Peering Into Galaxy Halos With Only a Few Clues: What Stellar Motions Can Really Tell Us
Gherghinescu and collaborators present a study that asks a surprisingly practical question: how much can astronomers truly learn about a galaxy’s dark matter halo when they only know where its stars are on the sky and how fast those stars move along our line of sight? This is a realistic limitation, because for most galaxies outside the Milky Way, astronomers cannot measure the full 3D motion of individual stars. The team develops axisymmetric, action-based dynamical models that can work with these restricted data and tests the limits of what the models can recover, especially when information about distances or proper motions is missing.
The Role of Stellar Haloes in Galaxy Formation
The paper begins by placing the work in the larger context of galaxy formation. In the ΛCDM model, galaxies grow through mergers and are dominated by dark matter at large scales. The shapes and mass profiles of dark matter halos differ depending on the nature of dark matter, so constraining these properties remains a major scientific goal. Because stellar halos preserve clues about a galaxy’s history, their distribution in phase space is an important probe of the gravitational potential.
A Dynamical Modeling Framework for Missing Data
To approach this problem, Gherghinescu introduces a dynamical modeling framework built upon earlier work that used full 6D stellar phase-space information. The authors adapt this approach to handle increasingly incomplete data, 5D, 4D, or the fully realistic 3D case where only on-sky positions and line-of-sight velocities are available. Their models include an axisymmetric galaxy potential (bulge, disc, and dark matter halo) and a double-power-law distribution function describing the stellar halo. A key innovation is an improved method for marginalizing over missing dimensions, allowing the model to use incomplete measurements without artificially biasing the results.
Testing the Method on Mock Galaxies and Simulations
The authors then apply their method to two main testbeds: (1) idealized mock galaxies, which allow for controlled experiments on how missing phase-space information affects the inferred mass distribution, and (2) the Auriga 23 simulated galaxy, which provides a more realistic case with complex stellar structures. These tests reveal clear trends. When full 6D information is available, the method successfully recovers both the total and dark matter mass distributions. As information is removed, the mass profiles remain well constrained overall, but uncertainties grow, especially in the inner regions of the galaxy. The inclination of the galaxy complicates matters further, influencing which velocity components are best recovered; for instance, the vertical velocity component is reliably recovered when the galaxy is face-on but becomes more biased when viewed edge-on.
A Fundamental Degeneracy in Dark Matter Halo Shapes
A crucial result of the paper is that while the overall mass distribution of a galaxy remains recoverable, the flattening of the dark matter halo (q_DM) becomes nearly impossible to constrain when only 3D or 4D phase-space data are available. The authors demonstrate that the inferred flattening becomes broadly distributed and essentially insensitive to the true value used in the mock galaxies. Importantly, however, this uncertainty does not significantly affect the recovered mass profile, suggesting a fundamental degeneracy rather than a methodological failure. Additional tests using Schwarzschild modeling, a completely different dynamical approach, confirm this conclusion.
Looking Forward
The paper concludes by emphasizing the capabilities and limitations of action-based dynamical modeling for external galaxies. While full characterization of dark matter halo shapes remains out of reach without more complete data, the method reliably recovers galaxy mass distributions even when only on-sky positions and line-of-sight velocities are available. Gherghinescu and co-authors note that combining their technique with other probes, such as stellar streams, may help break these degeneracies in future studies.
Source: Gherghinescu
