thick to thin disk structure, revealing insights into both major and minor merger events in the Milky Way.

The Age-Thickness Relation of the Milky Way Disk: A Tracer of Galactic Merging History

The paper "The Age-Thickness Relation of the Milky Way Disk: A Tracer of Galactic Merging History" presents an insightful analysis of the Milky Way's stellar disk and its connection to the galaxy's merger history. The authors investigate a novel method of examining the age-thickness relation of the disk as an indicator of past galactic interactions and merging events. They utilize comprehensive datasets from the LAMOST DR8 catalog and StarHorse ages from SDSS-IV DR12, focusing on Red Giants (RG), Red Clump Giants (RCG), and Metal-Poor Stars (MPS) samples, to drive this analysis.

Key Findings and Highlights

• Merger Events Timing: The paper identifies a key major merger event approximately 11.13 billion years ago, associated with a notable increase in disk thickness. This event aligns with the Gaia-Sausage Enceladus (GSE) merger. The authors also suggest possible earlier mergers occurring around 1.3 billion years after the Big Bang, although this remains less definite due to potential data limitations.
• Stellar Disk Evolution: The research demonstrates that the geometric thick disk of the Milky Way began forming approximately 11.13 billion years ago. A transition to a thin disk structure occurred over the subsequent 2.6 billion years, revealing the structural evolution through these epochs.
• Quantitative Analysis: Using the IllustrisTNG50 cosmological simulations, the authors showed that observing the increase in vertical dispersion among the stars could trace significant merger events. They identified multiple epochs of more recent interactions potentially linked to the Sagittarius dwarf galaxy, occurring around 5.20, 2.02, and 0.22 billion years ago.

Methodology

The authors employed age data from large astronomical surveys to calculate the vertical thickness of the Milky Way's stellar disk over time. By analyzing the dispersion in vertical positions of different age bins, they discerned periods where increased thickness suggested major merger events. The paper ties the resultant age-disk thickness relation to observed features in simulated galaxy evolution models, particularly those from the IllustrisTNG50 simulation. This application underscores the robustness of simulative predictions in interpreting observed galactic phenomena.

Implications and Speculation on Future Work

The paper provides a straightforward, observationally-based method to deduce the merger history of our galaxy through the age-thickness profile of its stellar disk. The implications are broad, enhancing our understanding of the Milky Way's past dynamics and interactions. Theoretically, this research suggests a refined approach for examining external galaxies, potentially providing comparative baselines for galactic evolution studies.

Future advancements in this area may emerge from improved age estimates and larger datasets from ongoing and upcoming sky surveys. With further refinements, the age-thickness methodology could offer an even more precise catalog of past merger events in our galaxy, and potentially aid in constructing robust models of galactic evolution that could be extrapolated to other galaxies, contributing to a detailed cosmological understanding of galaxy formation and development.

This paper's integration of extensive data sets and cosmological simulations presents a solid advancement in uncovering the Milky Way's evolution through its stellar disk, providing a concrete basis for subsequent astronomical endeavors examining the history of galactic mergers. As datasets expand and simulation capabilities improve, this method may evolve into a foundational tool for galaxy evolution studies.