Tracing the Origins of the Milky Way's Bulge

The Milky Way’s bulge is a boxy, peanut-shaped region near the Galactic center. Unlike a traditional spherical bulge, it forms primarily from disk stars trapped through gravitational instabilities. A puzzling observation arises here: metal-rich stars near the midplane display unexpectedly high line-of-sight velocity dispersion compared to their metal-poor counterparts. This trend flips at higher latitudes. Tristan Boin and colleagues investigated this phenomenon using APOGEE survey data and a detailed N-body simulation.

Observational Insights: Metallicity and Velocity Trends

Using APOGEE Data Release 17, the authors confirm the unusual velocity patterns of bulge stars. Metal-rich stars show steeper dispersion gradients near the midplane compared to metal-poor stars. At higher latitudes, however, the expected trend—where metal-poor stars dominate the velocity dispersion—reasserts itself. This behavior underscores a complex interaction between the metallicity and orbital characteristics of the bulge stars.

Modeling the Bulge: Simulating the Galactic Origins

The researchers employ an N-body simulation to mimic a Milky Way-like galaxy. Their model considers thin and thick disks with distinct metallicity and kinematic properties. Thin-disk stars, typically metal-rich, are kinematically colder and become tightly trapped in the boxy bulge. Thick-disk stars, often metal-poor, are kinematically hotter and exhibit more uniform dispersion profiles.

Results: Explaining the Inversion

The team’s model successfully reproduces the observed trends. The steep velocity dispersion of metal-rich stars near the midplane is attributed to the bar-like structure of the bulge. The orientation of this structure relative to the Sun plays a significant role in shaping these observed patterns. At higher latitudes, the dominance of metal-poor, thick-disk stars aligns with expectations from traditional kinematics.

Extreme Metal-Rich Stars: A Subset or Continuation?

The study also examines stars with extremely high metallicity ([Fe/H] > 0.3). These stars follow the same velocity trends as their moderately metal-rich counterparts, suggesting they are an integral part of the bar-like structure and not a dynamically distinct population.

Broader Implications and Model Limitations

The findings reinforce the idea that the Milky Way's bulge forms primarily from disk material rather than a classical, spheroidal bulge. However, the simulation lacks a gas component and ongoing star formation, which could further refine the velocity dispersions observed. Additionally, the absence of a stellar halo in the model may slightly underrepresent metal-poor star dispersion at higher latitudes.

Conclusion: A Galactic Dance of Disks

Boin and team’s study highlights how the interplay of disk components gives rise to the Milky Way’s bulge and its peculiar kinematics. The observed trends, driven by a combination of intrinsic disk properties and observational perspective, provide deeper insights into our galaxy's evolutionary history.

Source: Boin