Catching the Streams: Dynamical Moving Groups in the Milky Way’s Halo
In a perfectly smooth, spinning galaxy, the stars’ motions would look balanced and symmetrical. But the Milky Way tells a messier story. Astronomers have long noticed clumps of stars in the Solar neighborhood, called “moving groups” or “streams”, that share similar motions. These streams, like the famous Hercules and Sirius streams, are thought to arise when stars are trapped by patterns in the Milky Way’s bar and spiral arms. This paper, led by Adam Dillamore, looks beyond the usual streams in the disc to find similar features in the halo, the spherical cloud of stars surrounding the Milky Way. The authors show that even halo stars, with their wild, eccentric orbits, can be caught in the bar’s resonant “traps,” creating streams of stars moving in and out of the galaxy in a highly organized way.
Resonant Trapping Theory
The authors begin with an overview of how these dynamical traps work. In a galaxy, stars follow orbits defined by three main frequencies, how fast they move toward and away from the center, how fast they orbit around it, and how fast they oscillate above and below the plane. When these frequencies match up with the rotation speed of the Milky Way’s central bar in just the right ratio (called a resonance), stars can get trapped. Depending on the resonance, these trapped orbits can show up as streams of stars moving inwards or outwards in the galaxy. The paper explains how these resonances, like the corotation resonance (CR) and outer Lindblad resonance (OLR), carve out distinct regions in velocity space where stars tend to pile up.
Data and Simulations
To find these halo streams, the team used precise measurements of stellar positions, motions, and metallicities from the European Space Agency’s Gaia mission. They focused on stars near the Sun but moving at low angular momentum, characteristic of halo stars. They also ran detailed computer simulations, letting millions of virtual stars evolve in a realistic Milky Way potential that included a slowing bar. This allowed them to compare observed streams with predictions and confirm whether the bar could really trap halo stars into such structures.
Results: Streams in the Halo
The researchers found striking evidence of two inward-moving streams of halo stars at low angular momentum. In the velocity space (a plot of radial velocity vs. angular momentum), these streams appear as clear asymmetries, more stars moving inward than outward in certain regions. These correspond to features seen in previous Gaia studies as “chevrons,” which had been interpreted as remnants of ancient galaxy mergers. Dillamore and colleagues argue instead that these chevrons are not merger debris, but the result of resonant trapping by the Milky Way’s bar. They even named these two new halo streams “Iphicles” and “the Beret,” in keeping with the tradition of mythological and whimsical names.
Tracing Streams Through Space and Metallicity
The authors then tracked these streams beyond the local volume. They showed that the Iphicles and Beret streams persist over a wide range of distances from the center of the galaxy, angles around the galactic center, and even heights above the disk. Furthermore, they analyzed the stars’ metallicities (a measure of how many heavy elements they contain) and found the streams exist over a broad metallicity range. This suggests they are not remnants of any single globular cluster or dwarf galaxy, which would likely have more uniform metallicities, but rather dynamical features created by the galaxy’s own structure.
Fitting the Milky Way’s Potential
Finally, the team used the observed tracks of these streams to infer the shape and mass of the Milky Way’s gravitational potential and the pattern speed of the bar. By fitting orbits trapped in CR and OLR resonances to the observed streams, they measured the enclosed mass of the Milky Way within 20 kiloparsecs as about 2.17×10^11M⊙, and the bar’s pattern speed as approximately 32 km/s/kpc. This slower pattern speed aligns with recent studies, supporting the idea that the bar is slowing down over time.
Conclusions
This study demonstrates that the Milky Way’s bar is not just shaping the thin disc of the galaxy, but also dynamically sculpting the stellar halo. The inward-moving streams Iphicles and the Beret appear to arise from stars trapped in the bar’s resonances, revealing the bar’s influence even on highly eccentric halo stars. Moreover, these streams provide a powerful tool to probe the Milky Way’s mass and structure.
Source: Dillamore
