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

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 (naïvely) 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.

34 pages, 8 figures. Published in ApJ

References in corpus (16)

• Array Programming with NumPy
• The Gaia mission
• The 2.5 m Telescope of the Sloan Digital Sky Survey
• galpy: A Python Library for Galactic Dynamics
• APOGEE Data and Spectral Analysis from SDSS Data Release 16: Seven Years of Observations Including First Results from APOGEE-South
• The Local Dark Matter Density
• The Apache Point Observatory Galactic Evolution Experiment (APOGEE) Spectrographs
• Target Selection for the SDSS-IV APOGEE-2 Survey
• The APOGEE red-clump catalog: Precise distances, velocities, and high-resolution elemental abundances over a large area of the Milky Way's disk
• The GALAH Survey: Observational Overview and Gaia DR1 companion
• Weighing the stellar constituents of the Galactic halo with APOGEE red giant stars
• The mechanics of tidal streams
• Extended distribution functions for our Galaxy
• Cores and Cusps in the Dwarf Spheroidals
• Inferring the gravitational potential of the Milky Way with a few precisely measured stars
• Exploring the pertubed Milky Way disk and the substructures of the outer disk

Cited by in corpus (3)

• The Open Cluster Chemical Abundances and Mapping Survey: VI. Galactic Chemical Gradient Analysis from APOGEE DR17
• Kinematic signatures of impulsive supernova feedback in dwarf galaxies
• The Baryonic Tully-Fisher Relation in the Local Group and the Equivalent Circular Velocity of Pressure Supported Dwarfs