Bringing the Galaxy's dark halo to life

Abstract

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