When Galaxies Collide: How a Cosmic Merger Twists Stellar Streams
Stellar streams, long, delicate ribbons of stars pulled from ancient globular clusters, are some of the most beautiful and fragile structures in our galaxy. Claire Guillaume and collaborators explore what happens to these streams when the Milky Way experiences a major collision with another galaxy. Using high-resolution computer simulations, they show that such mergers can leave behind subtle but long-lasting distortions, changing how the leading and trailing arms of a stream evolve over billions of years.
A Galaxy in Motion
The study begins by setting the stage: stellar streams are powerful tools for mapping the Milky Way’s invisible mass, including dark matter. Because these streams are so thin and gravitationally “cold,” even small disturbances, such as encounters with giant molecular clouds, globular clusters, or dark matter clumps, can leave noticeable gaps or wiggles in their structure. Yet, most previous studies assumed the Milky Way was an isolated galaxy. Guillaume and her team instead explore what happens when this calm environment is shattered by a major galactic merger, like the Milky Way’s ancient encounter with the Gaia-Sausage-Enceladus galaxy.
Simulating a Galactic Collision
To study these interactions, the team used a detailed N-body simulation, a type of model that tracks millions of particles under gravity. Their “Milky Way–like” galaxy contained six components, including thin and thick stellar disks, a bulge, a dark halo, and 36 globular clusters, each of which formed a stellar stream. They then introduced a smaller satellite galaxy with one-tenth the mass of the host, placed on a prograde orbit, to mimic a realistic merger. For comparison, they also ran a “control” simulation of the same galaxy without the merger. Both systems evolved for ten billion years, long enough for the galaxies to collide and settle into a new state.
Measuring the Damage with Machine Learning
One of the paper’s innovations is how the team detected and measured stream asymmetries. Because the simulated streams often wrapped around the galaxy multiple times, traditional orbit-tracing methods could not easily follow them. Instead, Guillaume used a machine-learning framework called 1-DREAM (1D Recovery, Extraction, and Analysis of Manifolds) to identify and map the long, filament-like shapes of the streams. This method allowed the team to separate each stream into leading and trailing arms and measure their densities, lengths, and changes over time.
Finding the Imbalance
Even without a merger, stellar streams can develop natural asymmetries due to their elliptical orbits, stretching and contracting as they move closer to or farther from the galactic center. But when the simulated merger occurred, new and stronger distortions appeared. The satellite galaxy’s gravitational pull created gaps and density variations, especially in the leading arms of some streams, that persisted for billions of years. To quantify these effects, Guillaume introduced a new asymmetry metric, based on comparing the cumulative density profiles of each arm. This method captured subtle differences that previous length-based measurements missed. While the global asymmetry signal faded after about 2.5 billion years, some individual streams, particularly those on wide orbits, kept their imprints much longer.
Untangling the Many Influences
In the discussion, the authors caution that stellar streams can also be affected by many other factors, such as the Milky Way’s bar, spiral arms, or encounters with smaller satellites, making it hard to identify which feature came from which event. Their collisionless simulations, which ignore star-by-star interactions, cannot fully capture the internal evolution of globular clusters, but they still reveal how large-scale gravitational effects shape the streams’ overall structure. The results show that a galactic merger can easily mimic or obscure the effects of dark matter subhaloes, complicating attempts to use stream asymmetries as “dark matter detectors.”
Looking Back Through Cosmic History
Guillaume and her collaborators demonstrate that stellar streams can serve as cosmic archaeologists, recording not just the structure of dark matter but also the violent history of galactic mergers. Their work suggests that when interpreting the twists and asymmetries of these streams, astronomers must consider not only small-scale encounters but also the galaxy’s past collisions. Streams on wide orbits, which are less disrupted by the Milky Way’s disk, may be the best places to look for the fossil signatures of ancient mergers.
Source: Guillaume
