Unlocking the Secrets of the Stellar Halo: Dynamical Streams and the Galactic Bar
The Milky Way is a vast and complex galaxy, home to billions of stars that move in intricate patterns. Some of these stars travel together in groups known as "streams." While stellar streams in the Milky Way’s disk have been extensively studied, less is known about similar structures in the galaxy’s halo—the large, diffuse region surrounding the disk. In this study, Dillamore et al. investigate the motion of stars in the local stellar halo and explore how these stars may be influenced by the Milky Way’s central bar, a dense, elongated collection of stars at the galaxy’s core.
Resonant Trapping and the Galactic Bar
Stars in the Milky Way follow orbits that can be described by three key frequencies: one for their radial motion (moving toward and away from the center), one for their movement around the galaxy, and one for their vertical motion relative to the disk. When these frequencies align in certain ways, stars can become "trapped" in resonances, meaning their orbits are locked into a predictable pattern due to gravitational influences. The authors focus on two key resonances caused by the Milky Way’s rotating bar: the corotation resonance (CR) and the outer Lindblad resonance (OLR). These resonances can create moving groups of stars that are not random but rather guided by the gravitational forces at play in the galaxy.
Observational Data from Gaia
To explore these moving groups, the authors use data from the European Space Agency’s Gaia mission, which provides precise measurements of the positions, motions, and brightness of stars. They select stars from Gaia Data Release 3 (DR3) and transform their coordinates into a system centered on the Milky Way to better analyze their movements. They also apply filters to exclude stars near globular clusters to focus on halo stars.
Simulating the Galactic Halo
To compare their findings with theoretical expectations, the researchers run a test particle simulation. This simulation models the movement of stars in a Milky Way-like potential, incorporating a slowly decelerating central bar. By introducing a time-dependent gravitational influence from the bar, they can observe how stars react over billions of years. The simulation helps to confirm whether the structures seen in the Gaia data can indeed be explained by resonances with the bar.
Results: Trapped Orbits and Moving Groups
The study identifies two prominent moving groups in the local stellar halo, named Iphicles and the Beret. These groups consist of stars moving inward toward the galaxy's center, aligning with the expected positions of the CR and OLR. The authors analyze velocity distributions and find asymmetries in motion that suggest these stars are not just random members of the halo but are dynamically connected to the resonances of the bar. Their results show a strong agreement between observations and the predictions from their simulation.
Constraining the Milky Way’s Mass and Bar Pattern Speed
By fitting orbits to the moving groups, the researchers can estimate two important properties of the Milky Way: its mass enclosed within 20 kiloparsecs (kpc) and the pattern speed of the bar (how fast the bar rotates). They find a mass within 20 kpc consistent with previous studies. Their estimate for the pattern speed of the bar is slightly lower than other recent measurements but still within the expected range. These findings refine our understanding of the galaxy’s overall structure and dynamics.
Conclusion
This study demonstrates that moving groups in the stellar halo can be powerful tools for studying the Milky Way’s gravitational field. The identification of dynamical streams influenced by the bar adds a new dimension to our understanding of how stars in the halo move and interact with the rest of the galaxy. As future Gaia data releases and upcoming ground-based surveys provide even more precise stellar measurements, astronomers will be able to build even better models of the Milky Way, unlocking more secrets about its history and evolution.
Source: Dillamore
