Non-equilibrium in the Solar Neighbourhood using dynamical modelling with Gaia DR2

TL;DR

This paper develops a method to model non-equilibrium dynamics in the Milky Way's Solar Neighbourhood using Gaia DR2 data, revealing local perturbations likely caused by fast-moving stars or gas inhomogeneities.

Contribution

It introduces an orbital arc method that accounts for non-stationarities and massless perturbations, validated with simulations and applied to Gaia data.

Findings

The acceleration field near the Sun is perturbed, with phase space density growth of up to 5% per Myr.

Stars with larger angular momentum are primary carriers of local perturbations.

Perturbations are likely due to fast-moving stars or gas disc inhomogeneities.

Abstract

Matter distribution models of the Milky Way galaxy are usually stationary, although there are known to be wave-like perturbations in the disc at $\sim10\%$ level of the total density. Modelling of the overall acceleration field by allowing non-equilibrium is a complicated task. We must learn to distinguish whether density enhancements are persistent or not by their nature. In the present paper, we elaborate our orbital arc method to include the effects of massless perturbations and non-stationarities in the modelling. The method is tested by modelling of simulation data and shown to be valid. We apply the method to the Gaia DR 2 data within a region of $\sim 0.5$ kpc from the Sun and confirm that acceleration field in the Solar Neighbourhood has a perturbed nature -- the phase space density along the orbits of stars grow in the order of $h\lesssim 5\%$ per Myr due to non-stationarity. This result is a temporally local value and can be used only within the timeframe of a few Myrs. An attempt to pinpoint the origin of the perturbation shows that the stars having larger absolute angular momentum are the main carriers of the local perturbation. As they are faster than the average thin disc star, they are either originating further away and are close in their pericentre or they are perturbed locally by a fast co-moving perturber, such as gas disc inhomogenities.