Tracing Galactic History: Age and Motion in the Milky Way Disk

The study by Weixiang Sun et al. explores the connection between the ages of stars and their movements across the Milky Way's disk, using a large dataset of over 230,000 red clump (RC) stars. These stars serve as reliable markers for understanding the structure and evolution of the galaxy. By examining how the velocity dispersions (random motions) of stars vary with age and location, the research sheds light on the processes shaping the Milky Way, such as mergers, star formation, and disk heating mechanisms.

Data Overview

The study utilizes RC stars identified from two major surveys, LAMOST and Gaia, which provide highly accurate measurements of stellar properties like velocity, chemical composition, and age. After carefully filtering the data to remove sources of error and distinguish between the thin and thick disks of the galaxy, the researchers analyzed nearly 160,000 stars. These stars span a wide region, extending from 5 to 15 kiloparsecs (kpc) from the galaxy's center and up to 3 kpc above the disk plane.

Stellar Motions Across the Disk

The analysis revealed that stars' velocity dispersions increase with age and vary depending on their distance from the galactic center. The researchers modeled these changes using a power-law equation, which showed that older stars generally have higher velocity dispersions. Notably, there is a significant jump in velocity dispersion for stars older than 7–9 billion years, indicating the influence of violent galactic events, such as mergers with smaller galaxies.

Comparing the Thin and Thick Disks

The thin and thick disks of the Milky Way show distinct patterns of motion. Thin disk stars, typically younger, exhibit smoother increases in velocity dispersions, likely due to gradual heating by spiral arms and giant molecular clouds (GMCs). Thick disk stars, on the other hand, display more turbulent motions, suggesting they were shaped by dramatic events like galaxy mergers or chaotic star formation in the galaxy's early history. Interestingly, the thick disk stars, even at similar ages, have consistently higher dispersions than their thin disk counterparts.

Insights from the Galactic Radius

The velocity patterns vary across the galaxy's radius. In the thin disk, radial and vertical motions decline steadily with distance from the center, reflecting the effects of disk flaring and warping in the outer regions. Conversely, the thick disk exhibits a weak increasing trend in velocity dispersions with radius, hinting at different formation and heating mechanisms.

Key Findings and Implications

The study underscores the complex interplay between long-term, steady processes like GMC heating and spiral arm interactions in the thin disk, and short-term, intense events such as mergers in the thick disk. Additionally, a recent minor merger, possibly involving the Sagittarius dwarf galaxy, is suggested to have influenced the outer regions of the disk within the last 3 billion years.

Conclusion

By examining the intricate relationship between age and motion in stars, this research provides a clearer picture of the Milky Way's evolutionary history. The differences between the thin and thick disks highlight the galaxy's layered development, influenced by both gradual processes and significant upheavals. This study not only enhances our understanding of the Milky Way but also sets the stage for future investigations into the dynamics of other galaxies.

Source: Sun