Orbital Torus Imaging: Using Element Abundances to Map Orbits and Mass in the Milky Way

TL;DR

This paper introduces Orbital Torus Imaging, a new method that uses stellar element abundances and kinematics to map orbits and infer the Milky Way's mass distribution, offering robustness over traditional models.

Contribution

The paper presents a novel dynamical inference approach that leverages stellar labels and actions, reducing assumptions and dependence on survey selection functions.

Findings

Constrained the Milky Way disk mass at a few-percent level

Demonstrated the method with APOGEE data using only eight element ratios

Validated the robustness of the approach against traditional methods

Abstract

Many approaches to galaxy dynamics assume that the gravitational potential is simple and the distribution function is time-invariant. Under these assumptions there are traditional tools for inferring potential parameters given observations of stellar kinematics (e.g., Jeans models). However, spectroscopic surveys measure many stellar properties beyond kinematics. Here we present a new approach for dynamical inference, Orbital Torus Imaging, which makes use of kinematic measurements and element abundances (or other invariant labels). We exploit the fact that, in steady state, stellar labels vary systematically with orbit characteristics (actions), yet must be invariant with respect to orbital phases (conjugate angles). The orbital foliation of phase space must therefore coincide with surfaces along which all moments of all stellar label distributions are constant. Both classical-statistics and Bayesian methods can be built on this; these methods will be more robust and require fewer assumptions than traditional tools because they require no knowledge of the (spatial) survey selection function and they do not involve second moments of velocity distributions. We perform a classical-statistics demonstration with red giant branch stars from the APOGEE surveys: We model the vertical orbit structure in the Milky Way disk to constrain the local disk mass, scale height, and the disk--halo mass ratio (at fixed local circular velocity). We find that the disk mass can be constrained (na\"ively) at the few-percent level with Orbital Torus Imaging using only eight element-abundance ratios, demonstrating the promise of combining stellar labels with dynamical invariants.