Bringing the Galaxy's dark halo to life

TL;DR

This paper introduces a new method to create self-consistent models of Milky Way-like galaxies, revealing how dark halos respond to baryonic matter and affecting local dark matter velocity distributions.

Contribution

The paper develops an iterative action-based approach to model galaxy components and studies the dark halo's shape and velocity response to baryonic infall.

Findings

Dark halo flattens with minor-major axis ratios 0.75-0.95

Halo response depends on velocity anisotropy, with radially biased models forming a dark disc

Dark matter velocity distribution near the Sun is highly non-Gaussian, with increased dispersions

Abstract

We present a new method to construct fully self-consistent equilibrium models of multi-component disc galaxies similar to the Milky Way. We define distribution functions for the stellar disc and dark halo that depend on phase space position only through action coordinates. We then use an iterative approach to find the corresponding gravitational potential. We study the adiabatic response of the initially spherical dark halo to the introduction of the baryonic component and find that the halo flattens in its inner regions with final minor-major axis ratios $q$ = 0.75 - 0.95. The extent of the flattening depends on the velocity structure of the halo particles with radially biased models exhibiting a stronger response. In this latter case, which is according to cosmological simulations the most likely one, the new density structure resembles a "dark disc" superimposed on a spherical halo. We discuss the implications of these results for our recent estimate of the local dark matter density. The velocity distribution of the dark-matter particles near the Sun is very non-Gaussian. All three principal velocity dispersions are boosted as the halo contracts, and at low velocities a plateau develops in the distribution of $v_z$. For models similar to a state-of-the-art Galaxy model we find velocity dispersions around 155 km s$^{-1}$ for $v_z$ and the tangential velocity, $v_\varphi$, and 140 - 175 km s$^{-1}$ for the in-plane radial velocity, $v_R$, depending on the anisotropy of the model.