The North/South Asymmetry of the Galaxy: Possible Connection to the Vertical Phase Space Snail

TL;DR

This paper models the North/South asymmetry of the Galaxy using a phase snail structure in vertical phase space, linking it to disequilibrium features and estimating local dark matter density.

Contribution

It introduces a simple phase snail model to explain N/S asymmetry and fits it to observational data, connecting phase space features with Galactic potential and dark matter estimates.

Findings

The phase snail model reproduces observed N/S asymmetry.

Estimated local dark matter density is 0.0151 M_sun/pc^3.

Model aligns with observed vertical bulk motion and phase spiral features.

Abstract

The Galaxy is found to be in disequilibrium based on recent findings of the North/South (N/S) asymmetry and the phase mixing signatures, such as a phase spiral (snail) structure in the vertical phase space ($z-V_{z}$). We show that the N/S asymmetry in a tracer population of dwarfs may be quantitatively modeled with a simple phase snail model superimposed on a smooth equilibrium background. As the phase snail intersects with the $z$ axis, the number density is enhanced, and the velocity dispersion ($\sigma_{z}$) is decreased relative to the other side of the Galactic plane. Fitting only to the observed asymmetric N/S $\sigma_{z}$ profiles, we obtain reasonable parameters for the phase space snail and the potential utilized in modeling the background, despite the complex dependence of the model on the potential parameters and the significant selection effects of the data. Both the snail shape and the N/S number density difference given by our best-fit model are consistent with previous observations. The equilibrium background implies a local dark matter density of $0.0151^{+0.0050}_{-0.0051}$ ${\rm M}_{\odot}\,{\rm pc}^{-3}$. The vertical bulk motion of our model is similar to the observation, but with a $\sim$1.2 $\rm km\,s^{-1}$ shift. Our work demonstrates the strong correlation between the phase space snail and the N/S asymmetry. Future observational constraints will facilitate more comprehensive snail models to unravel the Milky Way potential and the perturbation history encoded in the snail feature.