Probing the Effect of Galactic Disequilibrium on Local Dark Matter Density Measurements

Abstract

The goal of this project is to estimate how our ability to measure the dark matter density in the disk local to the Sun is effected by the existence of disequilibria in the local disk plane. Throughout history, measurements of the local dark matter density, based on Jeans’ proposed analysis of stellar data sets in the Solar neighbourhood, has hinged on the assumption that the stars within the disk can be approximated to a collisionless fluid in dynamic equilibrium. However, recent measurements from satellites like SEGUE and Gaia [1] [2] indicate the existence of vertical velocity waves in the disk local to the Sun. One possible source of this disequilibrium has been postulated to be interactions with the Sagittarius galaxy [3]. Therefore, in this project we generate simulations of the Milky Way galaxy and a neighbouring satellite, modelled like Sagittarius. The two galaxies are placed in proximity to each other to simulate a collision, and the effects on the Milky Way are analysed over time scales longer than 10 Gyr. Then our ability to measure the local dark matter density in the unperturbed Milky Way simulation is compared to our estimates in the perturbed disk, using Jeans mass modelling method, under the assumption of dynamic equilibrium. Through this analysis we are able to determine that accurate estimates of the local dark matter density can be recovered on time scales greater than 5 Gyrs after the merger, with an accuracy that is comparable to our ability to do the same for an unperturbed disk.