Weighing the Galactic disk using phase-space spirals

Abstract

We present a new method for inferring the gravitational potential of the Galactic disk, using the time-varying structure of a phase-space spiral in the (z, w)-plane (where z and w represent vertical position and vertical velocity). Our method of inference extracts information from the shape of the spiral and disregards the bulk density distribution that is usually used to perform dynamical mass measurements. In this manner, it is complementary to traditional methods that are based on the assumption of a steady state. Our method consists of fitting an analytical model for the phase-space spiral to data, where the spiral is seen as a perturbation of the stellar number density in the (z, w)-plane. We tested our method on one-dimensional simulations, which were initiated in a steady state and then perturbed by an external force similar to that of a passing satellite. We were able to retrieve the true gravitational potentials of the simulations with high accuracy. The gravitational potential at 400–500 parsec distances from the disk mid-plane was inferred with an error of only a few percent. This is the first paper of a series in which we plan to test and refine our method on more complex simulations, as well as apply our method to Gaia data.

Snowmass2021 Cosmic Frontier White Paper: Dark Matter Physics from Halo Measurements

“…For example, the full three-dimensional gravitational potential can analysed with a method using "normalising flows" [355], which is model independent in terms of symmetry assumptions. It is also possible to relax the assumption of a steady state and extract information from the shape of time-varying dynamical structures of the Milky Way; this is exemplified by a new method [354,356] that infers the gravitational potential using the recently discovered phase-space spiral [357].…”

Snowmass2021 Cosmic Frontier White Paper: Prospects for obtaining Dark Matter Constraints with DESI

“…Such a particle could interact dissipatively forming a rotationally supported disk [129,130]. While searches for such a "dark disk" have been largely unsuccessful [131], this lack of success could in part be due to the assumption that the disk is in steady state [132].…”

“…It has recently been shown that the phase-space spiral, an inherently non-equilibrium feature, especially its morphological structure (its width, flattening and degree of winding), can be used to set extremely tight constraints on the potential of the Galaxy, especially the potential associated with a thin dark disk [132,138]. These authors used Gaia EDR3 kinematics to constrain the surface density of a thin dark disk with a scale height of less then 50 pc to be 5M ⊙ pc −2 much tighter than the constraints placed by previous steadystate models that also used Gaia data [139], essentially ruling out the existence of a "dark disk" and therefore more dissipative species of dark matter particles.…”

Charting galactic accelerations – II. How to ‘learn’ accelerations in the solar neighbourhood

“…F or e xample, Li & Widrow ( 2021 ) fit the phase spiral shape (both their mock spiral and the real Gaia DR2 spiral) to find the time elapsed since the spiral-inducing perturbation. Meanwhile, Widmark, Laporte & de Salas ( 2021b ) and Widmark et al ( 2021c ) use the spiral shape to derive the vertical potential in the Galactic disc, and from there the tightest constraints to date on a thin dark disc.…”

“…Not only can such a phase spiral be used to diagnose disequilibrium, but its shape can also reveal insights into the MW potential and perturbation history (e.g. Li & Widrow 2021 ;Widmark et al 2021b , c ) We compare the performance of our method to that of two other widely used methods: solution of the Jeans equations and fitting the vertical (one-dimensional) DF to parametrized models. Using the same mock data set as an input, our method measures accelerations the most accurately.…”