We have studied non-equilibrium galactic dynamics in the context of the phase-space spiral, which is a recently discovered perturbation to stars the Milky Way disk. Many open questions about the phase-space spiral still remain; for example, what was the mechanism that produced it, and how to the disk's properties affect its evolution? In order to make progress with these questions, we have studied the spiral's properties (such as amplitude, amount of winding, and rotation phase) and produced maps of how those properties vary with spatial location in the Milky Way disk. We have done so using data from the astrometric Gaia mission, supplemented with spectro-astrometric distance estimates and predictions for missing line-of-sight velocities using Bayesian neural networks.
These maps have shown surprising and highly informative results. The structure in winding and rotation phase are strongly correlated over large spatial scales, and also indicate strong and complex self-gravity effects are essential in order to understand the phase-space spiral's evolution, and in extension also to constrain the formation mechanism that sourced it. These maps are published in the Astrophysical Journal, in an article called "The phase spiral's origin and evolution: indications from its varying properties across the Milky Way disk". Work performed largely in parallel with this work, together with mainly the same collaborators at Columbia University, we have shown that similar spiral property structures arise in self-consistent three-dimensional N-body simulations, where a disk is perturbed by a passing satellite. This is published as an arXiv pre-print under the title "Phase spirals across galactic disks I: Exploring dynamical influences on winding", which is currently in review with the Astrophysical Journal.
We have also studied non-equilibrium galactic dynamics in the context of satellite dwarf galaxies. We have studied NGC 205, which is a satellite of our nearby Andromeda spiral galaxy neighbour, which exhibits features indicative of tidal perturbation, manifest as S-like shapes in its sky-projected surface brightness profile and in its line-of-sight velocity profile along its semi-major axis. Previous work (Howley et al. 2008) speculated that NGC 205's S-like velocity shape could arise if a rotation supported satellite is tidally stripped of material. We proposed a different mechanism, where NGC 205 experiences a strong and short-lived tidal impulse during its pericenter passage by Andromeda, and an S-like velocity shape arises as a very transient phenomena (roughly 20 million years), without significant stripping of material. In this new model, we could also match the line-of-sight velocity dispersion, which was not well matched in previous work.
We developed a novel method of precision inference, which draws information from the S-like velocity profile, in order to infer the satellite's total mass profile and orbit with respect to its host galaxy. While this method can achieve a high precision, it was stymied by the lack of available data for NGC 205. The velocity measurements were limited to a line along the semi-major axis. In order to obtain velocity measurements that are more spatially uniform, we applied for telescope time with Keck II, together with Prof. Marla Geha, who is an observational astronomer at Yale University. This new data will be available for analysis in the near future.
