Stellar Archaeology Disrupted: How the Milky Way’s Bar Smears Out Substructure
The Milky Way’s past is written in the stars—specifically, in the orbits of ancient globular clusters and tidal debris from long-gone dwarf galaxies. Astronomers have used a set of conserved quantities called integrals of motion (IoMs)—like energy (E) and angular momentum (Lz)—to detect these stellar fossils, assuming these values remain unchanged over billions of years. However, in their new paper, Adam Dillamore and Jason Sanders challenge this assumption, showing that the Galaxy’s rotating bar can significantly blur the signatures of substructure in IoM space.
Introduction & Background
Galaxies like the Milky Way were built through a series of mergers. Evidence of these past collisions can be found in substructures like globular clusters (GCs) and stellar streams. GCs are dense, spherical groups of stars that sometimes shed stars to form streams due to the Galaxy’s tidal forces. Traditionally, astronomers search for such structures in IoM space because stars from a common origin tend to cluster there—unless something has disrupted their orbits.
A major disruptor is the Milky Way’s rotating bar, a large structure of stars near the Galactic center that spins around the core. Previous studies suggested that the bar can perturb or even stretch stellar streams. Dillamore and Sanders explore whether the bar also erases substructure signals in the inner halo, where many old GCs reside.
Theory and Models
To understand the bar’s effects, the authors start with theory. In a rotating potential like that of the Galactic bar, traditional IoMs like E and Lz are no longer conserved. Instead, a different quantity—the Jacobi integral (HJ)—remains nearly constant. Dillamore and Sanders model how the bar changes the orbits of stars using diffusion theory, imagining each star undergoing small “kicks” in angular momentum as it interacts with the bar.
Their calculations show that these interactions stretch stellar debris along lines in IoM space with a slope determined by the bar’s rotation speed (called the pattern speed). As a result, what once appeared as tight clumps in (E, Lz) space becomes blurred streaks. The diffusion is strongest for low-energy, prograde (forward-moving) orbits—precisely the region occupied by many in situ GCs.
Simulations and Results
To test their predictions, the authors run two types of simulations. First, they model stars released from clusters over billions of years and track their motion in both barred and unbarred versions of the Galaxy. As expected, substructure remains compact in an unbarred, axisymmetric Galaxy. But once a rotating bar is introduced—especially one that slows down over time—the simulated debris spreads out dramatically in IoM space.
In one striking result, the authors find that about 75% of observed globular clusters and 25% of known stellar streams lie in regions where the bar can smear out substructure. For clusters on low-energy, prograde, or eccentric orbits, the angular momentum of debris can change by factors of 10–100—enough to erase the telltale clumping in IoM space. However, this blurring follows a predictable path: structures tend to stretch along lines with slopes tied to the average pattern speed of the bar, potentially offering a way to reconstruct the bar’s history.
Implications and a New Approach
Because of this smearing, clustering algorithms searching in (E, Lz, L⊥) space might miss bar-dispersed structures. Dillamore and Sanders suggest an alternative: use the Jacobi integral (HJ) and chemical information like metallicity ([Fe/H]) instead. Even when E and Lz are no longer reliable, HJ remains relatively stable—even in a slowing bar. The authors demonstrate that structures remain more tightly grouped in (HJ, [Fe/H]) space than in traditional IoMs, offering a more robust way to uncover ancient substructure.
Conclusion
This work highlights how the Galactic bar can distort the fossil record of the Milky Way. By combining theory, simulation, and real data, Dillamore and Sanders show that many ancient structures have likely been smeared out by the bar's influence—especially in the inner Galaxy. Their results call for a shift in how we search for substructure: not by relying solely on classical integrals of motion, but by turning to the Jacobi integral and chemical fingerprints. This approach could uncover pieces of our Galaxy’s past that would otherwise remain hidden.
Source: Dillamore
