Iron Snails: non-equilibrium dynamics and spiral abundance patterns

TL;DR

This paper demonstrates that spiral structures in phase-space density caused by galactic perturbations are also reflected in stellar labels like metallicity, allowing for new ways to analyze galaxy dynamics and history.

Contribution

It introduces a method to relate spiral patterns in phase-space density to those in stellar labels, providing a new approach to study galactic perturbations and their effects.

Findings

The Gaia Snail's amplitude and age from elemental abundances match those from phase-space density.

The best model dates the Snail's perturbation to about 400 million years ago.

Significant variations in age are observed with angular momentum.

Abstract

Galaxies are not in a dynamical steady state. They continually undergo perturbations, e.g., from infalling dwarf galaxies and dark-matter substructure. After a dynamical perturbation, stars phase mix towards a new steady state; in so doing they generally form spiral structures, such as spiral density waves in galaxy disks and the Gaia Snail observed in the vertical phase-space density in the solar neighborhood. Structures in phase-space density can be hard to measure accurately, because spatially varying selection effects imprint their own patterns on the density. However, stellar labels such as metallicity, or other element abundances, or stellar masses and ages, can be measured even in the face of complex or unknown spatial selection functions. We show that if the equilibrium galaxy has phase-space gradients in these labels, any perturbation that could raise a spiral wave in the phase-space density will raise a spiral wave in the distribution of labels as well. We work out the relationship between the spiral patterns in the density and in the labels. As an example, we analyze the Gaia Snail and show that its amplitude and dynamical age as derived from elemental abundances (mainly [Mg/Fe]) follow similar patterns to those derived from the phase-space density. Our best model dates the Snail's perturbation to about 400 Myr ago although we find significant variations with angular momentum in the best-fit age. Conceptually, the ideas presented here are related to Orbital Torus Imaging, chemical tagging, and other methods that use stellar labels to trace dynamics.